Archived NIST Technical Series Publication
The attached publication has been archived (withdrawn), and is provided solely for historical purposes.
It may have been superseded by another publication (indicated below).

Archived Publication
Series/Number:
Title:NIST Special Publication (SP) 800-63-2

Electronic Authentication Guideline

Publication Date(s):August 2013

Withdrawal Date:

June 22, 2017

Withdrawal Note:

SP 800-63-2 is superseded by the SP 800-63 suite, as follows. Sections 1-4 are superseded by SP
800-63-3. Section 5 is superseded by SP 800-63A. Sections 6-8 are superseded by SP 800-63B.
Section 9 is superseded by SP 800-63C.

Superseding Publication(s)The attached publication has been superseded by the following publication(s):Series/Number:
Title:
Author(s):Publication Date(s):URL/DOI:

SP 800-63-3, SP 800-63A, SP 800-63B, SP 800-63C
SP 800-63-3 (Digital Identity Guidelines); 800-63A (Digital Identity Guidelines: Enrollment and Identity
Proofing); 800-63B (Digital Identity Guidelines: Authentication and Lifecycle Management); 800-63C(Digital Identity Guidelines: Federation and Assertions)

National Institute of Standards and Technology
June 2017https://doi.org/10.6028/NIST.SP.800-63-3; https://doi.org/10.6028/NIST.SP.800-63a;https://doi.org/10.6028/NIST.SP.800-63b; https://doi.org/10.6028/NIST.SP.800-63c

Additional Information (if applicable)
Contact:
Latest revision of theattached publication:
Related information:
Withdrawal
announcement (link):

Applied Cybersecurity Division (Information Technology Laboratory)
SP 800-63-3, SP 800-63A, SP 800-63B, SP 800-63C (as of June 26,
2017)https://pages.nist.gov/800-63-3/
http://csrc.nist.gov/publications/

N/A

Date updated: :ƵŶĞϮϲ͕ϮϬϭϳNIST Special Publication 800-63-2

Electronic Authentication
Guideline
William E. Burr
Donna F. Dodson
Elaine M. Newton
Ray A. Perlner
W. Timothy Polk
Sarbari Gupta
Emad A. Nabbus

http://dx.doi.org/10.6028/NIST.SP.800-63-2

C O M P U T E R

S E C U R I T Y

NIST Special Publication 800-63-2

Electronic Authentication
Guideline
William E. Burr
Donna F. Dodson
Elaine M. Newton
Ray A. Perlner
W. Timothy Polk
Computer Security Division
Information Technology Laboratory
Sarbari Gupta
Emad A. Nabbus
Electrosoft Services, Inc.
Reston, VA

http://dx.doi.org/10.6028/NIST.SP.800-63-2

August 2013

U.S. Department of Commerce
Penny Pritzker, Secretary
National Institute of Standards and Technology
Patrick D. Gallagher, Under Secretary of Commerce for Standards and Technology and Director

Authority
This publication has been developed by NIST to further its statutory responsibilities under theFederal Information Security Management Act (FISMA), Public Law (P.L.) 107-347. NIST is
responsible for developing information security standards and guidelines, including minimum
requirements for Federal information systems, but such standards and guidelines shall not apply
to national security systems without the express approval of appropriate Federal officials
exercising policy authority over such systems. This guideline is consistent with the requirementsof the Office of Management and Budget (OMB) Circular A-130, Section 8b(3), Securing Agency
Information Systems, as analyzed in Circular A-130, Appendix IV: Analysis of Key Sections.
Supplemental information is provided in Circular A-130, Appendix III, Security of Federal
Automated Information Resources.
Nothing in this publication should be taken to contradict the standards and guidelines mademandatory and binding on Federal agencies by the Secretary of Commerce under statutory
authority. Nor should these guidelines be interpreted as altering or superseding the existing
authorities of the Secretary of Commerce, Director of the OMB, or any other Federal official.
This publication may be used by nongovernmental organizations on a voluntary basis and is not
subject to copyright in the United States. Attribution would, however, be appreciated by NIST.
National Institute of Standards and Technology Special Publication 800-63-2
Natl. Inst. Stand. Technol. Spec. Publ. 800-63-2, 112 pages (August 2013)
CODEN: NSPUE2

http://dx.doi.org/10.6028/NIST.SP.800-63-2
Certain commercial entities, equipment, or materials may be identified in this document in order todescribe an experimental procedure or concept adequately. Such identification is not intended to imply
recommendation or endorsement by NIST, nor is it intended to imply that the entities, materials, or
equipment are necessarily the best available for the purpose.
There may be references in this publication to other publications currently under development by NIST
in accordance with its assigned statutory responsibilities. The information in this publication, including
concepts and methodologies, may be used by Federal agencies even before the completion of suchcompanion publications. Thus, until each publication is completed, current requirements, guidelines,
and procedures, where they exist, remain operative. For planning and transition purposes, Federal
agencies may wish to closely follow the development of these new publications by NIST.
Organizations are encouraged to review all draft publications during public comment periods andprovide feedback to NIST. All NIST Computer Security Division publications, other than the ones
noted above, are available at http://csrc.nist.gov/publications.

Comments on this publication may be submitted to:
National Institute of Standards and Technology
Attn: Computer Security Division, Information Technology Laboratory
100 Bureau Drive (Mail Stop 8930) Gaithersburg, MD 20899-8930
Email: eauth-comments@nist.gov

ii

Reports on Computer Systems Technology
The Information Technology Laboratory (ITL) at the National Institute of Standards andTechnology (NIST) promotes the U.S. economy and public welfare by providing technicalleadership for the Nation’s measurement and standards infrastructure. ITL develops tests, test
methods, reference data, proof of concept implementations, and technical analyses to advance thedevelopment and productive use of information technology. ITL’s responsibilities include thedevelopment of management, administrative, technical, and physical standards and guidelines forthe cost-effective security and privacy of other than national security-related information inFederal information systems. The Special Publication 800-series reports on ITL’s research,guidelines, and outreach efforts in information system security, and its collaborative activities
with industry, government, and academic organizations.

Abstract
This recommendation provides technical guidelines for Federal agencies implementing
electronic authentication and is not intended to constrain the development or use ofstandards outside of this purpose. The recommendation covers remote authentication ofusers (such as employees, contractors, or private individuals) interacting with government
IT systems over open networks. It defines technical requirements for each of four levelsof assurance in the areas of identity proofing, registration, tokens, management processes,
authentication protocols and related assertions. This publication supersedes NIST SP
800-63-1.

Keywords
authentication; authentication assurance; credential service provider; electronic
authentication; electronic credentials; identity proofing; passwords; PKI; tokens.

iii

Acknowledgements
The authors, William Burr, Donna Dodson, Elaine Newton, Ray Perlner, Tim Polk of the
National Institute of Standards and Technology (NIST), and Sarbari Gupta, and Emad
Nabbus of Electrosoft, would like to acknowledge the contributions of our many
reviewers, including Jim Fenton from OneID, Hildegard Ferraiolo from NIST, and Erika
McCallister from NIST, as well as those from Enspier, Orion Security, and MITRE.

iv

Executive Summary
Electronic authentication (e-authentication) is the process of establishing confidence inuser identities electronically presented to an information system. E-authentication
presents a technical challenge when this process involves the remote authentication ofindividual people over an open network, for the purpose of electronic government andcommerce. The guidelines in this document assume the authentication and transaction
take place across an open network such as the Internet. In cases where the authenticationand transaction take place over a controlled network, agencies may take these security
controls into account as part of their risk assessment.
This recommendation provides technical guidelines to agencies to allow an individual toremotely authenticate his or her identity to a Federal IT system. This document may
inform but does not restrict or constrain the development or use of standards forapplication outside of the Federal government, such as e-commerce transactions. These
guidelines address only traditional, widely implemented methods for remoteauthentication based on secrets. With these methods, the individual to be authenticatedproves that he or she knows or possesses some secret information.
Current government systems do not separate functions related to identity proofing inregistration from credential issuance. In some applications, credentials (used inauthentication) and attribute information (established through identity proofing) could be
provided by different parties. While a simpler model is used in this document, it does notpreclude agencies from separating these functions.
These technical guidelines supplement OMB guidance, E-Authentication Guidance forFederal Agencies [OMB M-04-04] and supersede NIST SP 800-63-1. OMB M-04-04
defines four levels of assurance, Levels 1 to 4, in terms of the consequences ofauthentication errors and misuse of credentials. Level 1 is the lowest assurance level, andLevel 4 is the highest. The OMB guidance defines the required level of authentication
assurance in terms of the likely consequences of an authentication error. As theconsequences of an authentication error become more serious, the required level ofassurance increases. The OMB guidance provides agencies with the criteria for
determining the level of e-authentication assurance required for specific applications andtransactions, based on the risks and their likelihood of occurrence of each application ortransaction.
OMB guidance outlines a 5-step process by which agencies should meet their eauthentication assurance requirements:
1. Conduct a risk assessment of the government system.
2. Map identified risks to the appropriate assurance level.
3. Select technology based on e-authentication technical guidance.

v

4. Validate that the implemented system has met the required assurance level.
5. Periodically reassess the information system to determine technology refresh
requirements.
This document provides guidelines for implementing the third step of the above process.
After completing a risk assessment and mapping the identified risks to the required
assurance level, agencies can select appropriate technology that, at a minimum, meets thetechnical requirements for the required level of assurance. In particular, the document
states specific technical requirements for each of the four levels of assurance in thefollowing areas:•

Identity proofing and registration of Applicants (covered in Section 5),•

Tokens (typically a cryptographic key or password) for authentication(covered in Section 6),•

Token and credential management mechanisms used to establish and maintaintoken and credential information (covered in Section 7),•

Protocols used to support the authentication mechanism between the Claimant
and the Verifier (covered in Section 8),•

Assertion mechanisms used to communicate the results of a remoteauthentication if these results are sent to other parties (covered in Section 9).

A summary of the technical requirements for each of the four levels is provided below.
Level 1 - Although there is no identity proofing requirement at this level, theauthentication mechanism provides some assurance that the same Claimant who
participated in previous transactions is accessing the protected transaction or data. It
allows a wide range of available authentication technologies to be employed and permits
the use of any of the token methods of Levels 2, 3, or 4. Successful authentication
requires that the Claimant prove through a secure authentication protocol that he or she
possesses and controls the token.
Plaintext passwords or secrets are not transmitted across a network at Level 1. However
this level does not require cryptographic methods that block offline attacks byeavesdroppers. For example, simple password challenge-response protocols are allowed.
In many cases an eavesdropper, having intercepted such a protocol exchange, will be able
to find the password with a straightforward dictionary attack.
At Level 1, long-term shared authentication secrets may be revealed to Verifiers. At
Level 1, assertions and assertion references require protection frommanufacture/modification and reuse attacks.
Level 2 – Level 2 provides single factor remote network authentication. At Level 2,
identity proofing requirements are introduced, requiring presentation of identifying
materials or information. A wide range of available authentication technologies can beviemployed at Level 2. For single factor authentication, Memorized Secret Tokens, PreRegistered Knowledge Tokens, Look-up Secret Tokens, Out of Band Tokens, and Single
Factor One-Time Password Devices are allowed at Level 2. Level 2 also permits any of
the token methods of Levels 3 or 4. Successful authentication requires that the Claimant
prove through a secure authentication protocol that he or she controls the token. Online
guessing, replay, session hijacking, and eavesdropping attacks are resisted. Protocols arealso required to be at least weakly resistant to man-in-the middle attacks as defined in
Section 8.2.2.
Long-term shared authentication secrets, if used, are never revealed to any other party
except Verifiers operated by the Credential Service Provider (CSP); however, session(temporary) shared secrets may be provided to independent Verifiers by the CSP. In
addition to Level 1 requirements, assertions are resistant to disclosure, redirection,
capture and substitution attacks. Approved cryptographic techniques are required for all
assertion protocols used at Level 2 and above.
Level 3 – Level 3 provides multi-factor remote network authentication. At least two
authentication factors are required. At this level, identity proofing procedures require
verification of identifying materials and information. Level 3 authentication is based on
proof of possession of the allowed types of tokens through a cryptographic protocol.
Multi-factor Software Cryptographic Tokens are allowed at Level 3. Level 3 also permits
any of the token methods of Level 4. Level 3 authentication requires cryptographic
strength mechanisms that protect the primary authentication token against compromise by
the protocol threats for all threats at Level 2 as well as verifier impersonation attacks.
Various types of tokens may be used as described in Section 6.
Authentication requires that the Claimant prove, through a secure authentication protocol,that he or she controls the token. The Claimant unlocks the token with a password orbiometric, or uses a secure multi-token authentication protocol to establish two-factor
authentication (through proof of possession of a physical or software token in
combination with some memorized secret knowledge). Long-term shared authentication
secrets, if used, are never revealed to any party except the Claimant and Verifiers
operated directly by the CSP; however, session (temporary) shared secrets may beprovided to independent Verifiers by the CSP. In addition to Level 2 requirements,
assertions are protected against repudiation by the Verifier.
Level 4 – Level 4 is intended to provide the highest practical remote network
authentication assurance. Level 4 authentication is based on proof of possession of a key
through a cryptographic protocol. At this level, in-person identity proofing is required.
Level 4 is similar to Level 3 except that only “hard” cryptographic tokens are allowed.
The token is required to be a hardware cryptographic module validated at Federal
Information Processing Standard (FIPS) 140-2 Level 2 or higher overall with at least
FIPS 140-2 Level 3 physical security. Level 4 token requirements can be met by using
the PIV authentication key of a FIPS 201 compliant Personal Identity Verification (PIV)
Card.

vii

Level 4 requires strong cryptographic authentication of all communicating parties and allsensitive data transfers between the parties. Either public key or symmetric key
technology may be used. Authentication requires that the Claimant prove through asecure authentication protocol that he or she controls the token. All protocol threats at
Level 3 are required to be prevented at Level 4. Protocols shall also be strongly resistantto man-in-the-middle attacks. Long-term shared authentication secrets, if used, are neverrevealed to any party except the Claimant and Verifiers operated directly by the CSP;
however, session (temporary) shared secrets may be provided to independent Verifiers by
the CSP. Approved cryptographic techniques are used for all operations. All sensitive
data transfers are cryptographically authenticated using keys bound to the authentication
process.
At Level 4, “bearer” assertions (as defined in Section 9) are not used to establish theidentity of the Claimant to the Relying Party (RP). “Holder-of-key” assertions (as definedin Section 9) may be used, provided that the assertion contains a reference to a key that ispossessed by the Subscriber and is cryptographically linked to the Level 4 token used toauthenticate to the Verifier. The RP should maintain records of the assertions it receives,to support detection of a compromised verifier impersonating the subscriber.

viii

Table of Contents
1. Purpose......................................................................................................................... 1
2. Introduction .................................................................................................................. 1
3. Definitions and Abbreviations ..................................................................................... 6
4. E-Authentication Model............................................................................................. 16
4.1. Overview ............................................................................................................. 16
4.2. Subscribers, Registration Authorities and Credential Service Providers............ 19
4.3. Tokens ................................................................................................................. 20
4.4. Credentials .......................................................................................................... 224.5. Authentication Process........................................................................................ 23
4.6. Assertions ............................................................................................................ 24
4.7. Relying Parties .................................................................................................... 25
4.8. Calculating the Overall Authentication Assurance Level ................................... 25
5. Registration and Issuance Processes .......................................................................... 27
5.1. Overview ............................................................................................................. 27
5.2. Registration and Issuance Threats ...................................................................... 28
5.2.1. Threat Mitigation Strategies ........................................................................ 29
5.3. Registration and Issuance Assurance Levels ...................................................... 30
5.3.1. General Requirements per Assurance Level ................................................ 31
5.3.2. Requirements for Educational and Financial Institutions and other
Organizations ............................................................................................................ 37
5.3.3. Requirements for Certificates Issued under FPKI and Mapped Policies ..... 39
5.3.4. Requirements for One-Time Use ................................................................. 39
5.3.5. Requirements for Derived Credentials......................................................... 40
6. Tokens ........................................................................................................................ 42
6.1. Overview ............................................................................................................. 42
6.1.1. Single-factor versus Multi-factor Tokens .................................................... 43
6.1.2. Token Types................................................................................................. 43
6.1.3. Token Usage ................................................................................................ 45
6.1.4. Multi-Stage Authentication Using Tokens .................................................. 46
6.1.5. Assurance Level Escalation ......................................................................... 46
6.2. Token Threats ..................................................................................................... 47
6.2.1. Threat Mitigation Strategies ........................................................................ 49
6.3. Token Assurance Levels ..................................................................................... 50
6.3.1. Requirements per Assurance Level ............................................................. 50
7. Token and Credential Management ........................................................................... 57
7.1. Overview ............................................................................................................. 57
7.1.1. Categorizing Credentials .............................................................................. 57
7.1.2. Token and Credential Management Activities ............................................ 58
7.2. Token and Credential Management Threats ....................................................... 60
7.2.1. Threat Mitigation Strategies ........................................................................ 62
7.3. Token and Credential Management Assurance Levels ....................................... 62
7.3.1. Requirements per Assurance Level ............................................................. 62
7.3.2. Relationship of PKI Policies to E-Authentication Assurance Levels .......... 68
8. Authentication Process............................................................................................... 69
8.1. Overview ............................................................................................................. 69ix

8.2. Authentication Process Threats........................................................................... 70
8.2.1. Other Threats ............................................................................................... 71
8.2.2. Threat Mitigation Strategies ........................................................................ 72
8.2.3. Throttling Mechanisms ................................................................................ 75
8.2.4. Phishing and Pharming (Verifier Impersonation): SupplementaryCountermeasures ....................................................................................................... 77
8.3. Authentication Process Assurance Levels .......................................................... 79
8.3.1. Threat Resistance per Assurance Level ....................................................... 79
8.3.2. Requirements per Assurance Level ............................................................. 80
9. Assertions ................................................................................................................... 83
9.1. Overview ............................................................................................................. 83
9.1.1. Cookies ........................................................................................................ 87
9.1.2. Security Assertion Markup Language (SAML) Assertions ......................... 88
9.1.3. Kerberos Tickets .......................................................................................... 88
9.2. Assertion Threats ................................................................................................ 89
9.2.1. Threat Mitigation Strategies ........................................................................ 91
9.3. Assertion Assurance Levels ................................................................................ 94
9.3.1. Threat Resistance per Assurance Level ....................................................... 94
9.3.2. Requirements per Assurance Level ............................................................. 95
10. References ................................................................................................................ 99
10.1. General References ........................................................................................... 99
10.2. NIST Special Publications .............................................................................. 100
10.3. Federal Information Processing Standards ..................................................... 101
10.4. Certificate Policies .......................................................................................... 102
Appendix A: Estimating Entropy and Strength .............................................................. 103
Password Entropy ....................................................................................................... 103
A.1 Randomly Selected Passwords.......................................................................... 104
A.2 User Selected Passwords ................................................................................... 105
A.3 Other Types of Passwords................................................................................. 108
Appendix B: Mapping of Federal PKI Certificate Policies and PIV Credentials to Eauthentication Assurance Levels..................................................................................... 111

xElectronic Authentication Guideline

1. Purpose
This recommendation provides technical guidelines to agencies for the implementation ofelectronic authentication (e-authentication).

2. Introduction
Electronic authentication (e-authentication) is the process of establishing confidence inuser identities electronically presented to an information system. E-authentication
presents a technical challenge when this process involves the remote authentication ofindividual people over a network. This recommendation provides technical guidelines toagencies to allow an individual person to remotely authenticate his/her identity to a
Federal Information Technology (IT) system. This recommendation also provides
guidelines for Registration Authorities (RAs), Verifiers, Relying Parties (RPs) and
Credential Service Providers (CSPs).
Current government systems do not separate the functions of authentication and attributeproviders. In some applications, these functions are provided by different parties. While a
combined authentication and attribute provider model is used in this document, it does
not preclude agencies from separating these functions.
These technical guidelines supplement OMB guidance, E-Authentication Guidance forFederal Agencies [OMB M-04-04] and supersede NIST SP 800-63-1. OMB M-04-04
defines four levels of assurance, Levels 1 to 4, in terms of the consequences ofauthentication errors and misuse of credentials. Level 1 is the lowest assurance level andLevel 4 is the highest. The guidance defines the required level of authentication assurance
in terms of the likely consequences of an authentication error. As the consequences of anauthentication error become more serious, the required level of assurance increases. The
OMB guidance provides agencies with criteria for determining the level of eauthentication assurance required for specific electronic transactions and systems, basedon the risks and their likelihood of occurrence.
OMB guidance outlines a 5 step process by which agencies should meet their eauthentication assurance requirements:
1. Conduct a risk assessment of the government system – No specific risk
assessment methodology is prescribed for this purpose; however, the e-RA
tool 1 at <http://www.idmanagement.gov/> is an example of a suitable tool andmethodology, while NIST Special Publication (SP) 800-30 [SP 800-30] offersa general process for Risk Assessment and Risk Mitigation.

1

At the time of publication, the specific URL for this tool is at<http://www.idmanagement.gov/drilldown.cfm?action=era>. Alternatively, the tool can be found by
searching for “Electronic Risk and Requirements Assessment (e-RA)” at<http://www.idmanagement.gov/>.

1

Electronic Authentication Guideline

2. Map identified risks to the appropriate assurance level – Section 2.2 of OMB
M-04-04 provides the guidance necessary for agencies to perform thismapping.
3. Select technology based on e-authentication technical guidance – After theappropriate assurance level has been determined, OMB guidance states thatagencies should select technologies that meet the corresponding technicalrequirements, as specified by this document. Some agencies may possess
existing e-authentication technology. Agencies should verify that any existing
technology meets the requirements specified in this document.
4. Validate that the implemented system has met the required assurance level –As some implementations may create or compound particular risks, agencies
should conduct a final validation to confirm that the system achieves therequired assurance level for the user-to-agency process. NIST SP 800-53A[SP 800-53A] provides guidelines for the assessment of the implemented
system during the validation process. Validation should be performed as partof a security authorization process as described in NIST SP 800-37, Revision
1 [SP 800-37].
5. Periodically reassess the information system to determine technology refresh
requirements – The agency shall periodically reassess the information system
to ensure that the identity authentication requirements continue to be satisfied.
NIST SP 800-37, Revision 1 [SP 800-37] provides guidelines on thefrequency, depth and breadth of periodic reassessments. As with the initial
validation process, agencies should follow the assessment guidelines specified
in SP 800-53A [SP 800-53A] for conducting the security assessment.
This document provides guidelines for implementing the third step of the above process.
In particular, this document states specific technical requirements for each of the fourlevels of assurance in the following areas:•

Registration and identity proofing of Applicants (covered in Section 5);•

Tokens (typically a cryptographic key or password) for authentication(covered in Section 6);•

Token and credential management mechanisms used to establish and maintaintoken and credential information (covered in Section 7);•

Protocols used to support the authentication mechanism between the Claimant
and the Verifier (covered in Section 8);•

Assertion mechanisms used to communicate the results of a remoteauthentication, if these results are sent to other parties (covered in Section 9).

The overall authentication assurance level is determined by the lowest assurance level
achieved in any of the areas listed above.
Agencies may adjust the level of assurance using additional risk mitigation measures.
Easing credential assurance level requirements may increase the size of the enabledcustomer pool, but agencies shall ensure that this does not corrupt the system’s choice of2

Electronic Authentication Guideline

the appropriate assurance level. Alternatively, agencies may consider partitioning thefunctionality of an e-authentication enabled application to allow less sensitive functions
to be available at a lower level of authentication and attribute assurance, while moresensitive functions are available only at a higher level of assurance.
These technical guidelines cover remote electronic authentication of human users to IT
systems over a network. They do not address the authentication of a person who is
physically present, for example, for access to buildings, although some credentials andtokens that are used remotely may also be used for local authentication. These technicalguidelines establish requirements that Federal IT systems and service providers
participating in authentication protocols be authenticated to Subscribers. However, theseguidelines do not specifically address machine-to-machine (such as router-to-router)
authentication, or establish specific requirements for issuing authentication credentials
and tokens to machines and servers when they are used in e-authentication protocols withpeople.
The paradigm of this document is that individuals are enrolled and undergo a registration
process in which their identity is bound to a token. Thereafter, the individuals are
remotely authenticated to systems and applications over a network, using the token in anauthentication protocol. The authentication protocol allows an individual to demonstrate
to a Verifier that he or she has possession and control of the token 2, in a manner that
protects the token secret from compromise by different kinds of attacks. Higher
authentication assurance levels require use of stronger tokens, better protection of thetoken and related secrets from attacks, and stronger registration procedures.
This document focuses on tokens that are difficult to forge because they contain some
type of secret information that is not available to unauthorized parties and that ispreferably not used in unrelated contexts. Certain authentication technologies,
particularly biometrics and knowledge based authentication, use information that isprivate rather than secret. While they are discussed to a limited degree, they are largely
avoided because their security is often weak or difficult to quantify 3, especially in the
remote situations that are the primary scope of this document.
Knowledge based authentication achieves authentication by testing the personal
knowledge of the individual against information obtained from public databases. As thisinformation is considered private but not actually secret, confidence in the identity of anindividual can be hard to achieve. In addition, the complexity and interdependencies ofknowledge based authentication systems are difficult to quantify. However, knowledge
based authentication techniques are included as part of registration in this document. In
addition, pre-registered knowledge techniques are accepted as an alternative to passwords
at lower levels of assurance.

2

See Section 3 for the definition of “token” as used in this document, which is consistent with the original
version of SP 800-63, but there are a variety of definitions used in the area of authentication.
3
For example, see article by V. Griffith and M. Jakobsson, entitled “Messin’ with Texas – Deriving
Mother’s Maiden Names Using Public Records,” in RSA CryptoBytes, Winter 2007.

3

Electronic Authentication Guideline

Biometric characteristics do not constitute secrets suitable for use in the conventional
remote authentication protocols addressed in this document either. In the local
authentication case, where the Claimant is observed by an attendant and uses a capturedevice controlled by the Verifier, authentication does not require that biometrics be kept
secret. This document supports the use of biometrics to “unlock” conventional
authentication tokens, to prevent repudiation of registration, and to verify that the same
individual participates in all phases of the registration process.
This document identifies minimum technical requirements for remotely authenticating
identity. Agencies may determine based on their risk analysis that additional measures
are appropriate in certain contexts. In particular, privacy requirements and legal risks maylead agencies to determine that additional authentication measures or other process
safeguards are appropriate. When developing e-authentication processes and systems,
agencies should consult OMB Guidance for Implementing the Privacy Provisions of theE-Government Act of 2002 [OMB M-03-22]. See the Guide to Federal Agencies on
Implementing Electronic Processes [DOJ 2000] for additional information on legal risks,
especially those that are related to the need to satisfy legal standards of proof and prevent
repudiation, as well as Use of Electronic Signatures in Federal Organization
Transactions [GSA ESIG].
Additionally, Federal agencies implementing these guidelines should adhere to therequirements of Title III of the E-Government Act, entitled the Federal Information
Security Management Act [FISMA], and the related NIST standards and guidelines.
FISMA directs Federal agencies to develop, document, and implement agency-wide
programs to provide information security for the information and information systems
that support the operations and assets of the agency. This includes the security
authorization of IT systems that support e-authentication. It is recommended that nonFederal entities implementing these guidelines follow equivalent standards of security
management, certification and accreditation to ensure the secure operations of their eauthentication systems.
NIST SP 800-63-1 updated NIST SP 800-63 to reflect current (token) technologies andrestructured to provide a better understanding of the e-authentication architectural model
used here. Additional (minimum) technical requirements were specified for the CSP,
protocols utilized to transport authentication information, and assertions if implemented
within the e-authentication model. Other changes to NIST SP 800-63 included:
•
•
•
•
•
•

Recognition of more types of tokens, including pre-registered knowledge token, lookup secret token, out-of-band token, as well as some terminology changes for moreconventional token types;
Detailed requirements for assertion protocols and Kerberos;
A new section on token and credential management;
Simplification of guidelines for password entropy and throttling;
Emphasis that the document is aimed at Federal IT systems;
Recognition of different models, including a broader e-authentication model (incontrast to the simpler model common among Federal IT systems shown in Figure 1)
and an additional assertion model, the Proxy Model, presented in Figure 6;
4

Electronic Authentication Guideline••

Clarification of differences between Levels 3 and 4 in Table 12; andNew guidelines that permit leveraging existing credentials to issue derived
credentials.

The subsequent sections of NIST SP 800-63-1 presented a series of recommendations forthe secure implementation of RAs, CSPs, Verifiers, and RPs. It should be noted that
secure implementation of any one of these can only provide the desired level of assurance
if the others are also implemented securely. Therefore, the following assumptions weremade in NIST SP 800-63-1:• RAs, CSPs, and Verifiers are trusted entities. Agencies implementing any of
the above trusted entities have some assurance that all other trusted entities
with which the agency interacts are also implemented appropriately for thedesired security level.
•

The RP is not considered a trusted entity. However, in some authentication
systems the Verifier maintains a relationship with the RP to facilitate securecommunications and may employ security controls which only attain their fullvalue when the RP acts responsibly. The Subscriber also trusts the RP toproperly perform the requested service and to follow all relevant privacy
policy.

•It is assumed that there exists a process of certification through which
agencies can obtain the above assurance for trusted entities which they do not
implement themselves.

•

A trusted entity is considered to be implemented appropriately if it complies
with the recommendations in this document and does not behave maliciously.

•

While it is generally assumed that trusted entities will not behave maliciously,
this document does contain some recommendations to reduce and isolate any
damage done by a malicious or negligent trusted entity.

NIST SP 800-63-2 is a limited update of Special Publication 800-63-1 and substantive
changes are made only in section 5. Registration and Issuance Processes. The substantive
changes in the revised draft are intended to facilitate the use of professional credentials in
the identity proofing process, and to reduce the need to use postal mail to an address ofrecord to issue credentials for level 3 remote registration. Other changes to section 5 areminor explanations and clarifications.

5Electronic Authentication Guideline

3. Definitions and Abbreviations
There are a variety of definitions used in the area of authentication. We have kept terms
consistent with the original version of SP 800-63. Pay careful attention to how the terms
are defined here.
Active Attack

Address of Record

Approved

ApplicantAssertion

Assertion Reference

Assurance

Asymmetric Keys

AttackAttacker
Attribute
Authentication

An attack on the authentication protocol where the Attacker transmits
data to the Claimant, Credential Service Provider, Verifier, or Relying
Party. Examples of active attacks include man-in-the-middle,
impersonation, and session hijacking.
The official location where an individual can be found. The address ofrecord always includes the residential street address of an individualand may also include the mailing address of the individual. In very
limited circumstances, an Army Post Office box number, Fleet Post
Office box number or the street address of next of kin or of anothercontact individual can be used when a residential street address for theindividual is not available.
Federal Information Processing Standard (FIPS) approved or NIST
recommended. An algorithm or technique that is either 1) specified in a
FIPS or NIST Recommendation, or 2) adopted in a FIPS or NIST
Recommendation.
A party undergoing the processes of registration and identity proofing.
A statement from a Verifier to a Relying Party (RP) that contains
identity information about a Subscriber. Assertions may also containverified attributes.
A data object, created in conjunction with an assertion, which identifies
the Verifier and includes a pointer to the full assertion held by the
Verifier.
In the context of OMB M-04-04 and this document, assurance isdefined as 1) the degree of confidence in the vetting process used toestablish the identity of an individual to whom the credential
was issued, and 2) the degree of confidence that the individual who
uses the credential is the individual to whom the credential was issued.
Two related keys, a public key and a private key that are used toperform complementary operations, such as encryption and decryption
or signature generation and signature verification.
An attempt by an unauthorized individual to fool a Verifier or a
Relying Party into believing that the unauthorized individual inquestion is the Subscriber.
A party who acts with malicious intent to compromise an information
system.
A claim of a named quality or characteristic inherent in or ascribed tosomeone or something. (See term in [ICAM] for more information.)
The process of establishing confidence in the identity of users orinformation systems.
6Electronic Authentication Guideline

Authentication
ProtocolAuthentication
Protocol Run
Authentication Secret

Authenticity
Bearer Assertion

Bit
Biometrics

Certificate Authority(CA)
Certificate Revocation
List (CRL)
Challenge-Response
ProtocolClaimant

A defined sequence of messages between a Claimant and a Verifier that
demonstrates that the Claimant has possession and control of a valid
token to establish his/her identity, and optionally, demonstrates to the
Claimant that he or she is communicating with the intended Verifier.
An exchange of messages between a Claimant and a Verifier that
results in authentication (or authentication failure) between the twoparties.
A generic term for any secret value that could be used by an Attacker toimpersonate the Subscriber in an authentication protocol.
These are further divided into short-term authentication secrets, which
are only useful to an Attacker for a limited period of time, and longterm authentication secrets, which allow an Attacker to impersonate
the Subscriber until they are manually reset. The token secret is the
canonical example of a long term authentication secret, while the token
authenticator, if it is different from the token secret, is usually a shortterm authentication secret.
The property that data originated from its purported source.
An assertion that does not provide a mechanism for the Subscriber toprove that he or she is the rightful owner of the assertion. The RP has toassume that the assertion was issued to the Subscriber who presents theassertion or the corresponding assertion reference to the RP.
A binary digit: 0 or 1.
Automated recognition of individuals based on their behavioral andbiological characteristics.
In this document, biometrics may be used to unlock authentication
tokens and prevent repudiation of registration.
A trusted entity that issues and revokes public key certificates.
A list of revoked public key certificates created and digitally signed bya Certificate Authority. See [RFC 5280].
An authentication protocol where the Verifier sends the Claimant achallenge (usually a random value or a nonce) that the Claimant
combines with a secret (such as by hashing the challenge and a sharedsecret together, or by applying a private key operation to the challenge)
to generate a response that is sent to the Verifier. The Verifier can
independently verify the response generated by the Claimant (such as
by re-computing the hash of the challenge and the shared secret andcomparing to the response, or performing a public key operation on the
response) and establish that the Claimant possesses and controls thesecret.
A party whose identity is to be verified using an authentication
protocol.

7

Electronic Authentication Guideline

Claimed Address

Completely
Automated Public
Turing test to tellComputers andHumans Apart(CAPTCHA)
CookieCredential

Credential Service
Provider (CSP)

Cross Site Request
Forgery (CSRF)

The physical location asserted by an individual (e.g. an applicant)where he/she can be reached. It includes the residential street address ofan individual and may also include the mailing address of theindividual.
For example, a person with a foreign passport, living in the U.S., willneed to give an address when going through the identity proofing
process. This address would not be an “address of record” but a“claimed address.”
An interactive feature added to web-forms to distinguish use of theform by humans as opposed to automated agents. Typically, it requires
entering text corresponding to a distorted image or from a soundstream.

A character string, placed in a web browser’s memory, which isavailable to websites within the same Internet domain as the server thatplaced them in the web browser.
Cookies are used for many purposes and may be assertions or may
contain pointers to assertions. See Section 9.1.1 for more information.
An object or data structure that authoritatively binds an identity (andoptionally, additional attributes) to a token possessed and controlled bya Subscriber.
While common usage often assumes that the credential is maintainedby the Subscriber, this document also uses the term to refer toelectronic records maintained by the CSP which establish a binding
between the Subscriber’s token and identity.
A trusted entity that issues or registers Subscriber tokens and issues
electronic credentials to Subscribers. The CSP may encompass
Registration Authorities (RAs) and Verifiers that it operates. A CSP
may be an independent third party, or may issue credentials for its own
use.
An attack in which a Subscriber who is currently authenticated to an
RP and connected through a secure session, browses to an Attacker’s
website which causes the Subscriber to unknowingly invoke unwanted
actions at the RP.
For example, if a bank website is vulnerable to a CSRF attack, it may
be possible for a Subscriber to unintentionally authorize a large moneytransfer, merely by viewing a malicious link in a webmail message
while a connection to the bank is open in another browser window.

8Electronic Authentication Guideline

Cross Site Scripting(XSS)

Cryptographic Key

Cryptographic Token
Data Integrity
Derived Credential

Digital Signature

Eavesdropping Attack

Electronic
Authentication (EAuthentication)
Entropy

A vulnerability that allows attackers to inject malicious code into anotherwise benign website. These scripts acquire the permissions ofscripts generated by the target website and can therefore compromise
the confidentiality and integrity of data transfers between the website
and client. Websites are vulnerable if they display user supplied datafrom requests or forms without sanitizing the data so that it is not
executable.
A value used to control cryptographic operations, such as decryption,
encryption, signature generation or signature verification. For the
purposes of this document, key requirements shall meet the minimum
requirements stated in Table 2 of NIST SP 800-57 Part 1.
See also Asymmetric keys, Symmetric key.
A token where the secret is a cryptographic key.
The property that data has not been altered by an unauthorized entity.
A credential issued based on proof of possession and control of a token
associated with a previously issued credential, so as not to duplicate theidentity proofing process.
An asymmetric key operation where the private key is used to digitally
sign data and the public key is used to verify the signature. Digital
signatures provide authenticity protection, integrity protection, andnon-repudiation.
An attack in which an Attacker listens passively to the authentication
protocol to capture information which can be used in a subsequent
active attack to masquerade as the Claimant.
The process of establishing confidence in user identities electronicallypresented to an information system.

A measure of the amount of uncertainty that an Attacker faces todetermine the value of a secret. Entropy is usually stated in bits. See
Appendix A.
Extensible Mark-up
Extensible Markup Language, abbreviated XML, describes a class of
Language (XML)data objects called XML documents and partially describes the
behavior of computer programs which process them.
The FBCA is the entity operated by the Federal Public Key Infrastructure
Federal Bridge
Certification Authority (FPKI) Management Authority that is authorized by the Federal PKI
Policy Authority to create, sign, and issue public key certificates to(FBCA)
Principal CAs.
Federal Information
Title III of the E-Government Act requiring each federal agency to
Security Management develop, document, and implement an agency-wide program to provide
Act (FISMA)
information security for the information and information systems that
support the operations and assets of the agency, including those
provided or managed by another agency, contractor, or other source.

9Electronic Authentication Guideline

Federal Information
Processing Standard(FIPS)

Guessing Entropy

Hash Function

Holder-of-Key
Assertion

Identity
Identity Proofing

Kerberos

Under the Information Technology Management Reform Act (Public
Law 104-106), the Secretary of Commerce approves standards andguidelines that are developed by the National Institute of Standards andTechnology (NIST) for Federal computer systems. These standards andguidelines are issued by NIST as Federal Information Processing
Standards (FIPS) for use government-wide. NIST develops FIPS when
there are compelling Federal government requirements such as forsecurity and interoperability and there are no acceptable industry
standards or solutions. See background information for more details.
FIPS documents are available online through the FIPS home page:
http://www.nist.gov/itl/fips.cfm
A measure of the difficulty that an Attacker has to guess the average
password used in a system. In this document, entropy is stated in bits.
When a password has n-bits of guessing entropy then an Attacker has
as much difficulty guessing the average password as in guessing an nbit random quantity. The Attacker is assumed to know the actual
password frequency distribution. See Appendix A.
A function that maps a bit string of arbitrary length to a fixed length bit
string. Approved hash functions satisfy the following properties:
1. (One-way) It is computationally infeasible to find any input thatmaps to any pre-specified output, and2. (Collision resistant) It is computationally infeasible to find any two
distinct inputs that map to the same output.
An assertion that contains a reference to a symmetric key or a publickey (corresponding to a private key) held by the Subscriber. The RP
may authenticate the Subscriber by verifying that he or she can indeed
prove possession and control of the referenced key.
A set of attributes that uniquely describe a person within a given
context.
The process by which a CSP and a Registration Authority (RA) collectand verify information about a person for the purpose of issuingcredentials to that person.
A widely used authentication protocol developed at MIT. In “classic”
Kerberos, users share a secret password with a Key Distribution Center(KDC). The user, Alice, who wishes to communicate with another user,
Bob, authenticates to the KDC and is furnished a “ticket” by the KDC
to use to authenticate with Bob.
When Kerberos authentication is based on passwords, the protocol isknown to be vulnerable to off-line dictionary attacks by eavesdroppers
who capture the initial user-to- KDC exchange. Longer password
length and complexity provide some mitigation to this vulnerability,
although sufficiently long passwords tend to be cumbersome for users.

10

Electronic Authentication Guideline

Knowledge Based
Authentication

Man-in-the-Middle
Attack (MitM)
Message
Authentication Code(MAC)
Min-entropy

Multi-Factor

Network

Nonce

Off-line Attack

Online Attack

Authentication of an individual based on knowledge of information
associated with his or her claimed identity in public databases.
Knowledge of such information is considered to be private rather thansecret, because it may be used in contexts other than authentication to a
Verifier, thereby reducing the overall assurance associated with theauthentication process.
An attack on the authentication protocol run in which the Attacker
positions himself or herself in between the Claimant and Verifier sothat he can intercept and alter data traveling between them.
A cryptographic checksum on data that uses a symmetric key to detectboth accidental and intentional modifications of the data. MACs
provide authenticity and integrity protection, but not non-repudiation
protection.
A measure of the difficulty that an Attacker has to guess the mostcommonly chosen password used in a system. In this document,
entropy is stated in bits. When a password has n-bits of min-entropy
then an Attacker requires as many trials to find a user with thatpassword as is needed to guess an n-bit random quantity. The Attacker
is assumed to know the most commonly used password(s). See
Appendix A.
A characteristic of an authentication system or a token that uses morethan one authentication factor.
The three types of authentication factors are something you know,
something you have, and something you are.
An open communications medium, typically the Internet, that is used totransport messages between the Claimant and other parties. Unlessotherwise stated, no assumptions are made about the security of the
network; it is assumed to be open and subject to active (i.e.,
impersonation, man-in-the-middle, session hijacking) and passive (i.e.,eavesdropping) attack at any point between the parties (e.g., Claimant,
Verifier, CSP or RP).
A value used in security protocols that is never repeated with the same
key. For example, nonces used as challenges in challenge-response
authentication protocols must not be repeated until authentication keys
are changed. Otherwise, there is a possibility of a replay attack. Usinga nonce as a challenge is a different requirement than a randomchallenge, because a nonce is not necessarily unpredictable.
An attack where the Attacker obtains some data (typically byeavesdropping on an authentication protocol run or by penetrating asystem and stealing security files) that he/she is able to analyze in asystem of his/her own choosing.
An attack against an authentication protocol where the Attacker either
assumes the role of a Claimant with a genuine Verifier or actively alters
the authentication channel.

11Electronic Authentication Guideline

Online Guessing
AttackPassive Attack

Password
Personal Identification
Number (PIN)
Personal Identity
Verification (PIV)
Card

Personally Identifiable
Information (PII)

Pharming

Phishing

Possession and control
of a token
Practice Statement

Private Credentials

Private Key

An attack in which an Attacker performs repeated logon trials byguessing possible values of the token authenticator.
An attack against an authentication protocol where the Attacker
intercepts data traveling along the network between the Claimant andVerifier, but does not alter the data (i.e., eavesdropping).
A secret that a Claimant memorizes and uses to authenticate his or her
identity. Passwords are typically character strings.
A password consisting only of decimal digits.
Defined by [FIPS 201] as a physical artifact (e.g., identity card, smartcard) issued to federal employees and contractors that contains stored
credentials (e.g., photograph, cryptographic keys, digitized fingerprint
representation) so that the claimed identity of the cardholder can beverified against the stored credentials by another person (humanreadable and verifiable) or an automated process (computer readable
and verifiable).
Defined by GAO Report 08-536 as “Any information about anindividual maintained by an agency, including (1) any information thatcan be used to distinguish or trace an individual‘s identity, such as
name, social security number, date and place of birth, mother‘s maiden
name, or biometric records; and (2) any other information that is linked
or linkable to an individual, such as medical, educational, financial, andemployment information.”
An attack in which an Attacker corrupts an infrastructure service such
as DNS (Domain Name Service) causing the Subscriber to be
misdirected to a forged Verifier/RP, which could cause the Subscriber
to reveal sensitive information, download harmful software orcontribute to a fraudulent act.
An attack in which the Subscriber is lured (usually through an email) tointeract with a counterfeit Verifier/RP and tricked into revealing
information that can be used to masquerade as that Subscriber to thereal Verifier/RP.
The ability to activate and use the token in an authentication protocol.
A formal statement of the practices followed by the parties to anauthentication process (i.e., RA, CSP, or Verifier). It usually describes
the policies and practices of the parties and can become legally binding.
Credentials that cannot be disclosed by the CSP because the contents
can be used to compromise the token. (For more discussion, see Section
7.1.1.)The secret part of an asymmetric key pair that is used to digitally signor decrypt data.

12

Electronic Authentication Guideline

Protected Session

Pseudonym
Public Credentials
Public Key
Public Key Certificate

Public Key
Infrastructure (PKI)

Registration

Registration Authority(RA)

Relying Party (RP)RemoteReplay Attack

A session wherein messages between two participants are encrypted
and integrity is protected using a set of shared secrets called session
keys.
A participant is said to be authenticated if, during the session, he, sheor it proves possession of a long term token in addition to the session
keys, and if the other party can verify the identity associated with thattoken. If both participants are authenticated, the protected session issaid to be mutually authenticated.
A false name.
In this document, all unverified names are assumed to be pseudonyms.
Credentials that describe the binding in a way that does notcompromise the token. (For more discussion, see Section 7.1.1.)
The public part of an asymmetric key pair that is used to verify
signatures or encrypt data.
A digital document issued and digitally signed by the private key of a
Certificate authority that binds the name of a Subscriber to a public
key. The certificate indicates that the Subscriber identified in thecertificate has sole control and access to the private key. See also [RFC5280].
A set of policies, processes, server platforms, software andworkstations used for the purpose of administering certificates andpublic-private key pairs, including the ability to issue, maintain, andrevoke public key certificates.
The process through which an Applicant applies to become a
Subscriber of a CSP and an RA validates the identity of the Applicant
on behalf of the CSP.
A trusted entity that establishes and vouches for the identity or
attributes of a Subscriber to a CSP. The RA may be an integral part of a
CSP, or it may be independent of a CSP, but it has a relationship to the
CSP(s).
An entity that relies upon the Subscriber's token and credentials or a
Verifier's assertion of a Claimant’s identity, typically to process atransaction or grant access to information or a system.
(As in remote authentication or remote transaction) An informationexchange between network-connected devices where the information
cannot be reliably protected end-to-end by a single organization’s
security controls.
Note: Any information exchange across the Internet is consideredremote.
An attack in which the Attacker is able to replay previously captured
messages (between a legitimate Claimant and a Verifier) to masquerade
as that Claimant to the Verifier or vice versa.

13

Electronic Authentication Guideline

Risk Assessment

Salt

Secondary
Authenticator

Secure Sockets Layer(SSL)

Security AssertionMark-up Language(SAML)SAML Authentication
Assertion
Session Hijack Attack

Shared SecretSocial Engineering
Special Publication(SP)

Strongly Bound
Credentials
Subscriber
Symmetric Key

The process of identifying the risks to system security anddetermining the probability of occurrence, the resulting impact,
and additional safeguards that would mitigate this impact. Part of
Risk Management and synonymous with Risk Analysis.
A non-secret value that is used in a cryptographic process, usually toensure that the results of computations for one instance cannot bereused by an Attacker.
A temporary secret, issued by the Verifier to a successfullyauthenticated Subscriber as part of an assertion protocol. This secret issubsequently used, by the Subscriber, to authenticate to the RP.
Examples of secondary authenticators include bearer assertions,assertion references, and Kerberos session keys.
An authentication and security protocol widely implemented inbrowsers and web servers. SSL has been superseded by the newer
Transport Layer Security (TLS) protocol; TLS 1.0 is effectively SSL
version 3.1.
An XML-based security specification developed by the Organization
for the Advancement of Structured Information Standards (OASIS) for
exchanging authentication (and authorization) information between
trusted entities over the Internet. See [SAML].
A SAML assertion that conveys information from a Verifier to an RP
about a successful act of authentication that took place between the
Verifier and a Subscriber.
An attack in which the Attacker is able to insert himself or herself
between a Claimant and a Verifier subsequent to a successfulauthentication exchange between the latter two parties. The Attacker isable to pose as a Subscriber to the Verifier or vice versa to control
session data exchange. Sessions between the Claimant and the Relying
Party can also be similarly compromised.
A secret used in authentication that is known to the Claimant and the
Verifier.
The act of deceiving an individual into revealing sensitive information
by associating with the individual to gain confidence and trust.
A type of publication issued by NIST. Specifically, the Special
Publication 800-series reports on the Information Technology
Laboratory's research, guidelines, and outreach efforts in computer
security, and its collaborative activities with industry, government, andacademic organizations.
Credentials that describe the binding between a user and token in atamper-evident fashion. (For more discussion, see Section 7.1.1.)
A party who has received a credential or token from a CSP.
A cryptographic key that is used to perform both the cryptographic
operation and its inverse, for example to encrypt and decrypt, or createa message authentication code and to verify the code.

14Electronic Authentication Guideline

Token

Token Authenticator

Token SecretTransport Layer
Security (TLS)

Trust Anchor

Unverified Name
Valid
Verified Name
Verifier

Verifier Impersonation
AttackWeakly Bound
Credentials
Zeroize

Zero-knowledge
Password Protocol

Something that the Claimant possesses and controls (typically acryptographic module or password) that is used to authenticate theClaimant’s identity.
The output value generated by a token. The ability to generate valid
token authenticators on demand proves that the Claimant possesses andcontrols the token. Protocol messages sent to the Verifier are dependent
upon the token authenticator, but they may or may not explicitly
contain it.
The secret value, contained within a token, which is used to derive
token authenticators.
An authentication and security protocol widely implemented inbrowsers and web servers. TLS is defined by [RFC 2246], [RFC 3546],
and [RFC 5246]. TLS is similar to the older Secure Sockets Layer(SSL) protocol, and TLS 1.0 is effectively SSL version 3.1. NIST SP
800-52, Guidelines for the Selection and Use of Transport Layer
Security (TLS) Implementations specifies how TLS is to be used ingovernment applications.
A public or symmetric key that is trusted because it is directly built into
hardware or software, or securely provisioned via out-of-band means,rather than because it is vouched for by another trusted entity (e.g. in a
public key certificate).
A Subscriber name that is not verified as meaningful by identity
proofing.
In reference to an ID, the quality of not being expired or revoked.
A Subscriber name that has been verified by identity proofing.
An entity that verifies the Claimant’s identity by verifying theClaimant’s possession and control of a token using an authentication
protocol. To do this, the Verifier may also need to validate credentials
that link the token and identity and check their status.
A scenario where the Attacker impersonates the Verifier in anauthentication protocol, usually to capture information that can be used
to masquerade as a Claimant to the real Verifier.
Credentials that describe the binding between a user and token in amanner than can be modified without invalidating the credential. (Formore discussion, see Section 7.1.1.)
Overwrite a memory location with data consisting entirely of bits withthe value zero so that the data is destroyed and not recoverable. This isoften contrasted with deletion methods that merely destroy reference todata within a file system rather than the data itself.
A password based authentication protocol that allows a claimant toauthenticate to a Verifier without revealing the password to the
Verifier. Examples of such protocols are EKE, SPEKE and SRP.

15

Electronic Authentication Guideline

4. E-Authentication Model
4.1. Overview
In accordance with [OMB M-04-04], e-authentication is the process of establishingconfidence in user identities electronically presented to an information system. Systems
can use the authenticated identity to determine if that individual is authorized to perform
an electronic transaction. In most cases, the authentication and transaction take placeacross an open network such as the Internet; however, in some cases access to the
network may be limited and access control decisions may take this into account.
The e-authentication model used in these guidelines reflects current technologies andarchitectures used in government. More complex models that separate functions, such as
issuing credentials and providing attributes, among larger numbers of parties are also
possible and may have advantages in some classes of applications. While a simpler modelis used in this document, it does not preclude agencies from separating these functions.
E-authentication begins with registration. The usual sequence for registration proceeds asfollows. An Applicant applies to a Registration Authority (RA) to become a Subscriber ofa Credential Service Provider (CSP). If approved, the Subscriber is issued a credential by
the CSP which binds a token to an identifier (and possibly one or more attributes that the
RA has verified). The token may be issued by the CSP, generated directly by the
Subscriber, or provided by a third party. The CSP registers the token by creating acredential that binds the token to an identifier and possibly other attributes that the RA
has verified. The token and credential may be used in subsequent authentication events.
The name specified in a credential may either be a verified name or an unverified name.
If the RA has determined that the name is officially associated with a real person and the
Subscriber is the person who is entitled to use that identity, the name is considered averified name. If the RA has not verified the Subscriber’s name, or the name is known todiffer from the official name, the name is considered a pseudonym. The process used toverify a Subscriber’s association with a name is called identity proofing, and is performedby an RA that registers Subscribers with the CSP. At Level 1, identity proofing is not
required so names in credentials and assertions are assumed to be pseudonyms. At Level2, identity proofing is required, but the credential may assert the verified name or apseudonym. In the case of a pseudonym, the CSP shall retain the name verified during
registration. Level 2 credentials and assertions shall specify whether the name is averified name or a pseudonym. This information assists Relying Parties (RPs) in making
access control or authorization decisions. In most cases, only verified names may be
specified in credentials and assertions at Levels 3 and 4. 4 (The required use of a verified

4

Note that [FIPS 201] permits authorized pseudonyms in limited cases and does not differentiate between
credentials using authorized pseudonyms. Nothing in these guidelines should be interpreted as contravening
the contents of the FIPS or constraining the use of these authorized pseudonymous credentials. See
Appendix B for the level of PIV credentials.

16

Electronic Authentication Guidelinename at higher levels of assurance is derived from OMB M-04-04 and is specific toFederal IT systems, rather than a general e-authentication requirement.)
In this document, the party to be authenticated is called a Claimant and the party
verifying that identity is called a Verifier. When a Claimant successfully demonstrates
possession and control of a token to a Verifier through an authentication protocol, theVerifier can verify that the Claimant is the Subscriber named in the corresponding
credential. The Verifier passes on an assertion about the identity of the Subscriber to the
Relying Party (RP). That assertion includes identity information about a Subscriber, suchas the Subscriber name, an identifier assigned at registration, or other Subscriber
attributes that were verified in the registration process (subject to the policies of the CSP
and the needs of the application). Where the Verifier is also the RP, the assertion may be
implicit. The RP can use the authenticated information provided by the Verifier to makeaccess control or authorization decisions.
Authentication establishes confidence in the Claimant’s identity, and in some cases in the
Claimant’s personal attributes (for example the Subscriber is a US Citizen, is a student at
a particular university, or is assigned a particular number or code by an agency ororganization). Authentication does not determine the Claimant’s authorizations or accessprivileges; this is a separate decision. RPs (e.g., government agencies) will use a
Subscriber’s authenticated identity and attributes with other factors to make access
control or authorization decisions.
As part of authentication, mechanisms such as device identity or geo-location could be
used to identify or prevent possible authentication false positives. While thesemechanisms do not directly increase the assurance level for authentication, they canenforce security policies and mitigate risks. In many cases, the authentication process andservices will be shared by many applications and agencies. However, it is the individual
agency or application acting as the RP that shall make the decision to grant access orprocess a transaction based on the specific application requirements.
The various entities and interactions that comprise the e-authentication model used here
are illustrated below in Figure 1. The shaded box on the left shows the registration,
credential issuance, maintenance activities, and the interactions between the
Subscriber/Claimant, the RA and the CSP. The usual sequence of interactions is asfollows:
1. An individual Applicant applies to an RA through a registration process.
2.

The RA identity proofs that Applicant.

3. On successful identity proofing, the RA sends the CSP a registration
confirmation message.
4. A secret token and a corresponding credential are established between the
CSP and the new Subscriber.

17

Electronic Authentication Guideline

5. The CSP maintains the credential, its status, and the registration data collected
for the lifetime of the credential (at a minimum). 5 The Subscriber maintains
his or her token.
Other sequences are less common, but could also achieve the same functional
requirements.
The shaded box on the right side of Figure 1 shows the entities and the interactions
related to using a token and credential to perform e-authentication. When the Subscriber
needs to authenticate to perform a transaction, he or she becomes a Claimant to a
Verifier. The interactions are as follows:
1. The Claimant proves to the Verifier that he or she possesses and controls thetoken through an authentication protocol.
2. The Verifier interacts with the CSP to validate the credential that binds the
Subscriber’s identity to his or her token.
3. If the Verifier is separate from the RP (application), the Verifier provides 6 anassertion about the Subscriber to the RP, which uses the information in theassertion to make an access control or authorization decision.
4. An authenticated session is established between the Subscriber and the RP.
In some cases the Verifier does not need to directly communicate with the CSP tocomplete the authentication activity (e.g., some uses of digital certificates). Therefore, thedashed line between the Verifier and the CSP represents a logical link between the twoentities rather than a physical link. In some implementations, the Verifier, RP and the
CSP functions may be distributed and separated as shown in Figure 1; however, if thesefunctions reside on the same platform, the interactions between the components are local
messages between applications running on the same system rather than protocols overshared untrusted networks.
As noted above, CSPs maintain status information about credentials they issue. CSPs
will generally assign a finite lifetime when issuing credentials to limit the maintenanceperiod. When the status changes, or when the credentials near expiration, credentials
may be renewed or re-issued; or, the credential may be revoked and/or destroyed.
Typically, the Subscriber authenticates to the CSP using his or her existing, unexpired
token and credential in order to request re-issuance of a new token and credential. If the
Subscriber fails to request token and credential re-issuance prior to their expiration or
revocation, he or she may be required to repeat the registration process to obtain a new
token and credential. The CSP may choose to accept a request during a grace period after
expiration.

5

CSPs may be required to maintain this information beyond the lifetime of the credential to support
auditing or satisfy archiving requirements.
6
Many assertion protocols require assertions to be forwarded through the Claimant’s local system before
reaching the Relying Party. For Details, see Section 10.

18

Electronic Authentication Guideline

Figure 1 - The NIST SP 800-63-1 E-Authentication Architectural Model

4.2. Subscribers, Registration Authorities and Credential Service
Providers
The previous section introduced the different participants in the conceptual eauthentication model. This section provides additional details regarding the relationships
and responsibilities of the participants involved with Registration, Credential Issuanceand Maintenance (see the box on the left hand side of Figure 1).
A user may be referred to as the Applicant, Subscriber, or Claimant, depending on thestage in the lifecycle of the credential. An Applicant requests credentials from a CSP. If
the Applicant is approved and credentials are issued by a CSP, the user is then termed a
Subscriber of that CSP. A user may be a Subscriber of multiple CSPs to obtainappropriate credentials for different applications. A Claimant participates in anauthentication protocol with a Verifier to prove they are the Subscriber named in aparticular credential.
The CSP establishes a mechanism to uniquely identify each Subscriber, register the
Subscriber’s tokens, and track the credentials issued to that Subscriber for each token.
The Subscriber may be given credentials to go with the token at the time of registration,or credentials may be generated later as needed. Subscribers have a duty to maintaincontrol of their tokens and comply with the responsibilities to the CSP. The CSP (or theRA) maintains registration records for each Subscriber to allow recovery of registration
records.

19

Electronic Authentication Guideline

There is always a relationship between the RA and CSP. In the simplest and perhaps themost common case, the RA and CSP are separate functions of the same entity. However,an RA might be part of a company or organization that registers Subscribers with anindependent CSP, or several different CSPs. Therefore a CSP may have an integral RA,
or it may have relationships with multiple independent RAs, and an RA may haverelationships with different CSPs as well.
Section 5 specifies requirements for the registration, identity proofing and issuanceprocesses.

4.3. Tokens
The classic paradigm for authentication systems identifies three factors as the cornerstoneof authentication:•

Something you know (for example, a password)•

Something you have (for example, an ID badge or a cryptographic key)•

Something you are (for example, a fingerprint or other biometric data)

Multi-factor authentication refers to the use of more than one of the factors listed above.
The strength of authentication systems is largely determined by the number of factors
incorporated by the system. Implementations that use two factors are considered to bestronger than those that use only one factor; systems that incorporate all three factors arestronger than systems that only incorporate two of the factors. (As discussed in Section
4.1, other types of information, such as location data or device identity, may be used by
an RP or Verifier to reject or challenge a claimed identity, but they are not consideredauthentication factors.)
In e-authentication, the base paradigm is slightly different: the Claimant possesses andcontrols a token that has been registered with the CSP and is used to prove the bearer’s
identity. The token contains a secret the Claimant can use to prove that he or she is the
Subscriber named in a particular credential. 7 In e-authentication, the Claimant
authenticates to a system or application over a network by proving that he or she haspossession and control of a token. The token provides an output called a token
authenticator. This output is used in the authentication process to prove that the Claimantpossesses and controls the token (refer to Section 6.1 for more details), demonstratingthat the Claimant is the person to whom the token was issued. Depending on the type oftoken, this authenticator may or may not be unique for individual authentication
operations.

7

The stipulation that every token contains a secret is specific to these E-authentication guidelines. As
noted elsewhere authentication techniques where the token does not contain a secret may be applicable toauthentication problems in other environments (e.g., physical access).

20

Electronic Authentication Guideline

The secrets contained in tokens are based on either public key pairs (asymmetric keys) or
shared secrets.
A public key and a related private key comprise a public key pair. The private key isstored on the token and is used by the Claimant to prove possession and control of thetoken. A Verifier, knowing the Claimant’s public key through some credential (typicallya public key certificate), can use an authentication protocol to verify the Claimant’s
identity, by proving that the Claimant has possession and control of the associated private
key token.
Shared secrets stored on tokens may be either symmetric keys or passwords. While they
can be used in similar protocols, one important difference between the two is how they
relate to the subscriber. While symmetric keys are generally stored in hardware orsoftware that the Subscriber controls, passwords tend to be memorized by the Subscriber.
As such, keys are something the Subscriber has, while passwords are something he or sheknows. Since passwords are committed to memory, they usually do not have as many
possible values as cryptographic keys, and, in many protocols, are vulnerable to network
attacks that are impractical for keys. Moreover the entry of passwords into systems(usually through a keyboard) presents the opportunity for very simple keyboard logging
attacks, and it may also allow those nearby to learn the password by watching it being
entered. Therefore, keys and passwords demonstrate somewhat separate authentication
properties (something you have rather than something you know). However, when using
either public key pairs or shared secrets, the Subscriber has a duty to maintain exclusive
control of his or her token, since possession and control of the token is used toauthenticate the Claimant’s identity. Token threats are discussed more in Section 6.2.
In this document, e-authentication tokens always contain a secret. So, some of the classic
authentication factors do not apply directly to e-authentication. For example, an ID badge
is something you have, and is useful when authenticating to a human (e.g., a guard), but is
not a token for e-authentication. Authentication factors classified as something you know
are not necessarily secrets, either. Knowledge based authentication, where the claimant
is prompted to answer questions that can be confirmed from public databases, also does
not constitute an acceptable secret for e-authentication. More generally, something you
are does not generally constitute a secret. Accordingly, this recommendation does not
permit the use of biometrics as a token.
However, this recommendation does accept the notional model that authentication
systems that incorporate all three factors offer better security than systems that only
incorporate two of the factors. An e-authentication system may incorporate multiple
factors in either of two ways. The system may be implemented so that multiple factors
are presented to the Verifier, or some factors may be used to protect a secret that will bepresented to the Verifier. If multiple factors are presented to the Verifier, each will need
to be a token (and therefore contain a secret). If a single factor is presented to the
Verifier, the additional factors are used to protect the token and need not themselves betokens.

21Electronic Authentication Guideline

For example, consider a piece of hardware (the token) which contains a cryptographic
key (the token secret) where access is protected with a fingerprint. When used with thebiometric, the cryptographic key produces an output (the token authenticator) which isused in the authentication process to authenticate the Claimant. An impostor must steal
the encrypted key (by stealing the hardware) and replicate the fingerprint to use thetoken. This specification considers such a device to effectively provide two factor
authentication, although the actual authentication protocol between the Verifier and theClaimant simply proves possession of the key.
As noted above, biometrics, when employed as a single factor of authentication, do notconstitute acceptable secrets for e-authentication, but they do have their place in thisspecification. Biometric characteristics are unique personal attributes that can be used toverify the identity of a person who is physically present at the point of verification. They
include facial features, fingerprints, DNA, iris and retina scans, voiceprints and manyother characteristics. This publication recommends that biometrics be used in theregistration process for higher levels of assurance to later help prevent a Subscriber who
is registered from repudiating the registration, to help identify those who commit
registration fraud, and to unlock tokens.
Section 6 provides guidelines on the various types of tokens that may be used forelectronic authentication.

4.4. Credentials
As described in the preceding sections, e-authentication credentials bind a token to the
Subscriber’s name as part of the issuance process. Credentials are issued and maintainedby the CSP; Verifiers use the credentials to authenticate the Claimant’s identity based on
possession and control of the corresponding token. This section provides additional
background regarding the relationship of credentials in the e-authentication model withtraditional (paper) credentials and describes common e-authentication credentials.
Paper credentials are documents that attest to the identity or other attributes of anindividual or entity called the subject of the credentials. Some common paper credentials
include passports, birth certificates, driver’s licenses, and employee identity cards. The
authenticity of paper credentials is established in a variety of ways: traditionally perhaps
by a signature or a seal, special papers and inks, high quality engraving, and today bymore complex mechanisms, such as holograms, that make the credentials recognizableand difficult to copy or forge. In some cases, simple possession of the credentials is
sufficient to establish that the physical holder of the credential is indeed the subject of thecredentials. More commonly, the credentials contain information such as the subject’s
description, a picture of the subject or the handwritten signature of the subject, which canbe used to authenticate that the holder of the credentials is indeed the subject of thecredentials. When these paper credentials are presented in-person, the information
contained in those credentials can be checked to verify that the physical holder of thecredential is the subject.

22Electronic Authentication Guideline

E-authentication credentials may be considered the electronic analog to paper credentials.
In both cases, a valid credential authoritatively binds an identity to the necessary
information for verifying that a person is entitled to claim that identity. However, the use
cases differ in several significant aspects.
The Subject simply possesses and presents the paper credentials in most authentication
scenarios. Since they are generally easy to copy, mere possession of a valid electronic
credential is rarely a sufficient basis for successful authentication. The e-authentication
Claimant possesses a token and presents a token authenticator, but is not necessarily inpossession of the electronic credentials. For example, password database entries are
considered to be credentials for the purpose of this document but are possessed by the
Verifier. X.509 public key certificates are a classic example of credentials the Claimantcan (and often does) possess.
As was the case for paper credentials, in order to authenticate a Claimant using anelectronic credential, the Verifier shall also validate the credential itself (i.e. confirm thatthe credential was issued by an authorized CSP and has not subsequently expired or been
revoked.) There are two ways this can be done: If the credential has been signed by the
CSP, the verifier can validate it by checking the signature. Otherwise, validation may bedone interactively by querying the CSP directly through a secure protocol.
In the remainder of this document, the term “credentials” refers to electronic credentials
unless explicitly noted. Section 7 provides guidelines for token and credential
management activities that are applicable to electronic authentication.

4.5. Authentication Process
The authentication process begins with the Claimant demonstrating possession andcontrol of a token that is bound to the asserted identity to the Verifier through anauthentication protocol. Once possession and control has been demonstrated, the Verifier
verifies that the credential remains valid, usually by interacting with the CSP.
The exact nature of the interaction between the Verifier and the Claimant during theauthentication protocol is extremely important in determining the overall security of thesystem. Well designed protocols can protect the integrity and confidentiality of traffic
between the Claimant and the Verifier both during and after the authentication exchange,
and it can help limit the damage that can be done by an Attacker masquerading as alegitimate Verifier. Additionally, mechanisms located at the Verifier can mitigate online
guessing attacks against lower entropy secrets like passwords and PINs by limiting therate at which an Attacker can make authentication attempts or otherwise delaying
incorrect attempts. (Generally, this is done by keeping track of and limiting the number ofunsuccessful attempts, since the premise of an online guessing attack is that most
attempts will fail.)23

Electronic Authentication Guideline

The Verifier is a functional role, but is frequently implemented in combination with the
CSP and/or the RP. If the Verifier is a separate entity from the CSP, it is often desirable
to ensure that the Verifier does not learn the subscriber’s token secret in the process ofauthentication, or at least to ensure that the Verifier does not have unrestricted access tosecrets stored by the CSP.
Section 8 provides guidelines for the various types of protocols used by the Verifier toauthenticate the Claimant/Subscriber within the e-authentication model.

4.6. Assertions
Upon completion of the authentication process, the Verifier generates an assertion
containing the result of the authentication and provides it to the RP. If the Verifier isimplemented in combination with the RP, the assertion is implicit. If the Verifier is aseparate entity from the RP, the assertion is used to pass information about the Claimantor the authentication process from the Verifier to the RP. Assertions may be
communicated directly to the RP, or can be forwarded through the Claimant, which has
further implications for system design.
An RP trusts an assertion based on the source, the time of creation, and attributes
associated with the Claimant. The Verifier is responsible for providing a mechanism bywhich the integrity of the assertion can be confirmed. The RP is responsible forauthenticating the source (the Verifier) and for confirming the integrity of the assertion.
When the Verifier passes the assertion through the Claimant, the Verifier shall protect the
integrity of the assertion in such a way that it cannot be modified by the Claimant.
However, if the Verifier and the RP communicate directly, a protected session may be
used to provide the integrity protection. When sending assertions across an open
network, the Verifier is responsible for ensuring that any sensitive Subscriber information
contained in the assertion can only be extracted by an RP that it trusts to maintain theinformation’s confidentiality.
Examples of assertions include:•

Cookies – Cookies are character strings, placed in memory, which areavailable to websites within the same Internet domain as the server that placed
them in the web browser. Cookies are used for many purposes and may beassertions or may contain pointers to assertions. 8

8There are specific requirements that agencies must follow when implementing cookies. See OMBMemorandum M-10-22, OMB Guidance for Online Use of Web Measurement and Customization
Technologies, available at:
http://www.whitehouse.gov/sites/default/files/omb/assets/memoranda_2010/m10-22.pdf as well as OMBMemorandum M-03-22, OMB Guidance for Implementing the Privacy Provisions of the E-Government
Act of 2002, available at: http://www.whitehouse.gov/omb/memoranda/m03-22.html.

24

Electronic Authentication Guideline•

SAML Assertions – SAML assertions are specified using a mark-up language
intended for describing security assertions. They can be used by a Verifier tomake a statement to an RP about the identity of a Claimant. SAML assertions
may optionally be digitally signed.

•

Kerberos Tickets – Kerberos Tickets allow a ticket granting authority to issue
session keys to two authenticated parties using symmetric key based
encapsulation schemes.

Section 9 provides guidelines for the use of assertions in authentication protocols.

4.7. Relying Parties
An RP relies on results of an electronic authentication protocol to establish confidence in
the identity or attributes of a Subscriber for the purpose of some transaction. RPs will use
a Subscriber’s authenticated identity, the overall authentication assurance level, and otherfactors to make access control or authorization decisions. The Verifier and the RP may be
the same entity, or they may be separate entities. If they are separate entities, the RP
normally receives an assertion from the Verifier. The RP ensures that the assertion camefrom a Verifier trusted by the RP. The RP also processes any additional information in
the assertion, such as personal attributes or expiration times.
Section 9 provides guidelines for the assertions that may be used by RPs to establish
confidence in the identities of Claimants when the RP and the Verifier are not co-located.

4.8. Calculating the Overall Authentication Assurance Level
The overall authentication assurance level is based on the low watermark of the assurance
levels for each of the components of the architecture. For instance, to achieve an overall
assurance level of 3:•

The registration and identity proofing process shall, at a minimum, use Level
3 processes or higher.

•

The token (or combination of tokens) used shall have an assurance level of 3
or higher.

•

The binding between the identity proofing and the token(s), if proofing isdone separately from token issuance, shall be established at level 3.

•

The authentication protocols used shall have a Level 3 assurance level orhigher.

•

The token and credential management processes shall use a Level 3 assurance
level or higher.

25

Electronic Authentication Guideline•

Authentication assertions (if used) shall have a Level 3 assurance or higher.

The low watermark is the basis for the overall level because the lowest level will likely
be the target of the Attacker. For example, if a system uses a token for authentication thathas Level 2 assurance, but uses other mechanisms that have Level 3 assurance, theAttacker will likely focus on gaining access to the token since it is easier to attack asystem component meeting assurance Level 2 rather than attacking those meeting
assurance Level 3. (See Sections 5 through 9 for information on assurance levels foreach area.)26

Electronic Authentication Guideline

5. Registration and Issuance Processes
5.1. Overview
In the registration process, an Applicant undergoes identity proofing by a trusted RA. If
the RA is able to verify the Applicant’s identity, the CSP registers or gives the Applicant
a token and issues a credential as needed to bind that token to the identity or some related
attribute. The Applicant is now a Subscriber of the CSP and may use the token as a
Claimant in an authentication protocol. This section describes the requirements forregistration and for token and credential issuance.
The RA can be a part of the CSP, or the RA can be a separate and independent entity;
however, a trusted relationship always exists between the RA and CSP. Where the RA
and CSP are separate entities, the trust relationship is often contractual, but the trust
relationship may also be based on laws and regulations, such as when a notary performs
the RA function. The RA or CSP maintain records of the registration. The RA and CSP
can provide services on behalf of an organization or may provide services to the public.
The processes and mechanisms available to the RA for identity proofing may differ as aresult. Where the RA operates on behalf of an organization, the identity proofing process
may be able to leverage a pre-existing relationship (e.g., the Applicant is an employee or
student). Where the RA provides services to the public, the identity proofing process isgenerally limited to confirming publicly available information and previously issued
credentials.
The registration and identity proofing processes are designed based on the required
assurance level, to ensure that the RA/CSP knows the true identity of the Applicant.
Specifically, the requirements include measures to ensure that:•

A person with the Applicant’s claimed attributes exists, and those attributes
are sufficient to uniquely identify a single person;•

The Applicant whose token is registered is in fact the person who is entitled tothe identity;•

It is difficult for the Claimant to later repudiate the registration and dispute anauthentication using the Subscriber’s token.

An Applicant may appear in person to register, or the Applicant may register remotely.
Somewhat different processes and mechanisms apply to identity proofing in each case.
Remote registration is limited to Levels 1 through 3.
After successful identity proofing of the Applicant, the RA registers the Applicant, andthen the CSP is responsible for token and credential issuance for the new Subscriber(additional CSP responsibilities are discussed further in Section 7). Issuance includes
creation of the token. Depending on the type of token being used, the CSP will either
create a new token and supply the token to the Subscriber, or require the Subscriber toregister a token that the Applicant already possesses or has newly created. In either case,27

Electronic Authentication Guideline

the mechanism for transporting the token from the token origination point to the
Subscriber may need to be secured to ensure that the confidentiality and integrity of the
newly established token is maintained and that token is in possession of correct
Applicant.
The CSP is also responsible for the creation of a credential that binds the Subscriber’s
identity to his or her token. Optionally, the CSP may include other verified attributes
about the Subscriber within the credential, such as his or her organizational affiliation,
policies, or constraints for token use.
In models where the registration and identity proofing take place separately fromcredential issuance, the CSP is responsible for verifying that the credential is being issued
to the same person who was identity proofed by the RA. In this model, issuance must be
strongly bound to registration and identity proofing so that an Attacker cannot pose as a
newly registered Subscriber and attempt to collect a token/credential meant for the actual
Subscriber. This attack, and similar attacks, can be thwarted by the methods described in
Section 5.3.1 (below Table 3), which describes which techniques are consideredappropriate for establishing the necessary binding at the various assurance levels.
5.2. Registration and Issuance Threats
There are two general categories of threats to the registration process: impersonation andeither compromise or malfeasance of the infrastructure (RAs and CSPs). This
recommendation concentrates on addressing impersonation threats. Infrastructure threats
are addressed by normal computer security controls (e.g., separation of duties, record
keeping, independent audits) and are outside the scope of this document 9.
The threats to the issuance process include impersonation attacks and threats to thetransport mechanism for the token and credential issuance. Table 1 lists the threats related
to registration and issuance.

9

See NIST SP800-53, Recommended Security Controls For Federal Information Systems for appropriate
security controls.

28

Electronic Authentication Guideline

Table 1 - Registration and Issuance Threats
Activity
10
Registration

Issuance

Threat/Attack
Impersonationof claimed
identity
Repudiation ofregistration

ExampleAn Applicant claims an incorrect identity byusing a forged driver's license.

Disclosure

A key created by the CSP for a Subscriber
is copied by an Attacker as it is transported
from the CSP to the Subscriber during
token issuance.

Tampering

A new password created by the Subscriber
is modified by an Attacker as it is being
submitted to the CSP during the credential
issuance phase.

Unauthorizedissuance

A person claiming to be the Subscriber (butin reality is not the Subscriber) is issued
credentials for that Subscriber.

A Subscriber denies registration, claimingthat he or she did not register that token.

5.2.1. Threat Mitigation Strategies
Registration threats can be deterred by making impersonation more difficult toaccomplish or increasing the likelihood of detection. This recommendation deals
primarily with methods for making impersonation more difficult; however, it doesprescribe certain methods and procedures that may help to prove who carried out animpersonation. At each level, methods are employed to determine that a person with theclaimed identity exists, that the Applicant is the person who is entitled to the claimed
identity, and that the Applicant cannot later repudiate the registration. As the level ofassurance increases, the methods employed provide increasing resistance to casual,systematic and insider impersonation. Table 2 lists strategies for mitigating threats to theregistration and issuance processes.

10

Some impostors may attempt to register as any Subscriber in the system and other impostors may wish toregister as a specific Subscriber.

29

Electronic Authentication Guideline

Table 2 - Registration and Issuance Threat Mitigation Strategies
Activity
Registration

Threat/Attack
Impersonationof claimed
identity

Repudiation ofregistration

Issuance

Mitigation Strategy
RAs request documentation that provides a
specified level of confidence (or assurance)in the identity of the Applicant and makes it
more difficult for imposters to successfully
pass the identity proofing step.
Government issued documents such as
driver’s licenses, and passports presented
by the Applicant are often used to assert
the identity of the Applicant.
Have the Applicant provide non-government
issued documentation (e.g. electricity bills in
the name of the Applicant with the current
address of the Applicant printed on the bill,or a credit card bill) to help in achieving ahigher level of confidence in the identity of
the Applicant.
Have the Applicant sign a form
acknowledging participation in theregistration activity.

Disclosure

Issue the token in person, physically mail itin a sealed envelope to a secure location,or use a protected session to send thetoken electronically.

Tampering

Issue credentials in person, physicallymailing storage media in a sealed envelope,or through the use of a communication
protocol that protects the integrity of thesession data.
Establish a procedure that allows the
Subscriber to authenticate the CSP as thesource of any token and credential data thathe or she may receive.
Establish procedures to ensure that theindividual who receives the token is thesame individual who participated in theregistration procedure.
Implement a dual-control issuance processthat ensures two independent individuals
shall cooperate in order to issue a token
and/or credential.

Unauthorizedissuance

5.3. Registration and Issuance Assurance Levels
The following sections list the NIST recommendations for registration and issuance forthe four levels corresponding to the OMB guidance. As noted in the OMB guidance,
Levels 1 and 2 recognize the use of pseudonymous credentials. When pseudonymous

30Electronic Authentication Guidelinecredentials are used to imply membership in a group, the level of proofing shall beconsistent with the requirements for the credential of that level. Explicit requirements forregistration processes for pseudonymous credentials are not specified, as they are uniqueto the membership criteria for each specific group.
5.3.1. General Requirements per Assurance Level
For levels 2 and above records of registration shall be maintained either by the RA or by
the CSP, depending on the context. Either the RA or the CSP shall maintain a record ofeach individual whose identity has been verified and the steps taken to verify his or her
identity, including any information collected from the Applicant in compliance with thesections below. The CSP shall have the capability to provide records of identity proofing
to RPs if required 11. The identity proofing and registration processes shall be performed
according to applicable written policy or practice statement that specifies the particular
steps taken to verify identities.
For Levels 2 and above, if the RA and CSP are remotely located and communicate over a
network, the entire registration transaction between the RA and CSP shall occur over amutually authenticated protected session. Equivalently, the transaction may consist oftime-stamped or sequenced messages signed by their source and encrypted for theirrecipient. In either case, Approved cryptography is required.
The CSP shall be able to uniquely identify each Subscriber and the associated tokens andthe credentials issued to that Subscriber. The CSP shall be capable of conveying thisinformation to Verifiers. At Level 1, the name associated with the Subscriber is provided
by the Applicant and accepted without verification. At Level 2, the identifier associatedwith the Subscriber may be pseudonymous but the RA or CSP shall retain the actual
identity of the Subscriber. In addition, pseudonymous Level 2 credentials shall be
distinguishable from Level 2 credentials that contain verified names.
At Level 3 and above, the name associated with the Subscriber shall be verified. At alllevels, personally identifiable information (PII) collected as part of the registration
process shall be protected, and all privacy requirements shall be satisfied.
The following text establishes registration requirements specific to each level, except as
noted in the following subsections. There are no level-specific requirements at Level 1.
Both in-person and remote registration are permitted for Levels 2 and 3. Remoteregistration requirements are designed to permit fully automated solutions. However,
implementations may also leverage call centers or online assistance as a substitute orcomplement for fully automated solutions. Explicit requirements are specified for eachscenario in Levels 2 and 3. At Level 4, only in-person registration is permitted.

11

It is beyond the scope of this document to specify what circumstances make it is necessary and/or
appropriate for the CSP to provide this information. Refer to applicable privacy laws, rules of evidence etc.

31

Electronic Authentication Guideline

At Level 2 and higher, the Applicant supplies his or her full legal name, an address ofrecord, and date of birth, and may, subject to the policy of the RA or CSP, also supply
other PII. Detailed level-by-level identity proofing requirements are stated in Table 3.
In some contexts, once an agency has met the minimum registration requirements for anassurance level, the agency may choose to use additional knowledge based authentication
methods to increase confidence in the registration process. For example, an Applicant
could be asked to supply non-public information on his or her past dealing with theagency that could help confirm the Applicant’s identity.
The sensitive data collected during the registration and identity proofing stage shall beprotected at all times (i.e., transmission, storage) to ensure their security andconfidentiality. Additionally, the results of the identity proofing step (which may include
background investigations of the Applicant) have to be protected to ensure source
authentication, confidentiality, and integrity.

32Electronic Authentication Guideline

Table 3 - Identity Proofing Requirements by Assurance Level
In-Person

RemoteBasis forissuing
credentials

Possession of a valid current primary government
picture ID 13 that contains Applicant’s picture, and either
address of record or nationality of record (e.g., driver’s
license or Passport)

RA and CSP
actions

RA inspects photo-ID; compares picture to Applicant;and records the ID number, address and date of birth(DoB). (RA optionally reviews personal information inrecords to support issuance process “a” below.)

Possession of a valid current government ID 14 (e.g., adriver’s license or Passport) number and a financial orutility account number (e.g. checking account, savings
account, utility account, loan or credit card, or tax ID)
confirmed via records of either the government ID or
account number. Note that confirmation of thefinancial or utility account may require supplemental
information from the applicant.
•
RA inspects both ID number and accountnumber supplied by Applicant (e.g., for correct
number of digits). Verifies information provided
by Applicant including ID number OR accountnumber through record checks either with theapplicable agency or institution or through credit
bureaus or similar databases, and confirms that:
name, DoB, address and other personal
information in records are on balance consistent
with the application and sufficient to identify aunique individual. For utility account numbers,
confirmation shall be performed by verifying
knowledge of recent account activity. (Thistechnique may also be applied to some financial
accounts.)•
Address/phone number confirmation andnotification: 15a) CSP issues credentials in a manner that
confirms the ability of the Applicant toreceive mail at a physical address
associated with the Applicant in records; orb) If personal information in records includes atelephone number or e-mail address, the
CSP issues credentials in a manner that
confirms the ability of the Applicant toreceive telephone communications or text
message at phone number or e-mailaddress associated with the Applicant inrecords. Any secret sent over an
unprotected session shall be reset upon firstuse and shall be valid for a maximum
lifetime of seven days; orc) CSP issues credentials. RA or CSP sends
notice to an address of record confirmed in
the records check. 16

Level 212

If the photo-ID appears valid and the photo matches
Applicant then:a) If personal information in records includes atelephone number or e-mail address, the
CSP issues credentials in a manner that
confirms the ability of the Applicant toreceive telephone communications or text
message at phone number or e-mail address
associated with the Applicant in records. Any
secret sent over an unprotected session
shall be reset upon first use; orb) If ID confirms address of record, RA
authorizes or CSP issues credentials. Notice
is sent to address of record, or;c) If ID does not confirm address of record,
CSP issues credentials in a manner that
confirms the claimed address.

12

A token at this Level may also be obtained by authenticating to the CSP using mechanisms at the same ora higher Level (e.g., PIV). See 5.3.5 for more information.
13
The following resources offer examples of what some agencies consider to be primary or secondary ID:• USCIS Form I-9, "Lists of Acceptable Documents", http://www.uscis.gov/files/form/i-9.pdf• Instructions for First Time Passport Applicants
http://travel.state.gov/passport/get/first/first_830.html#step4first• Secondary Evidence of Identification
http://travel.state.gov/passport/get/secondary_evidence/secondary_evidence_4314.html
14
Agencies issuing credentials to foreign nationals residing in foreign countries determine what constitutes
a valid Government issued ID as required.
15
Requirements that use USPS mail for address confirmation and/or notification have a legal basis: Title 18
U.S. Code: Criminal Procedure, Section 1708: Theft or receipt of stolen mail matter generally.
16
Agencies are encouraged to use methods a) and b) where possible to achieve better security. Method c)
is especially weak when not used in combination with knowledge of account activity

33

Electronic Authentication Guideline

Level 3

12

Basis forissuing
credentials

Possession of verified current primary Government
Picture ID that contains Applicant’s picture and either
address of record or nationality of record (e.g., driver’s
license or passport)

RA and CSP
actions

RA inspects photo-ID and verifies via the issuinggovernment agency or through credit bureaus orsimilar databases. Confirms that: name, DoB, address
and other personal information in record are consistent
with the application. Compares picture to Applicant andrecords ID number.
If ID is valid and photo matches Applicant, then:a) If personal information in records includes atelephone number, the CSP issues
credentials in a manner that confirms theability of the Applicant to receive telephone
communications at a number associated with
the Applicant in records, while recording theApplicant’s voice or using alternative meansthat establish an equivalent level of nonrepudiation; orb) If ID confirms address of record, RA
authorizes or CSP issues credentials. Notice
is sent to address of record, or;c) If ID does not confirm address of record,
CSP issues credentials in a manner that
confirms the claimed address.

34

Possession of a valid Government ID (e.g., a driver’s
license or Passport) number and a financial or utility
account number (e.g., checking account, savings
account, utility account, loan or credit card) confirmed
via records of both numbers. Note that confirmation of
the financial or utility account may require
supplemental information from the Applicant.
•RA verifies information provided by Applicant
including ID number AND account number
through record checks either with the applicableagency or institution or through credit bureaus orsimilar databases, and confirms that: name,
DoB, address and other personal information inrecords are consistent with the application andsufficient to identify a unique individual. At aminimum, the records check for both the ID
number AND the account number should confirm
the name and address of the Applicant. For
utility account numbers, confirmation shall be
performed by verifying knowledge of recent
account activity. (This technique may also be
applied to some financial accounts.)•
Address confirmation:a) CSP issues credentials in a manner that
confirms the ability of the applicant toreceive mail at a physical address
associated with the Applicant in records;
or15b) If personal information in records includes
both an electronic address and a physicaladdress that are linked together with theApplicant’s name, and are consistent with
the information provided by the applicant,then the CSP may issue credentials in amanner that confirms ability of the Applicant
to receive messages (SMS, voice or e-mail)sent to the electronic address. Any secretsent over an unprotected session shall bereset upon first use and shall be valid for a
maximum lifetime of seven days.

Electronic Authentication Guideline

Level 4

12

Basis forissuing
credentials

RA and CSP
actions

In-person appearance and verification of:a) a current primary Government Picture ID that
contains Applicant’s picture, and either
address of record or nationality of record(e.g., driver’s license or passport), and;b) either a second, independent Government ID
document that contains current corroborating
information (e.g., either address of record or
nationality of record), OR verification of afinancial account number (e.g., checking
account, savings account, loan or creditcard) confirmed via records.
•
Primary Photo ID:
RA inspects photo-ID and verifies via the issuinggovernment agency or through credit bureaus orsimilar databases. Confirms that: name, DoB,
address, and other personal information in record
are consistent with the application. Compares
picture to Applicant and records ID number.
•
Secondary Government ID or financial accounta) RA inspects secondary Government ID and
if apparently valid, confirms that theidentifying information is consistent with theprimary Photo-ID, or;b) RA verifies financial account number
supplied by Applicant through record checks
or through credit bureaus or similar
databases, and confirms that: name, DoB,
address, and other personal information inrecords are on balance consistent with theapplication and sufficient to identify a unique
individual.

Not Applicable

Not Applicable[Note: Address of record shall be confirmed
through validation of either the primary or
secondary ID.]••

Current Biometric
RA records a current biometric (e.g., photographor fingerprints) to ensure that Applicant cannot
repudiate application.
Credential Issuance
CSP issues credentials in a manner that confirms
address of record.

Remote registration at Levels 2 and 3 requires confirmation of a financial or utility
account number. The requirement for a financial account or utility account number may
be satisfied by a cellular or landline telephone service account under the following
conditions:•the phone is associated in Records with the Applicant's name and address ofrecord; and•the applicant demonstrates that they are able to send or receive messages atthe phone number.

Registration, identity proofing, token creation/issuance, and credential issuance are
separate processes that can be broken up into a number of separate physical encounters orelectronic transactions. (Two electronic transactions are considered to be separate if they35

Electronic Authentication Guideline

are not part of the same protected session.) In these cases, the following methods shall be
used to ensure that the same party acts as Applicant throughout the processes:•

At Level 1, there is no specific requirement, however some effort should bemade to uniquely identify and track applications.

•

At Level 2: For electronic transactions, the Applicant shall identify
himself/herself in any new transaction (beyond the first transaction or
encounter) by presenting a temporary secret which was established during aprior transaction or encounter, or sent to the Applicant’s phone number, email
address, or physical address of record. For physical transactions, the Applicant
shall identify himself/herself in person by either using a secret as describedabove, or by biometric verification (comparing a captured biometric sample toa reference biometric sample that was enrolled during a prior encounter).

•

At Level 3: For electronic transactions, the Applicant shall identify
himself/herself in each new electronic transaction by presenting a temporary
secret which was established during a prior transaction or encounter, or sent to
the Applicant’s phone number, email address, or physical address of record.
Permanent secrets shall only be issued to the Applicant within a protectedsession.
For physical transactions, the Applicant shall identify himself/herself inperson by either using a secret as described above, or through the use of abiometric that was recorded during a prior encounter. Temporary secrets shallnot be reused. If the CSP issues permanent secrets during a physicaltransaction, then they shall be loaded locally onto a physical device that isissued in person to the Applicant or delivered in a manner that confirms theaddress of record.

•

At Level 4: Only physical transactions apply. The Applicant shall identify
himself/herself in person in each new physical transaction through the use of abiometric that was recorded during a prior encounter. 17 If the CSP issues
permanent secrets, then they shall be loaded locally onto a physical device
that is issued in person or delivered in a manner that confirms the address ofrecord.

A common reason for breaking up the registration process as described above is to allow
the subscriber to register or obtain tokens for use in two or more environments. This ispermissible as long as the tokens individually meet the appropriate assurance level.
However, if the exact number of tokens to be issued is not agreed upon early in theregistration process, then the tokens should be distinguishable so that Verifiers will beable to detect whether any suspicious activity occurs during the first few uses of a newly
issued token.
If a valid credential has already been issued, the CSP may issue another credential ofequivalent or lower assurance. In this case, proof of possession and control of the original
token may be substituted for repeating the identity proofing steps. (This is a special case17

Special arrangements can be made for Applicants who are unable to provide the required biometrics.

36

Electronic Authentication Guidelineof a derived credential. See Section 5.3.5 for procedures when the derived credential isissued by a different CSP.) Any requirements for credential delivery at the appropriate
Level shall still be satisfied.

5.3.2. Requirements for Educational and Financial Institutions andother Organizations
The relationships of many organizations (e.g., corporations, healthcare organizations,educational institutions and financial institutions) to the individuals who are employees,
affiliates, associates, students and customers are often regulated or supervised bygovernment, while law and regulation place burdens on these organizations to know theidentities of such individuals. The strength of these relationships and the obligations oforganizations to know identities vary considerably, for example employers have legal
obligations to withhold and pay taxes on employees and are regulated by a variety of
local, state and Federal entities, but the certainty enforced in many employment situations
is not high. Retail stores are not broadly required to know their customers, but financial
institutions are. Healthcare organizations are regulated at many levels and are expected
to know the identities and professional qualifications of their professional staff, as arelegal and accounting firms. This section identifies several areas where theseorganizations may leverage their existing relationships with individuals to act as CAs or
CSPs for those individuals and issue credentials for use with Federal entities.
At Level 2, employers and educational institutions who verify the identity of theiremployees or students by means comparable to those stated above for Level 2 may electto become an RA or CSP and issue credentials to employees or students, either in-person
by inspection of a corporate or school issued picture ID, or through online processes,
where notification is via the distribution channels normally used for sensitive, personalcommunications.
Federal or State laws and regulations impose requirements for institutions in certain
businesses to confirm the educational and licensing credentials for selected employees oraffiliates. For example, a health care organization that has accepted the Medicare"Conditions for Participation" is required to examine the credentials for each candidate
for the medical staff. Where such institutions rigorously confirm the identity, education,and licensing credentials of a licensed professional through an in-person appearance
before employment or affiliation, this specification permits issuing e-authentication
credentials without repeating the identity proofing process.
The initial process for confirming the identity, education, and licensing credentials of alicensed professional through an in-person process must include the following steps:•

Verification of a current primary Government Picture ID that contains
Applicant’s picture, and either address of record or nationality of record (e.g.,a driver’s license or passport);

37

Electronic Authentication Guideline•

Verification of post-secondary education/training of two or more years
appropriate for the position (e.g., an appropriate medical degree); and•Verification of current state or federal licensure (e.g., as a physician) based on
an examination process, with requirements for continuing education or active
professional participation as a condition of valid licensing.

(Examples of licensed professionals that would generally be eligible for streamlined
credential issuance include medical doctors, registered nurses, dentists, pharmacists,
optometrists, veterinarians, lawyers, and licensed professional engineers.)
Institutions that have performed a process satisfying these conditions may issue eauthentication tokens and credentials to those employees and affiliates with verified
credentials at Levels 2, 3, or 4 provided that:•the issuance process is either:
1. in-person, or
2. for Levels 2 and 3, the remote issuance process incorporates theaddress/phone number confirmation appropriate for that level, and•they meet the corresponding provisions of Sections 6 through 9 for that Level.

Federal law, including the Bank Secrecy Act and the USA PATRIOT Act, imposes a dutyon financial institutions to “know their customers” and report suspicious transactions to
help prevent money laundering and terrorist financing. Many financial institutions are
regulated by Federal agencies such as the Office of the Comptroller of the Currency(OCC) or other members of the Federal Financial Institutions Examination Council(FFIEC) and the Securities and Exchanges Commission (SEC). These regulators
normally require the institutions to implement a Customer Identification Program.
The following provisions apply to Federally regulated financial institutions, brokerages
and dealers subject to such Federal regulation, that implement such a Customer
Identification Program:•

At Level 2, such institutions may issue credentials to their customers via themechanisms normally used for online banking or brokerage credentials andmay use online banking or brokerage credentials and tokens as Level 2 eauthentication credentials and tokens, provided they meet the provisions ofSections 6 through 9 for Level 2.

•

At Level 3, such institutions may issue credentials to their customers via themechanisms normally used for online banking or brokerage credentials andmay use online banking or brokerage credentials and tokens as Level 3 eauthentication credentials and tokens, provided:
1. The customers have been in good standing with the institution for a periodof at least 1 year prior to the issuance of e-authentication credentials, and
2. The credentials and tokens meet the provisions of Sections 6 through 9 forLevel 3.
38

Electronic Authentication Guideline•

At Level 4, such institutions may issue credentials to their customers via themechanisms normally used for online banking or brokerage credentials andmay use online banking or brokerage credentials and tokens as Level 4 eauthentication credentials, provided:
1. The customers have appeared in-person before a representative of thefinancial institution, and the representative has inspected a Government
issued primary Photo-ID and compared the picture to the customer; and2. The credentials and tokens meet all additional provisions of Section 5, as
well as all provisions in Sections 6 through 9 for Level 4, as appropriate.

5.3.3. Requirements for Certificates Issued under FPKI and Mapped
Policies
The identity proofing and certificate issuance processes specified in the Federal PKI
Certificate Policies [FBCA1, FBCA2, FBCA3] are considered equivalent to therequirements specified in Section 5.3.1 in accordance with Appendix B.
At Level 2, agencies may rely on any CA whose policy satisfies the identity proofing andregistration requirements specified for Level 2, in addition to any CA cross-certified with
the Federal Bridge CA under one of the certificate policies identified in Appendix B as a
Level 2 certificate or a policy mapped to one of those policies through cross-certificates.
For Levels 3 and 4, agencies shall only accept PKI certificates issued by a CA crosscertified with the Federal Bridge CA under one of the certificate policies identified inAppendix B as a Level 3 or Level 4 certificate or a policy mapped to one of those policies
through cross-certificates.
The identity proofing and certificate issuance processes specified in Federal Information
Processing Standard (FIPS) 201, ‘Personal Identity Verification’ [FIPS201], meet andexceed the Level 4 requirements specified in the preceding section.

5.3.4. Requirements for One-Time Use
For infrequently used applications, issuance and maintenance of credentials would beprohibitively expensive. Claimants can be authenticated for immediate one-time accessto an application for Levels 1 through 3. At Level 1, there is no requirement for identity
proofing before one-time use. At Levels 2 and 3, application owners act as the RA/CSP
in the remote registration processes described in Section 5.3.1, using processes that do
not require confirmation of the address of record and omitting credential issuance.
For immediate one-time access at Level 2, application owners can use the registration
processes specified in Table 3 that:39

Electronic Authentication Guideline••

Confirm "the ability of the Applicant to receive telephone communications ortext message at phone number or e-mail address associated with the Applicant
in records”; orSubsequently send a “notice to an address of record confirmed in the records
check.”For immediate one-time access at Level 3, application owners can use the registration
process specified in Table 3 that:• Confirms "the ability of the Applicant to receive telephone communications at aphone number associated with the Applicant in records while recording theApplicant’s voice or using alternative means that establish an equivalent level of
non-repudiation.”
5.3.5. Requirements for Derived Credentials
Where the Applicant already possesses recognized authentication credentials, the CSP
may choose to identity proof the Claimant by verifying possession and control of thetoken associated with the credentials and issue a new derived credential.
Before issuing any derived credential the CSP shall verify the original credential status
and shall verify that the corresponding token is possessed and controlled by the Claimant.
The status of the original credential should be re-checked at a later date (e.g. after aweek) to confirm that it was not compromised at the time of issuance of the derived
credential. (This guards against the case where an Attacker requests the desired credential
before revocation information can be updated.) Further, the CSP shall record the detailsof the original credential used as the basis for derived credential issuance. If the derived
credential is revoked, the CSP that issued the derived credential may notify the issuer ofthe original credential, if the reason for revocation might motivate action by the issuer ofthe original credential and applicable law, regulation, and agreements permit such
notification.
The CSP may issue a derived level 4 credential for a suitable Level 4 capable token,
based on an original level 4 credential. Before issuing the derived Level 4 credential, the
CSP shall:•Obtain and verify a copy of a biometric recorded when the original credential wasissued. An example of such a biometric is the signed biometric data object on a
PIV card, however if the biometric reference is not available from the Level 4
token, it may be obtained elsewhere, as long as its authenticity is assured;•Compare a fresh biometric sample obtained in person from the Applicant to thereference biometric retained from the original Level 4 credentials and determinethat they match, and;•

Determine that the token that contains the token secret associated with the derived
credential meets the requirements of Table 6 for a Level 4 token.
40Electronic Authentication Guideline

The CSP may issue a Level 3 derived credential based on proof of possession of a Level
4 token. Issuance may be in person or remote. If Level 3 credentials are electronicallytransmitted, or physically shipped with a token to a claimant, then token activation shallrequire proof of possession of both the derived token and the original Level 4 token.
The CSP may issue a Level 2 derived credential based on proof of possession of a Level
3 or 4 token. Issuance may be in person or remote. If Level 2 credentials areelectronically transmitted, or physically shipped with a token to a claimant, then token
activation shall require proof of possession of both the derived token and the original
Level 3 or Level 4 token.
In some cases, there may be a desire to tightly couple the revocation status of the derived
credential to the original. In this case, it is the responsibility of the CSP that issued thederived credential to ensure that a tight coupling is maintained. For example, the issuer ofthe derived credential could regularly monitor the status of the primary credential. 18 19

18This document does not require or prevent CSPs from linking the expiration of the original and derived
credentials. However, where the revocation status is tightly coupled, this may simplify revocation
procedures.
19
Requirements for derived credentials issued by the same CSP are at the end of Section 5.3.1.

41

Electronic Authentication Guideline

6. Tokens
The concept of a token was introduced in Section 4. This section provides a more indepth treatment of e-authentication tokens. Section 6.1 describes classes of tokens
recognized by this recommendation and how they can be combined in practice. Section
6.2 identifies threats and mitigation strategies applicable to tokens. Section 6.3 maps
recognized classes of tokens to assurance levels and identifies any required threat
mitigation strategies.
6.1. Overview
In the e-authentication context, a token contains a secret to be used in authentication
processes. Tokens are possessed by a Claimant and controlled through one or more of
the traditional authentication factors (something you know, have, or are). Figure 2
depicts an abstract model for a token.
The outer box shown in Figure 2 is the token. Tokens may exist in hardware (e.g., asmart card), software (e.g., a software cryptographic module), or may only exist in humanmemory. The inner box represents the token secret that is stored within the token. The
output of a token is the token authenticator, which is the value that is provided to theprotocol stack for transmission to the Verifier to prove that the Claimant possesses andcontrols the token. The token authenticator may be the token secret, or a transformationof the token secret.
There are two optional inputs to the token: token input data; and token activation data.
Token input data, such as a challenge or nonce, may be required to generate the token
authenticator. Token input data may be supplied by the user or be a feature of the token
itself (e.g. the clock in an OTP device). Token activation data, such as a PIN or
biometric, may be required to activate the token and permit generation of anauthenticator. Token activation data is needed when a Claimant controls the token
through something you know or something you are. (Where the token is something you
know, such as a password or memorized secret, token activation is implicit.)
The authenticator is generated through the use of the token. In the general case, anauthenticator is generated by performing a mathematical function using the token secretand one or more optional token input values (a nonce or challenge):
Authenticator = Function (<token secret> [, <nonce>] [, <challenge>] )As noted above, in the trivial case, the authenticator may be the token secret itself (e.g.,where the token is a password).

42

Electronic Authentication Guideline

Figure 2 - Token Model
6.1.1. Single-factor versus Multi-factor Tokens
Tokens are characterized by the number and types of authentication factors that they use.
(See Section 4.3 for discussion on three types of authentication factors.) For example, apassword is something you know, a biometric is something you are, and a cryptographic
identification device is something you have. Tokens may be single-factor or multi-factor
tokens as described below:• Single-factor Token – A token that uses one of the three factors to achieveauthentication. For example, a password is something you know. There areno additional factors required to activate the token, so this is considered single
factor.
•Multi-factor Token – A token that uses two or more factors to achieveauthentication. For example, a private key on a smart card that is activated via
PIN is a multi-factor token. The smart card is something you have, andsomething you know (the PIN) is required to activate the token.

This document does not differentiate between tokens that require two factors and threefactors, as two factors are sufficient to achieve the highest level recognized in thisdocument. Other applications or environments may require such a differentiation.

6.1.2. Token Types
These guidelines recognize the following types of tokens for e-authentication.
•

Memorized Secret Token – A secret shared between the Subscriber and the
CSP. Memorized Secret Tokens are typically character strings (e.g.,
passwords and passphrases) or numerical strings (e.g., PINs.) The token
authenticator presented to the Verifier in an authentication process is the43Electronic Authentication Guideline

secret itself (e.g. the password or passphrase itself). Memorized secret tokens
are something you know.

20•

Pre-registered Knowledge Token – A series of responses to a set of prompts
or challenges. These responses may be thought of as a set of shared secrets.
The set of prompts and responses are established by the Subscriber and CSP
during the registration process. The token authenticator is the set ofmemorized responses to pre-registered prompts during a single run of theauthentication process. An example of a Pre-registered Knowledge Token
would be establishing responses for prompts such as “What was your first
pet’s name?” During the authentication process, the Claimant is asked toprovide the appropriate responses to a subset of the prompts. Alternatively, a
Subscriber might select and memorize an image during the registration
process. In an authentication process, the Claimant is prompted to identify thecorrect images from a set(s) of similar images. Transactions from previously
authenticated sessions could be accepted as Pre-registered Knowledge
Tokens. Pre-registered Knowledge Tokens are something you know.

•

Look-up Secret Token – A physical or electronic token that stores a set ofsecrets shared between the Claimant and the CSP. The Claimant uses thetoken to look up the appropriate secret(s) needed to respond to a prompt fromthe Verifier (the token input). For example, a Claimant may be asked by the
Verifier to provide a specific subset of the numeric or character strings printed
on a card in table format. The token authenticator is the secret(s) identified by
the prompt. Look-up secret tokens are something you have.

•

Out of Band Token – A physical token that is uniquely addressable and canreceive a Verifier-selected secret for one-time use. The device is possessedand controlled by the Claimant and supports private communication 20 over achannel that is separate from the primary channel for e-authentication. The
token authenticator is the received secret and is presented to the Verifier using
the primary channel for e-authentication. For example, a Claimant attempts tolog into a website and receives a text message on his or her cellular phone,
PDA, pager, or land line (pre-registered with the CSP during the registration
phase) with a random authenticator to be presented as a part of the electronic
authentication protocol. Out of Band Tokens are something you have.

•Single-factor (SF) One-Time Password (OTP) Device – A hardware device
that supports the spontaneous generation of one-time passwords. This device
has an embedded secret that is used as the seed for generation of one-time
passwords and does not require activation through a second factor.
Authentication is accomplished by providing an acceptable one-time password
and thereby proving possession and control of the device. The token
authenticator is the one-time password. For example, a one-time password
device may display 6 characters at a time. SF OTP devices are something you
have.

Private communication means the Verifier’s message is sent directly to the Claimant’s device.

44

Electronic Authentication Guideline•

Single-factor (SF) Cryptographic Device – a hardware device that performs
cryptographic operations on input provided to the device. This device does not
require activation through a second factor of authentication. This device uses
embedded symmetric or asymmetric cryptographic keys. Authentication isaccomplished by proving possession of the device. The token authenticator ishighly dependent on the specific cryptographic device and protocol, but it isgenerally some type of signed message. For example, in TLS, there is a“certificate verify” message. SF Cryptographic Devices are something you
have.

•

Multi-factor (MF) Software Cryptographic Token – A cryptographic key isstored on disk or some other “soft” media and requires activation through asecond factor of authentication. Authentication is accomplished by proving
possession and control of the key. The token authenticator is highly dependent
on the specific cryptographic protocol, but it is generally some type of signed
message. For example, in TLS, there is a “certificate verify” message. The
MF software cryptographic token is something you have, and it may be
activated by either something you know or something you are.

•Multi-factor (MF) One-Time Password (OTP) Device – A hardware device
that generates one-time passwords for use in authentication and which
requires activation through a second factor of authentication. The second
factor of authentication may be achieved through some kind of integral entry
pad, an integral biometric (e.g., fingerprint) reader or a direct computerinterface (e.g., USB port). The one-time password is typically displayed on
the device and manually input to the Verifier as a password, although directelectronic input from the device to a computer is also allowed. The token
authenticator is the one-time password. For example, a one-time password
device may display 6 characters at a time. The MF OTP device is something
you have, and it may be activated by either something you know or something
you are.

•Multi-factor (MF) Cryptographic Device – A hardware device that contains a
protected cryptographic key that requires activation through a second
authentication factor. Authentication is accomplished by proving possessionof the device and control of the key. The token authenticator is highly
dependent on the specific cryptographic device and protocol, but it isgenerally some type of signed message. For example, in TLS, there is a“certificate verify” message. The MF Cryptographic device is something you
have, and it may be activated by either something you know or something youare.

6.1.3. Token Usage
An authentication process may involve a single token, or a combination of two or more
tokens, as described below.

45

Electronic Authentication Guideline•

Single-token authentication – The Claimant presents a single token
authenticator to prove his or her identity to the Verifier. For example, when a
Claimant attempts to log into a password protected website, the Claimant
enters a username and password. In this instance, only the password would beconsidered to be a token.

•

Multi-token authentication – The Claimant presents token authenticators
generated by two or more tokens to prove his or her identity to the Verifier.
The combination of tokens is characterized by the combination of factors usedby the tokens (both inherent in the manifestation of the tokens, and those used
to activate the tokens). A Verifier that requires a Claimant to enter a password
and use a single-factor cryptographic device is an example of multi-token
authentication. The combination is considered multi-factor, since thepassword is something you know and the cryptographic device is something
you have.

6.1.4. Multi-Stage Authentication Using Tokens
Multi-stage authentication processes, which use a single-factor token to obtain a second
token, do not constitute multi-factor authentication. The level of assurance associatedwith the compound solution is the assurance level of the weakest token.
For example, some cryptographic mobility solutions allow full or partial cryptographic
keys to be stored on an online server and downloaded to the Claimant’s local system after
successful authentication using a password or passphrase. Subsequently, the Claimant canuse the downloaded software cryptographic token to authenticate to a remote Verifier for
e-authentication. This type of solution is considered only as strong as the password
provided by the Claimant to obtain the cryptographic token.

6.1.5. Assurance Level Escalation
In certain circumstances, it may be desirable to raise the assurance level of an eauthentication session between a Subscriber and an RP in the middle of the application
session. This guideline recognizes a special case of multi-token authentication, where a
primary token is used to establish a secure session, and a secondary token is used later inthe session to present a second token authenticator. Even though the two tokens were notused at the same time, this document recognizes the result as a multi-token authentication
scheme (which may upgrade the overall level of assurance). In these authentication
scenarios, the level of assurance achieved by the two stages in combination is the same asa multi-token authentication scheme using the same set of tokens. Table 7 describes thehighest level of assurance achievable through a combination of two token types.

46

Electronic Authentication Guideline

6.2. Token Threats
An Attacker who can gain control of a token will be able to masquerade as the token’s
owner. Threats to tokens can be categorized based on attacks on the types ofauthentication factors that comprise the token:•

Something you have may be lost, damaged, stolen from the owner or cloned
by the Attacker. For example, an Attacker who gains access to the owner’s
computer might copy a software token. A hardware token might be stolen,tampered with, or duplicated.

•

Something you know may be disclosed to an Attacker. The Attacker might
guess a password or PIN. Where the token is a shared secret, the Attacker
could gain access to the CSP or Verifier and obtain the secret value. An
Attacker may observe the entry of a PIN or passcode, find a written record orjournal entry of a PIN or passcode, or may install malicious software (e.g., akeyboard logger) to capture the secret. Additionally, an Attacker may
determine the secret through off-line attacks on network traffic from anauthentication attempt. Finally, an Attacker may be able to gain information
about a Subscriber’s Pre-registered Knowledge researching the subscriber or
through other social engineering techniques. (For example, the subscriber
might refer to his or her first pet in a conversation or blog.)•

Something you are may be replicated. An Attacker may obtain a copy of thetoken owner’s fingerprint and construct a replica - assuming that the biometricsystem(s) employed do not block such attacks by employing robust livenessdetection techniquesThis document assumes that the Subscriber is not colluding with the Attacker who isattempting to falsely authenticate to the Verifier. With this assumption in mind, thethreats to the token(s) used for e-authentication are listed in Table 4, along with someexamples.

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Electronic Authentication Guideline

Table 4 – Token Threats
Token Threats/Attacks

Description

ExamplesTheft

A physical token is stolen by an
Attacker.

Discovery

The responses to token prompts
are easily discovered throughsearching various data sources.
The Subscriber’s token has been
copied with or without his or her
knowledge.

A hardware cryptographic device is stolen.
A One-Time Password device is stolen.
A look-up secret token is stolen.
A cell phone is stolen.
The question “What high school did you attend?” is asked as a Preregistered Knowledge Token, when the answer is commonly foundon social media websites.
Passwords written on paper are disclosed.
Passwords stored in an electronic file are copied.
Software PKI token (private key) copied.
Look-up token copied.
Passwords are learned by watching keyboard entry.
Passwords are learned by keystroke logging software.

Duplication

Eavesdropping

The token secret or authenticator
is revealed to the Attacker as the
Subscriber is submitting the token
to send over the network.

Offline cracking

The token is exposed using
analytical methods outside theauthentication mechanism.

Phishing or pharming

The token secret or authenticator
is captured by fooling the
Subscriber into thinking the
Attacker is a Verifier or RP.

Social engineering

The Attacker establishes a level oftrust with a Subscriber in order toconvince the Subscriber to reveal
his or her token or token secret.
The Attacker connects to the
Verifier online and attempts toguess a valid token authenticatorin the context of that Verifier.

Online guessing

A PIN is captured from PIN pad device.
A key is extracted by differential power analysis on stolen hardware
cryptographic token.
A software PKI token is subjected to dictionary attack to identify thecorrect password to use to decrypt the private key.
A password is revealed by Subscriber to a website impersonating
the Verifier.
A password is revealed by a bank Subscriber in response to an
email inquiry from a Phisher pretending to represent the bank.
A password is revealed by the Subscriber at a bogus Verifier
website reached through DNS re-routing.
A password is revealed by the Subscriber to an officemate asking
for the password on behalf of the Subscriber’s boss.
A password is revealed by a Subscriber in a telephone inquiry froman Attacker masquerading as a system administrator.
Online dictionary attacks are used to guess passwords.
Online guessing is used to guess token authenticators for a onetime password token registered to a legitimate Claimant.

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Electronic Authentication Guideline

6.2.1. Threat Mitigation Strategies
Token related mechanisms that assist in mitigating the threats identified above aresummarized in Table 5.
Table 5 - Mitigating Token Threats
Token Threat/Attack
Theft
Duplication
Discovery
Eavesdropping

Offline cracking

Phishing or pharming

Social engineering

Online guessing

Threat Mitigation Mechanisms- Use multi-factor tokens which need to be activated througha PIN or biometric.
- Use tokens that are difficult to duplicate, such as hardware
cryptographic tokens.
- Use methods in which the responses to prompts cannot beeasily discovered.
- Use tokens with dynamic authenticators where knowledgeof one authenticator does not assist in deriving a subsequentauthenticator.
- Use tokens that generate authenticators based on a token
input value.
- Establish tokens through a separate channel.
- Use a token with a high entropy token secret- Use a token that locks up after a number of repeated failed
activation attempts.
- Use tokens with dynamic authenticators where knowledgeof one authenticator does not assist in deriving a subsequentauthenticator.
- Use tokens with dynamic authenticators where knowledgeof one authenticator does not assist in deriving a subsequentauthenticator.
- Use tokens that generate high entropy authenticators.

There are several other strategies that may be applied to mitigate the threats described inTable 5:
• Multiple factors make successful attacks more difficult to accomplish. If an
Attacker needs to steal a cryptographic token and guess a password, then thework to discover both factors may be too high.
•Physical security mechanisms may be employed to protect a stolen token from
duplication. Physical security mechanisms can provide tamper evidence,
detection, and response.

•

Imposing password complexity rules may reduce the likelihood of a successful
guessing attack. Requiring the use of long passwords that don’t appear incommon dictionaries may force Attackers to try every possible password.

•

System and network security controls may be employed to prevent an Attacker
from gaining access to a system or installing malicious software.

•

Periodic training may be performed to ensure the Subscriber understands
when and how to report compromise (or suspicion of compromise) or49

Electronic Authentication Guideline

otherwise recognize patterns of behavior that may signify an Attacker
attempting to compromise the token.
•

Out of band techniques may be employed to verify proof of possession ofregistered devices (e.g., cell phones).

6.3. Token Assurance Levels
This section discusses the requirements for tokens used at various levels of assurance.
6.3.1. Requirements per Assurance Level
The following sections list token requirements for single and multi-token authentication.
6.3.1.1. Single Token Authentication
Table 6 lists the assurance levels that may be achieved by each of the token types whenused in a single-token authentication scheme. The requirements for each token are listed
per assurance level. If token requirements are listed only at one assurance level, the token
may be used at lower levels but shall satisfy the requirements given at whatever level islisted. If there is more than one box under “Verifier Requirements” for a given token
type, it is only necessary to satisfy the requirements in one box.

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Electronic Authentication Guideline

Table 6 - Token Requirements Per Assurance Level
Token Type
Memorized
Secret Token

Level

Token Requirements

Verifier Requirements

Level 1

The memorized secret may be auser chosen string consisting of 6
or more characters chosen from analphabet of 90 or more characters,a randomly generated PIN
consisting of 4 or more digits, or a21
secret with equivalent entropy.

The Verifier shall implement a
throttling mechanism that
effectively limits the number offailed authentication attempts an
Attacker can make on the
Subscriber’s account to 100 or
fewer in any 30-day period.
Note: While an implementation that
simply counted all failed
authentication attempts in eachcalendar month and locked out theaccount when the limit was
exceeded would technically meetthe requirement, this is a poor
choice for reasons of system
availability. See Section 8.2.3 formore detailed advice.

Level 2The memorized secret may be arandomly generated PIN consistingof 6 or more digits, a user
generated string consisting of 8 ormore characters chosen from analphabet of 90 or more characters,
or a secret with equivalent
21
entropy.
CSP implements dictionary orcomposition rule to constrain usergenerated secrets.

Pre-Registered
Knowledge
Token

Level 1The secret provides at least 14 bits
21
of entropy.

The entropy in the secret cannot
be directly calculated, e.g., user
21

For more information, see Table A.1 in Appendix A.

51

The Verifier shall implement a
throttling mechanism that
effectively limits the number offailed authentication attempts an
Attacker can make on the
Subscriber’s account to 100 or
fewer in any 30-day period.
Note: While an implementation that
simply counted all failed
authentication attempts in eachcalendar month and locked out theaccount when the limit was
exceeded would technically meetthe requirement, this is a poor
choice for reasons of system
availability. See Section 8.2.3 formore detailed advice.
The Verifier shall implement a
throttling mechanism that
effectively limits the number offailed authentication attempts an
Attacker can make on the
Subscriber’s account to 100 or
fewer in any 30-day period.
Note: While an implementation that
simply counted all failed
authentication attempts in eachcalendar month and locked out theaccount when the limit was
exceeded would technically meetthe requirement, this is a poor
choice for reasons of system
availability. See Section 8.2.3 formore detailed advice.
For these purposes, an empty
answer is prohibited.

Electronic Authentication Guideline

Token Type

Level

Token Requirements
chosen or personal knowledge
questions.
If the questions are not supplied by
the user, the user shall selectprompts from a set of at least fivequestions.

Level 2The secret provides at least 20 bits
21
of entropy.

The entropy in the secret cannot
be directly calculated, e.g., userchosen or personal knowledge
questions.
If the questions are not supplied by
the user, the user shall selectprompts from a set of at least
seven questions.

Verifier Requirements
The Verifier shall verify theanswers provided for at least threequestions, and shall implement a
throttling mechanism that
effectively limits the number offailed authentication attempts an
Attacker can make on the
Subscriber’s account to 100 or
fewer in any 30-day period.
Note: While an implementation that
simply counted all failed
authentication attempts in eachcalendar month and locked out theaccount when the limit was
exceeded would technically meetthe requirement, this is a poor
choice for reasons of system
availability. See Section 8.2.3 formore detailed advice.
The Verifier shall implement a
throttling mechanism that
effectively limits the number offailed authentication attempts an
Attacker can make on the
Subscriber’s account to 100 or
fewer in any 30-day period.
Note: While an implementation that
simply counted all failed
authentication attempts in eachcalendar month and locked out theaccount when the limit was
exceeded would technically meetthe requirement, this is a poor
choice for reasons of system
availability. See Section 8.2.3 formore detailed advice.
For these purposes, an empty
answer is prohibited.
The Verifier shall verify theanswers provided for at least fivequestions, and shall implement a
throttling mechanism that
effectively limits the number offailed authentication attempts an
Attacker can make on the
Subscriber’s account to 100 or
fewer in any 30-day period.
Note: While an implementation that
simply counted all failed
authentication attempts in eachcalendar month and locked out theaccount when the limit was
exceeded would technically meetthe requirement, this is a poor
choice for reasons of system
availability. See Section 8.2.3 for52

Electronic Authentication Guideline

Token Type
Look-up SecretToken

Out of BandToken

SF One-Time
Password
DeviceSF
Cryptographic
Device22

Level

Level 2

Level 2

Level 2

Level 2

Token Requirements
The token authenticator has 64
21
bits of entropy.
The token authenticator has at
21
least 20 bits of entropy.

The token is uniquely addressable
and supports communication overa channel that is separate from theprimary channel for eauthentication.

Shall use Approved block cipher orhash function to combine asymmetric key stored on device
with a nonce to generate a onetime password.

The nonce may be a date andtime, or a counter generated on
the device.
The cryptographic module shall bevalidated at FIPS 140-2 Level 1 or
22higher.

Verifier Requirements
more detailed advice.
N/A
The Verifier shall implement a
throttling mechanism that
effectively limits the number offailed authentication attempts an
Attacker can make on the
Subscriber’s account to 100 or
fewer in any 30-day period.
Note: While an implementation that
simply counted all failed
authentication attempts in eachcalendar month and locked out theaccount when the limit was
exceeded would technically meetthe requirement, this is a poor
choice for reasons of system
availability. See Section 8.2.3 formore detailed advice.
The Verifier generated secret shall21
have at least 64 bits of entropy.
The Verifier generated secret shall21
have at least 20 bits of entropy
and the Verifier shall implement a
throttling mechanism that
effectively limits the number offailed authentication attempts an
Attacker can make on the
Subscriber’s account to 100 or
fewer in any 30-day period.
Note: While an implementation that
simply counted all failed
authentication attempts in eachcalendar month and locked out theaccount when the limit was
exceeded would technically meetthe requirement, this is a poor
choice for reasons of system
availability. See Section 8.2.3 formore detailed advice.
The one-time password shall havea limited lifetime, on the order ofminutes.
The cryptographic module
performing the verifier function
shall be validated at FIPS 140-2
22
Level 1 or higher.

Verifier generated token input(e.g., a nonce or challenge) has at
21
least 64 bits of entropy.

Products validated under subsequent versions of FIPS 140-2 are also acceptable.

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Electronic Authentication Guideline

Token TypeMF Software
Cryptographic
Token

Level

Token Requirements

Verifier Requirements

Level 3The cryptographic module shall bevalidated at FIPS 140-2 Level 1 or
22higher. Each authentication shallrequire entry of the password orother activation data and theunencrypted copy of theauthentication key shall be erased
after each authentication.

Verifier generated token input(e.g., a nonce or challenge) has at
21
least 64 bits of entropy.

MF OTP
Hardware Token

Level 4

Cryptographic module shall be
FIPS 140-2 validated Level 2 orhigher; with physical security at
22
FIPS 140-2 Level 3 or higher.

The one-time password shall havea limited lifetime of less than 2
minutes.

The one-time password shall be
generated by using an Approved
block cipher or hash function tocombine a symmetric key stored
on a personal hardware device
with a nonce to generate a onetime password.

MF Hardware
Cryptographic
Token

Level 4

The nonce may be a date andtime, a counter generated on thedevice. Each authentication shallrequire entry of a password orother activation data through an
integrated input mechanism.
Cryptographic module shall be
FIPS 140-2 validated, Level 2 orhigher; with physical security at
22
FIPS 140-2 Level 3 or higher.
Shall require the entry of apassword, PIN, or biometric toactivate the authentication key.
Shall not allow the export ofauthentication keys.

Verifier generated token input(e.g., a nonce or challenge) has at
21
least 64 bits of entropy.

6.3.1.2. Multi-Token Authentication
When two of the token types are combined for a multi-token authentication scheme,
Table 7 shows the highest possible assurance level that can be achieved by the
combination. 23

23Note that the table displays tokens that exhibit the properties of “something you have” and “something
you know”.

54Electronic Authentication Guideline

Table 7 - Assurance Levels for Multi-Token E-Authentication Schemes 24
SF Cryptographic
DeviceMF
Software
CryptographicToken

MF OTP
DeviceMF Cryptographic
DeviceLevel 3

Level 3

Level 3

Level 4

Level 4

Level 3

Level 3

Level 3

Level 3

Level 4

Level 4

Level 2

Level 2

Level 2

Level 2

Level 3

Level 4

Level 4

X

X

Level 2

Level 2

Level 2

Level 3

Level 4

Level 4

X

X

X

X

Level 2

Level 2

Level 3

Level 4

Level 4

X

X

X

X

X

Level 2

Level 3

Level 4

Level 4

X

X

X

X

X

X

Level 3

Level 4

Level 4

MF OTP Device

X

X

X

X

X

X

X

Level 4

Level 4MF
Cryptographic
Device

X

X

X

X

X

X

X

X

Level 4

Memorized
SecretToken

Preregistered
Knowledge
Token

Look-up
SecretToken

Out ofBandToken

SF OTP
DeviceMemorized
Secret Token

Level 2

Level 2

Level 3

Level 3Pre-registered
Knowledge
Token

X

Level 2

Level 3Look-up Secret
Token

X

X

Out of Band
Token

X

SF OTP Device
SF
Cryptographic
DeviceMF Software
Cryptographic
Token

24

The boxes marked with an “x” denote that the combination already appears in the table

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Electronic Authentication Guideline

The principles used in generating Table 7 are as follows. Level 3 can be achieved using
two tokens rated at Level 2 that represent two different factors of authentication. Since
this specification does not address the use of biometrics as a stand-alone token for remoteauthentication, achieving Level 3 with separate Level 2 tokens implies something you
have and something you know:
Token (Level 2, something you have) + Token (Level 2, something you know) → Token(Level 3)

In all other cases, combinations of tokens are considered to achieve the Level of thehighest rated token.
For example, a Memorized Secret Token combined with a Look-up Secret Token can beused to achieve Level 3 authentication, since the look-up secret token is “something you
have” and the Memorized Secret Token is “something you know”. However, combining a
MF software cryptographic token (which is rated at Level 3) and a Memorized Secret
Token (which is rated at Level 2) achieves an overall level of 3, since the addition of theMemorized Secret Token does not increase the assurance of the combination.
It should be noted that to achieve Level 4 with a single token or token combination, oneof the tokens needs to be usable with an authentication process that strongly resists manin-the-middle attacks. While it is possible to meet this requirement with a wide variety oftoken types, certain choices of tokens may complicate the task of designing a protocolthat meets Level 4 requirements for authentication process (as described in Section 8 ofthis document). In particular, one-time password devices that rely exclusively on thehuman user for input and output may be especially problematic and may need to besupplemented with a software cryptographic token to provide strong man-in-the-middle
resistance.

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Electronic Authentication Guideline

7. Token and Credential Management
7.1. Overview
As introduced in Section 4, credentials are objects that bind identity to a token. To
maintain the level of assurance provided by an e-authentication solution, credentials andtokens shall be managed to reflect any changes in that binding. This section discusses
token and credential management activities performed by the CSP subsequent to theregistration, identity proofing and issuance activities described in Section 5. This includes
the lifecycle management activities for the token and credential. The activities that mustbe performed by the CSP depend in part upon the nature of the credentials and the token
itself.
7.1.1. Categorizing Credentials
This specification categorizes credentials according to two orthogonal perspectives.
Some classes of credentials can be distributed to relying parties, while others cannot be
disclosed by the CSP without compromising the token itself. Another classification
indicates whether the binding represented in the credential is tamper-evident.
Credentials that describe the binding in a way that does not compromise the token arereferred to as Public Credentials. The classic example of a Public Credential is a publickey certificate; it is mathematically infeasible to calculate the user’s private key evenwith knowledge of the corresponding public key. Credentials that cannot be disclosed by
the CSP because the contents can be used to compromise the token are considered
Private Credentials. The classic example of a Private Credential is the hashed value of apassword, since this hash can be used to perform an offline attack on the password.
Credentials that describe the binding between a user and token in a tamper-evident
fashion are considered Strongly Bound Credentials. For example, modification of adigitally signed credential (such as a public key certificate) can be easily detected throughsignature verification. The binding between a user and token can be modified in Weakly
Bound Credentials without invalidating the credentials. Weakly bound credentials require
supplemental integrity protection and/or access controls to ensure that the binding
represented by the credential remains accurate. For example, replacing the value of ahashed password in a password file associates the user with a new password, so access tothis file is restricted to system users and processes.
Strongly bound credential mechanisms require little or no additional integrity protection,
whereas weakly bound credentials require additional integrity protection or access
controls to ensure that unauthorized parties cannot spoof or tamper with the binding of
the identity to the token representation within the credential.
Unencrypted password files are private credentials that are weakly bound, and hence
need to be afforded confidentiality as well as integrity protection. Signed password files
are private credentials that are strongly bound and therefore require confidentiality
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Electronic Authentication Guideline

protection but no additional integrity protection. An unsigned pairing of a public key andthe name of its owner or a self-signed certificate is an example of a public credential thatis weakly bound. Finally, a CA-signed public key certificate represents a publiccredential that is strongly bound.
CSPs and Verifiers are trusted to obey the requirements in this section as well as Section
8.
7.1.2. Token and Credential Management Activities
The CSP manages tokens and credentials. The RA establishes the Applicant’s identity,
and the CSP is responsible for generating credentials and supplying the Subscriber with atoken or allowing the Subscriber to register his or her own token as described in Section
5. The CSP is responsible for some or all of the following token and credential
management activities following issuance of the token and credential:•

Credential storage – After the credential has been created, the CSP may be
responsible for maintaining the credentials in storage. In cases where thecredentials are stored by the CSP, the level of security afforded to thecredential will depend on the type of credential issued. For private credentials,
additional confidentiality mechanisms are required in storage, whereas for
public credentials, this is not necessary. Similarly, for weakly boundcredentials, additional integrity protection is needed in storage, unlike strongly
bound credentials. Finally, credentials need to be available to allow CSPs andVerifiers to determine the identity of the corresponding token owner.

•

Token and credential verification services – In many e-authentication
scenarios, the Verifier and the CSP are not part of the same entity. In thesecases, the CSP is responsible for providing the Verifier with the informationneeded to facilitate the token and credential verification process. The CSP
may provide token and credential verification services to Verifiers. For
example, the Verifier may request the CSP to verify the password submitted
by the Claimant against the CSP’s local password database.

•Token and credential renewal /re-issuance – Certain types of tokens andcredentials may support the process of renewal or re-issuance. During
renewal, the usage or validity period of the token and credential is extended
without changing the Subscriber’s identity or token. During re-issuance, a newcredential is created for a Subscriber with a new identity and/or a new token.
The CSP establishes suitable policies for renewal and re-issuance of tokens
and credentials. The CSP may establish a time period prior to the expiration of
the credential, when the Subscriber can request renewal or re-issuancefollowing successful authentication using his or her existing, unexpired token
and credential. For example, a CSP may allow a digital certificate to be
renewed for another year prior to the expiry of the current certificate byproving possession and control of the existing token (i.e., the private key).

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Electronic Authentication Guideline

Once the Subscriber’s credentials have expired, the Subscriber may berequired to re-establish his or her identity with the CSP; this is typically thecase with CSPs that issue digital certificates. Conversely, the CSP may
establish a grace period for the renewal or re-issuance of an expired
credential, such that the Subscriber can request renewal/re-issuance of his or
her credential even after it has expired without the need to re-establish his or
her identity with the CSP. For example, if a Claimant attempts to login to ausername/password based system on which his or her password has already
expired, and the system supports a grace period, the user may be prompted tocreate a new password and supply the last password for verification purposes.
The use of expired tokens or credentials to invoke renewal/re-issuance is morepractical when the Verifier and CSP are part of the same entity.
The public key certificate for a Subscriber may be renewed with the same
public key, or may be re-issued with a new public key. Passwords are seldom
renewed so that the life of the existing password is extended for anotherperiod. Usually the account name/password credential for a Subscriber isrenewed by having the Subscriber select a new password.
•

Token and credential revocation and destruction – The CSP is responsible formaintaining the revocation status of credentials and destroying the credential
at the end of its life. Explicit and elaborate revocation mechanisms may berequired for “public credentials” since these credentials are disseminatedwidely, possibly with a preset validity period. For example, public key
certificates are revoked using Certificate Revocation Lists (CRLs) after thecertificates are distributed.
“Private credentials” are held closely by the CSP, and hence the revocationand destruction of these credentials is implemented easily through an updateof the CSP’s local credential stores. Credentials that bindusernames/passwords are instantaneously revoked and destroyed if the CSP
deletes its mapping between the username and the password. Certain types oftokens may need to be explicitly deleted or zeroized at the end of thecredential life in order to permanently disable the token and prevent its
unauthorized reuse. For example, a Multi-factor Hardware Cryptographic
Token may need to be zeroized to ensure that all of the information pertaining
to the Subscriber is deleted from the token.
The CSP may be responsible for ensuring that hardware tokens are collected
and cleared of any data when the Subscriber no longer has a need for its use.
The CSP may establish policies for token collection to avoid the possibility ofunauthorized use of the token after it is considered out of use. The CSP may
destroy such collected tokens, or zeroize them to ensure that there are no
remnants of information that can be used by an Attacker to derive the token
value. For example, a Subscriber who is issued a hardware OTP token by a
CSP may be required by policy to return the token to the CSP at the end of its
life, or when the Subscriber’s association with that CSP terminates.

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Electronic Authentication Guideline•

Records retention – The CSP or its representative is responsible formaintaining a record of the registration, history, and status of each token andcredential, including revocation. CSPs operated by or on behalf of executive
branch agencies shall also follow either the General Records Scheduleestablished by the National Archives and Records Administration or an
agency-specific schedule as applicable. All other entities shall comply with
their respective records retention policies in accordance with whatever laws
apply to those entities. A minimum record retention period is required at
Level 2 and above.

•

Security controls – The CSP is responsible for implementing and maintaining
appropriate security controls contained in NIST SP 800-53. The security
control baseline for CSPs is specified in terms of a FIPS 200 impact level foreach assurance level. (See Section 7.3, below.)

7.2. Token and Credential Management Threats
Tokens and credentials can only be as strong as the strength of the management
mechanisms used to secure them. The CSP is responsible for mitigating threats to the
management operations described in the last section. Token and credential management
threats are described below; they are categorized in accordance with the management
activity to which they apply.
These threats represent the potential to breach the confidentiality, integrity andavailability of tokens and credentials during the CSP activities, and are listed below.

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Table 8 - Token and Credential Management Threats
Token and Credential
Management Activity
Credential storage

Threat/Attack
Disclosure

Tampering

Token and credential
verification services

Disclosure

An Attacker is able to view
requests and responses
between the CSP and the
Verifier.
An Attacker is able tomasquerade as the CSP andprovide bogus responses to the
Verifier’s password verification
requests.
The password file or the CSP is
unavailable to provide
password and username
mappings.
Public key certificates for
Claimants are unavailable tothe Verifier because thedirectory systems are down (forexample for maintenance or asa result of a denial of service
attack).

Tampering

Unavailability

Token and credential
issuance/renewal/reissuance

ExampleUsernames and passwords
stored in a system file arerevealed.
The file that maps usernames
to passwords within the CSP is
hacked so that the mappings
are modified, and existing
passwords are replaced bypasswords known to the
Attacker.

Disclosure

Tampering

Unauthorized issuance61

Password renewed by the CSP
for a Subscriber is copied by an
Attacker as it is transported
from the CSP to the
Subscriber.
New password created by the
Subscriber is modified by an
Attacker as it is being
submitted to the CSP toreplace an expired password.
The CSP is compromised
through unauthorized physicalor logical access resulting inissuance of fraudulent
credentials.

Electronic Authentication Guideline

Token and Credential
Management Activity

Token and credential
revocation/destruction

Threat/Attack
Unauthorized renewal/reissuance

ExampleAttacker fools the CSP into reissuing the credential for acurrent Subscriber – the newcredential binds the current
Subscriber’s identity with atoken provided by the Attacker.
Attacker is able to takeadvantage of a weak credential
renewal protocol to extend thecredential validity period for acurrent Subscriber.

Delayed revocation/destructionof credentials

Stale CRLs allow accounts(that should have been locked
as a result of credential
revocation) to be used by an
Attacker.
User accounts are not deleted
when employees leave acompany leading to a possible
use of the old accounts byunauthorized persons.
A hardware token is used after
the corresponding credential
was revoked or expired.

Token use after
decommissioning

7.2.1. Threat Mitigation Strategies
Token and credential management related mechanisms that assist in mitigating the threats
identified above are summarized in Table 9.
7.3. Token and Credential Management Assurance Levels

7.3.1. Requirements per Assurance Level
The stipulations for management of tokens and credentials by the CSP and Verifier aredescribed below for each assurance level. The stipulations described at each level in thissection are incremental in nature; requirements stipulated at lower levels are implicitly
included at higher levels.

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Table 9 - Token and Credential Threat Mitigation Strategies
Token and Credential
Management Activity
Credential storage

Threat/Attack
Disclosure

Tampering

Token and credential
verification services

Disclosure

Use a communication protocol thatoffers confidentiality protection.
Ensure that Verifiers authenticate the
CSP prior to accepting a verification
response from that CSP.
Use a communication protocol thatoffers integrity protection.
Ensure that the CSP has a welldeveloped and tested Contingency
Plan.

Tampering

Unavailability

Token and credential
issuance/renewal/reissuance

Disclosure

Tampering

Unauthorized issuance

Unauthorized
renewal/re-issuance

Credential
revocation/destruction

Mitigation Strategy
Use access control mechanisms that
protect against unauthorized disclosureof credentials held in storage.
Use access control mechanisms that
protect against unauthorized tamperingof credentials and tokens.

Use a communication protocol thatprovides confidentiality protection ofsession data.
Use a communication protocol thatallows the Subscriber to authenticate
the CSP prior to engaging in token reissuance activities and protects theintegrity of the data passed.
Implement physical and logical access
controls to prevent compromise of the
CSP. See [FISMA] for details on
security controls.
Establish policy that Subscriber shallprove possession of the old token to
successfully negotiate the re-issuanceprocess. Any attempt to negotiate there-issuance process using an expired or
revoked token should fail.

Delayedrevocation/destructionof credentials

Revoke/Destroy credentials as soon as
notification that the credentials should
be revoked or destroyed.

Token use after
decommissioning

Destroy tokens after their corresponding
credentials have been revoked.

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Electronic Authentication Guideline

7.3.1.1. Level 1
At Level 1, the following shall be required:•

Credential storage – Files of shared secrets used by Verifiers at Level 1
authentication shall be protected by access controls that limit access toadministrators and only to those applications that require access. Such sharedsecret files shall not contain the plaintext passwords; typically they contain aone-way hash or “inversion” of the password. In addition, any method
allowed for the protection of long-term shared secrets at Level 2 or above may
be used at Level 1.

•

Token and credential verification services – Long term token secrets should
not be shared with other parties unless absolutely necessary.

•

Token and credential renewal / re-issuance – No stipulation•

Token and credential revocation and destruction – No stipulation•

Records retention – No stipulation•

Security controls – No stipulation
7.3.1.2. Level 2

At Level 2, the following shall be required:•

Credential storage – Files of shared secrets used by CSPs at Level 2 shall beprotected by access controls that limit access to administrators and only tothose applications that require access. Such shared secret files shall notcontain the plaintext passwords or secrets; two alternative methods may be
used to protect the shared secret:
1. Passwords may be concatenated to a variable salt (variable across a groupof passwords that are stored together) and then hashed with an Approved
algorithm so that the computations used to conduct a dictionary or
exhaustion attack on a stolen password file are not useful to attack othersimilar password files. The hashed passwords are then stored in the
password file. The variable salt may be composed using a global salt(common to a group of passwords) and the username (unique perpassword) or some other technique to ensure uniqueness of the salt within
the group of passwords.
2. Shared secrets may be encrypted and stored using Approved encryption
algorithms and modes, and the needed secret decrypted only when
immediately required for authentication. In addition, any method allowed
to protect shared secrets at Level 3 or 4 may be used at Level 2.

64Electronic Authentication Guideline•

Token and credential verification services – Long term shared authentication
secrets, if used, shall never be revealed to any other party except Verifiers
operated by the CSP; however, session (temporary) shared secrets may beprovided by the CSP to independent Verifiers.
Cryptographic protections are required for all messages between the CSP and
Verifier which contain private credentials or assert the validity of weakly
bound or potentially revoked credentials. Private credentials shall only be sent
through a protected session to an authenticated party to ensure confidentiality
and tamper protection.
The CSP may send the Verifier a message that either asserts that a weakly
bound credential is valid, or that a strongly bound credential has not beensubsequently revoked. In this case, the message shall be logically bound to thecredential, and the message, the logical binding, and the credential shall all betransmitted within a single integrity protected session between the Verifier
and the authenticated CSP. If revocation is an issue, the integrity protected
messages shall either be time stamped, or the session keys shall expire with an
expiration time no longer than that of the revocation list. Alternatively, thetime stamped message, binding, and credential may all be signed by the CSP,
although, in this case, the three in combination would comprise a strongly
bound credential with no need for revocation.

•

Token and credential renewal/re-issuance – The CSP shall establish suitablepolicies for renewal and re-issuance of tokens and credentials. Proof-ofpossession of the unexpired current token shall be demonstrated by the
Claimant prior to the CSP allowing renewal and re-issuance. Passwords shallnot be renewed; they shall be re-issued. After expiry of current token and any
grace period, renewal and re-issuance shall not be allowed. Upon re-issuance,
token secrets shall not be set to a default or reused in any manner. Allinteractions shall occur over a protected session such as SSL/TLS.

•

Token and credential revocation and destruction – CSPs shall revoke ordestroy credentials and tokens within 72 hours after being notified that acredential is no longer valid or a token is compromised to ensure that a
Claimant using the token cannot successfully be authenticated. If the CSP
issues credentials that expire automatically within 72 hours (e.g., issues freshcertificates with a 24 hour validity period each day) then the CSP is not
required to provide an explicit mechanism to revoke the credentials. CSPs that
register passwords shall ensure that the revocation or de-registration of thepassword can be accomplished in no more than 72 hours. CAs cross-certified
with the Federal Bridge CA at the Citizen and Commerce Class Basic,
Medium and High or Common Certificate Policy levels are considered tomeet credential status and revocation provisions of this level.

•

Records retention – A record of the registration, history, and status of eachtoken and credential (including revocation) shall be maintained by the CSP or
its representative. The record retention period of data for Level 2 credentials isseven years and six months beyond the expiration or revocation (whichever is65

Electronic Authentication Guideline

later) of the credential. CSPs operated by or on behalf of executive branchagencies shall also follow either the General Records Schedule established by
the National Archives and Records Administration or an agency-specificschedule as applicable. All other entities shall comply with their respective
records retention policies in accordance with whatever laws apply to thoseentities.
•

Security controls – The CSP must employ appropriately tailored security
controls from the low baseline of security controls defined in [SP 800-53] andmust ensure that the minimum assurance requirements associated with the low
baseline are satisfied.
7.3.1.3. Level 3

At Level 3, the following is required:•

Credential storage 25 – Files of long-term shared secrets used by CSPs or
Verifiers at Level 3 shall be protected by access controls that limit access toadministrators and only to those applications that require access. Such sharedsecret files shall be encrypted so that:
1. The encryption key for the shared secret file is encrypted under a key held
in a FIPS 140-2 Level 2 or higher validated hardware cryptographic
module or any FIPS 140-2 Level 3 or 4 cryptographic module anddecrypted only as immediately required for an authentication operation.
2. Shared secrets are protected as a key within the boundary of a FIPS 140-2
Level 2 or higher validated hardware cryptographic module or any FIPS
140-2 Level 3 or 4 cryptographic module and is not exported in plaintext
from the module.
Strongly bound credentials support tamper detection mechanisms such as
digital signatures, but weakly bound credentials can be protected against
tampering using access control mechanisms as described above.

•

Token and credential verification services – CSPs shall provide a secure
mechanism to allow Verifiers or RPs to ensure that the credentials are valid.
Such mechanisms may include on-line validation servers or the involvementof CSP servers that have access to status records in authenticationtransactions.
Temporary session authentication keys may be generated from long-term
shared secret keys by CSPs and distributed to third party Verifiers, as a part ofthe verification services offered by the CSP, but long-term shared secrets shallnot be shared with any third parties, including third party Verifiers. This type

25

With regard to references to FIPS 140-2, products validated under subsequent versions of FIPS 140-2 arealso acceptable.

66

Electronic Authentication Guidelineof third-party (or delegated) verification is used in the realm of GSM (Global
System for Mobile Communications) roaming; the locally available network
authenticates the “roaming” Subscriber using a temporary session
authentication key received from the Base Station. Such temporary session
authentication keys are typically created by cryptographically combining thelong term shared secret with a nonce challenge, to generate a session key. The
challenge and session key are securely transmitted to the Verifier. The
Verifier in turn sends only the challenge to the Claimant, and the Claimant
applies the challenge to the long-term shared secret to generate the session
key. Both Claimant and Verifier now share a session key, which can be used
for authentication. Such verification schemes are permitted at this levelprovided that Approved cryptographic algorithms are used for all operations.
Token and credential verification services categorized as FIPS 199“Moderate” or “High” for availability shall be protected in accordance with
the Contingency Planning (CP) controls specified in NIST SP 800-53 toprovide an adequate level of availability needed for the service.
•

Token and credential renewal /re-issuance – Renewal and re-issuance shall
only occur prior to expiration of the current credential. Claimants shallauthenticate to the CSP using the existing token and credential in order torenew or re-issue the credential. All interactions shall occur over a protectedsession such as SSL/TLS.

•

Credential revocation and destruction – CSPs shall have a procedure to
revoke credentials and tokens within 24 hours. The certificate statusprovisions of CAs cross-certified with the Federal Bridge CA at the Basic,
Medium, High or Common Certificate Policy levels are considered to meetcredential status and revocation provisions of this level. Verifiers shall ensurethat the tokens they rely upon are either freshly issued (within 24 hours) or
still valid. Shared secret based authentication systems may simply remove
revoked Subscribers from the verification database.

•

Records retention – All stipulations from Level 2 apply.

•

Security controls – The CSP must employ appropriately tailored security
controls from the moderate baseline of security controls defined in [SP 80053] and must ensure that the minimum assurance requirements associated withthe moderate baseline are satisfied.
7.3.1.4. Level 4

At Level 4, the following is required:•

Credential storage – No additional stipulation.

•

Token and credential verification services – No additional stipulation.

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Electronic Authentication Guideline•

Token and credential renewal/re-issuance – Sensitive data transfers shall be
cryptographically authenticated using keys bound to the authentication
process. All temporary or short-term keys derived during the original
authentication operation shall expire and re-authentication shall be required
after not more than 24 hours from the initial authentication.

•

Token and credential revocation and destruction – CSPs shall have aprocedure to revoke credentials within 24 hours. Verifiers or RPs shall ensurethat the credentials they rely upon are either freshly issued (within 24 hours)
or still valid. The certificate status provisions of CAs cross-certified with theFederal Bridge CA at the High and Common Certificate Policies shall beconsidered to meet credential status provisions of Level 4. [FBCA1]
It is generally good practice to destroy a token within 48 hours of the end ofits life or the end of the Subscriber’s association with the CSP. Destroying
includes either the physical destruction of the token or cleansing it of all
information related to the Subscriber.

•

Records retention – All stipulations from Levels 2 and 3 apply. The minimum
record retention period for Level 4 credential data is ten years and six months
beyond the expiration or revocation of the credential.

•

Security controls – The CSP must employ appropriately tailored security
controls from the moderate baseline of security controls defined in [SP 80053] and must ensure that the minimum assurance requirements associated withthe moderate baseline are satisfied.

7.3.2. Relationship of PKI Policies to E-Authentication Assurance
LevelsAppendix B specifies the mapping between the Federal PKI Certificate Policies and therequirements in Section 7.

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Electronic Authentication Guideline

8. Authentication Process
8.1. Overview
The authentication process establishes the identity of the Claimant to the Verifier with acertain degree of assurance. It is implemented through an authentication protocol
message exchange, as well as management mechanisms at each end that further constrain
or secure the authentication activity. One or more of the messages of the authentication
protocol may need to be carried on a protected session. This is illustrated in Figure 3.

Figure 3 - Authentication Process Model
An authentication protocol is a defined sequence of messages between a Claimant and a
Verifier that demonstrates that the Claimant has control of a valid token to establish hisor her identity, and optionally, demonstrates to the Claimant that he or she iscommunicating with the intended Verifier. An exchange of messages between a Claimant
and a Verifier that results in authentication (or authentication failure) between the twoparties is an authentication protocol run. During or after a successful authentication
protocol run, a protected communication session may be created between the two parties;
this protected session may be used to exchange the remaining messages of theauthentication protocol run, or to exchange session data between the two parties.
Management mechanisms may be implemented on the Claimant and the Verifier tofurther enhance the authentication process. For example, trust anchors may be establishedat the Claimant to enable the authentication of the Verifier using public key mechanisms
such as TLS. Similarly, mechanisms may be implemented on the Verifier to limit the rateof online guessing of passwords by an Attacker who is trying to authenticate as alegitimate Claimant. Further, detection of authentication transactions originating from an
unexpected location or channel for a Claimant, or indicating use of an unexpected
hardware or software configuration, may indicate increased risk levels and motivate
additional confirmation of the Claimant’s identity.

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Electronic Authentication Guideline

At the conclusion of the authentication protocol run, the verifier might issue a secondary
authentication credential, such as a cookie, to the Claimant and rely upon it toauthenticate the claimant in the near future. Requirements for doing this securely are in
Section 9.
8.2. Authentication Process Threats
In general, attacks that reveal long-term token secrets are worse than attacks that reveal
short-term authentication secrets or session data, because in the former, the Attacker canthen use the token secret to assume a Subscriber’s identity and do greater harm.
RAs, CSPs, and Verifiers are ordinarily trustworthy (in the sense of being correctly
implemented and not deliberately malicious). However, Claimants or their systems may
not be trustworthy (or else their identity claims could simply be trusted). Moreover, whileRAs, CSPs, and Verifiers are normally trustworthy, they are not invulnerable, and could
become corrupted. Therefore, authentication protocols that expose long-term
authentication secrets more than is absolutely required, even to trusted entities, should be
avoided. Table 10 lists the types of threats posed to the authentication process.
Table 10 - Authentication Process Threats
Type of Attack
Online guessing

Description
An Attacker performs repeated logon
trials by guessing possible values of
the token authenticator.

Phishing

A Subscriber is lured to interact witha counterfeit Verifier, and tricked into
revealing his or her token secret,sensitive personal data orauthenticator values that can beused to masquerade as the
Subscriber to the Verifier.
A Subscriber who is attempting toconnect to a legitimate Verifier, isrouted to an Attacker’s website
through manipulation of the domainname service or routing tables.

Pharming

Eavesdropping

ReplayAn Attacker listens passively to theauthentication protocol to capture
information which can be used in a
subsequent active attack tomasquerade as the Claimant.
An Attacker is able to replay
previously captured messages(between a legitimate Claimant and a
Verifier) to authenticate as that
Claimant to the Verifier.

70

ExampleAn Attacker navigates to a
web page and attempts to log
in using a Subscriber'susername and commonly usedpasswords, such as“password” and “secret”.
A Subscriber is sent an emailthat redirects him or her to a
fraudulent website and is
asked to log in using his or her
username and password.

A Subscriber is directed to a
counterfeit website throughDNS poisoning, and reveals or
uses his or her token believing
he or she is interacting withthe legitimate Verifier.
An Attacker captures thetransmission of a password or
password hash from a
Claimant to a Verifier.
An Attacker captures a
Claimant’s password or
password hash from an actual
authentication session, andreplays it to the Verifier to gain
access at a later time.

Electronic Authentication Guideline

Type of Attack
Session hijack

Man-in-the-middle

Description
An Attacker is able to insert himself
or herself between a Subscriber anda Verifier subsequent to a successfulauthentication exchange between
the latter two parties. The Attacker isable to pose as a Subscriber to the
Verifier/RP or vice versa to control
session data exchange.
The Attacker positions himself or
herself in between the Claimant andVerifier so that he or she canintercept and alter the content of theauthentication protocol messages.
The Attacker typically impersonates
the Verifier to the Claimant andsimultaneously impersonates theClaimant to the Verifier. Conducting
an active exchange with both parties
simultaneously may allow the
Attacker to use authentication
messages sent by one legitimate
party to successfully authenticate tothe other.

ExampleAn Attacker is able to takeover an already authenticatedsession by eavesdropping onor predicting the value ofauthentication cookies used tomark HTTP requests sent by
the Subscriber.
An Attacker breaks into arouter that forwards messages
between the Verifier and a
Claimant. When forwarding
messages, the Attacker
substitutes his or her own
public key for that of the
Verifier. The Claimant istricked into encrypting his or
her password so that the
Attacker can decrypt it.
An Attacker sets up a
fraudulent website
impersonating the Verifier.
When an unwary Claimant
tries to log in using his or her
one-time password device, theAttacker’s website
simultaneously uses theClaimant’s one-time password
to log in to the real Verifier.

8.2.1. Other Threats
Attacks are not limited to the authentication protocol itself. Other attacks include:•

Denial of Service attacks in which the Attacker overwhelms the Verifier byflooding it with a large amount of traffic over the authentication protocol;•

Malicious code attacks that may compromise or otherwise exploit
authentication tokens;•

Attacks that fool Claimants into using an insecure protocol, when the
Claimant thinks that he or she is using a secure protocol, or trick the Claimant
into overriding security controls (for example, by accepting server certificatesthat cannot be validated).

The purpose of flooding attacks is to overwhelm the resources used to support anauthentication protocol to the point where legitimate Claimants cannot reach the Verifier
or to slow down the process to make it more difficult for the Claimant to reach the
Verifier. For example, a Verifier that implements an authentication protocol that uses
encryption/decryption is sent a large number of protocol messages causing the Verifier to71Electronic Authentication Guideline

be crippled due to the use of excessive system resources to encrypt/decrypt. Nearly all
authentication protocols are susceptible to flooding attacks; possible ways to resist suchattacks is through the use of distributed Verifier architectures, use of load balancing
techniques to distribute protocol requests to multiple mirrored Verifier systems, or othersimilar techniques.
Malicious code could be introduced into the Claimant’s computer system for the purposeof compromising or otherwise exploiting the Claimant’s token. The malicious code may
be introduced by many means, including the threats detailed below. There are many
countermeasures (e.g., virus checkers and firewalls) that can mitigate the risk ofmalicious code on Claimant systems. General good practice to mitigate malicious code
threats is outside the scope of this document 26. Hardware tokens prevent malicious
software from extracting and copying the token secret. However, malicious code may still
misuse the token, particularly if activation data is presented to the token via the computer.

8.2.2. Threat Mitigation Strategies
The following are strategies that can be incorporated in authentication processes tomitigate the attacks listed in the previous section:

26•

Online guessing resistance – An authentication process is resistant to online
guessing attacks if it is impractical for the Attacker, with no a priori
knowledge of the token authenticator, to authenticate successfully by repeatedauthentication attempts with guessed authenticators. The entropy of theauthenticator, the nature of the authentication protocol messages, and othermanagement mechanisms at the Verifier contribute to this property. For
example, password authentication systems can make targeted password
guessing impractical by requiring use of high-entropy passwords and limiting
the number of unsuccessful authentication attempts, or by controlling the rate
at which attempts can be carried out. (See Appendix A and Table 6 in Section
6.3.1.). Similarly, to resist untargeted password attacks, a Verifier may
supplement these controls with network security controls.

•

Phishing and pharming resistance (verifier impersonation) – Anauthentication process is resistant to phishing and pharming (also known as
Verifier impersonation,) if the impersonator does not learn the value of atoken secret or a token authenticator that can be used to act as a Subscriber tothe genuine Verifier. In the most general sense, this assurance can be provided
by the same mechanisms that provide the strong man-in-the-middle resistancedescribed later in this section; however, long term secrets can be protectedagainst phishing and pharming simply by the use of a tamper resistant token,
provided that the long term secret cannot be reconstructed from a Token
Authenticator. To decrease the likelihood of phishing and pharming attacks, it

See SP 800-53, Recommended Security Controls For Federal Information Systems

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Electronic Authentication Guideline

is recommended that the Claimant authenticate the Verifier using
cryptographic mechanisms prior to submitting the token authenticator to the
supposed Verifier. Additionally, management mechanisms can be
implemented at the Verifier to send a Claimant personalized content after
successful authentication of the Claimant or the Claimant’s device. (Refer to
Section 8.2.4 for further details on personalization.) This allows the Claimant
to achieve a higher degree of assurance of the authenticity of the Verifier
before proceeding with the remainder of the session with the Verifier or RP. It
should be mentioned, however, that there is no foolproof way to prevent the
Claimant from revealing any sensitive information to which he or she hasaccess.
•Eavesdropping resistance – An authentication process is resistant toeavesdropping attacks if an eavesdropper who records all the messages
passing between a Claimant and a Verifier finds it impractical to learn the
Claimant’s token secret or to otherwise obtain information that would allow
the eavesdropper to impersonate the Subscriber in a future authentication
session. Eavesdropping-resistant protocols make it impractical 27 for an
Attacker to carry out an off-line attack where he or she records anauthentication protocol run and then analyzes it on his or her own system foran extended period to determine the token secret or possible token
authenticators. For example, an Attacker who captures the messages of apassword-based authentication protocol run may try to crack the password bysystematically trying every password in a large dictionary, and comparing itwith the protocol run data. Protected session protocols, such as TLS, provide
eavesdropping resistance.

•

Replay resistance – An authentication process resists replay attacks if it isimpractical to achieve a successful authentication by recording and replaying
a previous authentication message. Protocols that use nonces or challenges toprove the “freshness” of the transaction are resistant to replay attacks since the
Verifier will easily detect that the old protocol messages replayed do notcontain the appropriate nonces or timeliness data related to the current
authentication session.

•

Hijacking resistance – An authentication process and data transfer protocol
combination are resistant to hijacking if the authentication is bound to the datatransfer in a manner that prevents an adversary from participating actively inthe data transfer session between the Subscriber and the Verifier or RP
without being detected. This is a property of the relationship of theauthentication protocol and the subsequent session protocol used to transfer
data. This binding is usually accomplished by generating a per-session sharedsecret during the authentication process that is subsequently used by the27“Impractical” is used here in the cryptographic sense of nearly impossible, that is there is always a small
chance of success, but even the Attacker with vast resources will nearly always fail. For off-line attacks,
impractical means that the amount of work required to “break” the protocol is at least on the order of 280
cryptographic operations. For on-line attacks impractical means that the number of possible on-line trials isvery small compared to the number of possible key or password values.

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Electronic Authentication Guideline

Subscriber and the Verifier or RP to authenticate the transfer of all sessiondata.
It is important to note that web applications, even those protected by
SSL/TLS, can still be vulnerable to a type of session hijacking attack called
Cross Site Request Forgery (CSRF). In this type of attack, a malicious website
contains a link to the URL of the legitimate RP. The malicious website is
generally constructed so that a web browser will automatically send an HTTP
request to the RP whenever the browser visits the malicious website. If the
Subscriber visits the malicious website while he or she has an open SSL/TLS
session with the RP, the request will generally be sent in the same session andwith any authentication cookies intact. While the Attacker never gains accessto the session secret, the request may be constructed to have side effects, suchas sending an email message or authorizing a large transfer of money.
CSRF attacks may be prevented by making sure that neither an Attacker nor ascript running on the Attacker’s website has sufficient information to
construct a valid request authorizing an action (with significant consequences)by the RP. This can be done by inserting random data, supplied by the RP,
into any linked URL with side effects and into a hidden field within any formon the RP’s website. This mechanism, however, is not effective if the Attacker
can run scripts on the RP’s website (Cross Site Scripting or XSS). To prevent
XSS vulnerabilities, the RP should sanitize inputs from Claimants or
Subscribers to make sure they are not executable, or at the very least notmalicious, before displaying them as content to the Subscriber’s browser.
•

Man-in-the-middle resistance – Authentication protocols are resistant to a
man-in-the-middle attack when both parties (i.e., Claimant and Verifier) areauthenticated to the other in a manner that prevents the undetectedparticipation of a third party. There are two levels of resistance:
3. Weak man-in-the-middle resistance – A protocol is said to be weakly
resistant to man-in-the-middle attacks if it provides a mechanism for the
Claimant to determine whether he or she is interacting with the real
Verifier, but still leaves the opportunity for the non-vigilant Claimant toreveal a token authenticator (to an unauthorized party) that can be used tomasquerade as the Claimant to the real Verifier. For example, sending apassword over server authenticated TLS is weakly resistant to man-in the
middle attacks. The browser allows the Claimant to verify the identity of
the Verifier; however, if the Claimant is not sufficiently vigilant, thepassword will be revealed to an unauthorized party who can abuse theinformation. Weak man-in-the-middle resistance can also be provided by azero-knowledge password protocol, such as Encrypted Key Exchange(EKE), Simple Password Exponential Key Exchange (SPEKE), or Secure
Remote Password Protocol (SRP), which enables the Claimant toauthenticate to a Verifier without disclosing the token secret. However, it
is possible for the Attacker to trick the Claimant into passing his or her
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Electronic Authentication Guideline

password into a less secure protocol, thereby revealing the password to the
Attacker. Furthermore, if it is unreasonably difficult for the Claimant toverify that the proper protocol is being used, then the overall
authentication process does not even provide weak man-in-the-middle
resistance (for example, if a zero-knowledge password protocol isimplemented by an unsigned java applet displayed on a plaintext HTTP
page).
4.

Strong man-in-the-middle resistance: A protocol is said to be strongly
resistant to man-in-the-middle attack if it does not allow the Claimant toreveal, to an Attacker masquerading as the Verifier, information (token
secrets, authenticators) that can be used by the latter to masquerade as thetrue Claimant to the real Verifier. An example of such a protocol is client
authenticated TLS, where the browser and the web server authenticate one
another using PKI. Even an unwary Claimant cannot easily reveal to an
Attacker masquerading as the Verifier any information that can be used by
the Attacker to authenticate to the real Verifier. Specialized protocols
where the Claimant’s token device will only release an authenticator to apreset list of valid Verifiers may also be strongly resistant to man-in-themiddle attacks.

Note that systems can supplement the mitigation strategies listed above by enforcing
appropriate security policies. For example, device identity, system health checks, andconfiguration management can be used to mitigate the risk that the Claimant’s system has
been compromised.

8.2.3. Throttling Mechanisms
When using a token that produces low entropy token Authenticators, it is necessary to
implement controls at the Verifier to protect against online guessing attacks. An explicit
requirement for such tokens is given in Table 6: the Verifier shall effectively limit online
Attackers to 100 failed attempts on a single account in any 30 day period.
The simplest way of implementing a throttling mechanism (which is not therecommended approach) would be to keep a counter of failed attempts that is reset at the
beginning of each calendar month, and to lock the account for the rest of the month, when
the counter exceeds 50. Aside from the fact that this system would not technically meetthe requirement on the first of March in non-leap years, this throttling mechanism has anumber of more severe problems. Most notably, it leaves the Verifier open to a very easy
denial of service attack (on the first day of the month, an Attacker simply makes 50 failed
attempts on each Subscriber account he or she knows about, and the system is unusablefor the next 29 days.)

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Electronic Authentication Guideline

The above simple implementation is also sufficiently limiting that it may suffer fromusability problems, where the legitimate Subscriber is penalized for behavior that could
reasonably be identified as benign and should not be counted as failed attempts by an
Attacker. For example, if the Verifier records a dozen failed authentication attempts
followed by a successful attempt from the same IP address over a few minutes to a few
hours, it would be reasonable to assume that those attempts did not come from an
Attacker.
Additional techniques can be used to prioritize authentication attempts that are likely tocome from the Subscriber over those that are more likely to come from an Attacker.
•

Requiring the Claimant to complete a Completely Automated Public Turing
test to tell Computers and Humans Apart (CAPTCHA) before attempting
authentication.

•

Requiring the Claimant to wait for a short period of time (anything from 30
seconds to an hour, depending on how close the system is to its maximum
allowance for failed attempts) before attempting Authentication following afailed attempt.

•

Only accepting authentication requests from a white list of IP addresses atwhich the Subscriber has been successfully authenticated before.

Since these measures often create user inconvenience, it is best to allow a certain numberof failed authentication attempts before employing the above techniques. For example, asystem which enforces the 30-day failed attempt limit, by dividing the calendar into 10day sub-periods and only allowing 25 failed attempts in each sub-period, could allocate
failed attempts as follows: in a given 10 day period, the Verifier could allow 2 failed
attempts each day regardless of any other considerations, allow an additional 5 failed
attempts over the whole period with no additional protections, require CAPTCHAs forthe next 5 failed attempts (beyond the 2-per-day quota), and only allow the final 5
attempts to come from a white-listed IP address after the Claimant has completed a
CAPTCHA.
Finally, if the Verifier accepts authentication attempts for a large number of Subscribers,
it is possible that an Attacker will attempt on online attack on all Subscriber accounts
simultaneously, hoping to gain access to one of them, thus circumventing the throttlingmechanisms employed on the individual accounts. No specific guideline is given forprotecting against such attacks, but Verifiers with a large number of Subscribers shouldtake measures to detect such attacks and either respond to them automatically or alert
system administrators to the threat.

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Electronic Authentication Guideline

8.2.4. Phishing and Pharming (Verifier Impersonation):
Supplementary Countermeasures
It is important to note that phishing and pharming are attacks that use different techniques
to achieve the same goal. Effectively, the Claimant is tricked into believing that he or she
is interacting with the Verifier when in actuality, the Verifier is being impersonated by an
Attacker attempting to collect token information or other sensitive information.
In a successful phishing attack, the Attacker sends an official looking email to a
Subscriber claiming to be a Verifier. The email usually contains a link to a counterfeit
Verifier and will ask the Subscriber to click on the link and authenticate to the Verifier 28.
The Subscriber proceeds to authenticate to the counterfeit Verifier and the login
information and token authenticator is captured. At this point, the Subscriber is unawarethat he or she has been phished, and proceeds with the actions requested by the original
email. Once the Subscriber logs off, he or she is unaware that his or her login information
has been captured and that potentially sensitive data has been captured.
In a successful pharming attack, the Attacker corrupts either the domain name service(using a technique called DNS poisoning) or the local routing tables (by modifying thehost files on a Claimant’s computer to point to a bogus DNS server). When the
Subscriber attempts to connect to a legitimate Verifier on the Internet, the corrupted DNS
tables or routing tables take the Subscriber to a counterfeit Verifier on the Internet. The
Subscriber unknowingly reveals token authenticators and other sensitive information to
the counterfeit Verifier.
The strongest mechanism for preventing phishing and pharming of authentication secrets,
such as token authenticators, is to make sure that some authentication secrets are notdirectly accessible to the Claimant (as described in Section 8.2.2). However, to help
mitigate a wider variety of phishing and pharming attacks, the following techniques may
be used:•Out of band confirmation of transaction details – Details (e.g., accountnumber, amount) of sensitive transactions authorized by the Subscriber may
be sent by the RP to the Subscriber’s out of band token and displayed alongwith a confirmation code. The confirmation code may either be
cryptographically derived from the Subscriber’s token secret and thetransaction details, or it may be a random value that is sent to the Subscriber’sout of band token along with the transaction details. Alternatively, transaction
details may be typed in by the Subscriber as manual inputs to a one-time
password device. In order to complete the transaction, the Subscriber shallsend the correct one-time password or confirmation code to the Verifier orRP.

•Adding a “Last Login” feature by the Verifier to inform the Subscriber of hisor her last login – If the Subscriber logged in at 8:00am and then logs in at

28

Some phishing attacks may request the Subscriber to provide personally sensitive information so that the
Attacker may impersonate the Subscriber outside the scope of E-authentication.

77Electronic Authentication Guideline

4:00pm but the Last Login feature states that the last login was at 2:00pm, theSubscriber may suspect that he or she has been phished and take appropriate
action.
Personalization is the process of customizing a webpage or email for a user to enhance
the user experience. For the purpose of this document, personalization schemes can assist
the user to determine if he or she is interacting with the correct entity. It is important tonote that personalization is at best a low assurance mechanism for mitigating Phishingand Pharming threats, especially when delivered over a communication protocol that is
not strongly resistant to man-in-the-middle attacks. However, personalization may
provide additional assurance when combined with other techniques.
There are three types of personalization in the context of this guideline:•

Pre-authentication personalization – The Verifier displays to the Claimant
some personalized indicator (such as an image or user-chosen phrase picked atregistration) prior to the latter submitting the token authenticator to theformer. This indicator may be established by the Subscriber at the time ofregistration. When the Claimant views the personalized indicator, theClaimant has an increased sense of assurance that he or she is interacting withthe correct Verifier. For example, a Verifier may require the Claimant tosubmit the username first; in response, the Verifier provides the personalized
indicator for the claimed username. If the Claimant recognizes the
personalized indicator as his or her own, the Claimant submits his or her token
authenticator to the Verifier. Pre-authentication personalization does not
eliminate Phishing attacks, but requires the Attacker to use a more complex
technique to succeed in a Phishing attack.

•Post-authentication personalization – The Verifier displays a personalized
indicator to the Subscriber after successful authentication of the latter. The
personalized indicator provides assurance to the Subscriber that he or she has
in fact logged in to the correct site. This indicator may be established by the
Subscriber at the time of registration. For example, after a Subscriber
authenticates to the Verifier, the Verifier provides a personalized indicator(such as a picture, a phrase, or a greeting) that the Subscriber can readily
recognize as his or her own. If the personalized indicator is not shown, or is
not recognized by the Subscriber, the Subscriber suspects that he or she has
been phished and takes appropriate action. Post-authentication personalization
does not protect any secrets used by the Subscriber in the initial authentication
process. Nonetheless, if some or all of these secrets are protected by hardware
or software that runs a protocol with strong man-in-the-middle resistance, thenthe personalization will assist the Subscriber in recognizing that he or she isinteracting with a bogus site and refraining from revealing any further
sensitive information. If personalization appears before the Subscriber is
prompted for a password, but after the Verifier strongly authenticates the78Electronic Authentication Guideline

Subscriber’s local system, then the Subscriber’s password may also beprotected from phishing.
•

Personalization of email sent to the Subscriber by a valid Verifier – This typeof personalization is employed to help the Subscriber differentiate between
email from a valid Verifier, and email from a Phisher. For example, an email
from a Verifier may contain a picture which the Subscriber selected in theregistration process. This type of personalization forces the Phisher to use afairly difficult attack and in effect forces the Phisher to either use a targeted
attack against each Subscriber or hope that the Subscriber will not notice theincorrect or missing personalization identifier.
It is important to note that using a Subscriber’s name (first or last) as the only
method of personalization is a relatively weak method to thwart a phishingattack since it is fairly easy for an Attacker to gain this type of informationand display it in an email or display it after logging into a site. Information ofa non-public nature is a better candidate for use during personalization.

8.3. Authentication Process Assurance Levels
The stipulations for authentication process assurance levels are described in the following
sections.

8.3.1. Threat Resistance per Assurance Level
Authentication process assurance levels can be defined in terms of required threat
resistance. Table 11 lists the threat resistance requirements per assurance level:
Table 11 – Required Authentication Protocol Threat Resistance per Assurance Level
Authentication Process
Attacks/Threats
Online guessing
ReplaySession hijacking
Eavesdropping
Phishing/pharming(verifier
impersonation)
Man in the middle
30
Denial of service/flooding

Level 1
Yes
Yes
No
No

Threat Resistance Requirements
Level 2
Level 3
Level 4
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
29

No

No

Yes

No
No

Weak
No

Weak
No

29

Yes
StrongNo

Long term authentication secrets shall be protected at this level. Short term secrets may or may not beprotected.
30
Although there are techniques used to resist flood attacks, no protocol has comprehensive resistance tostop flooding.

79Electronic Authentication Guideline

8.3.2. Requirements per Assurance Level
This section states the requirements levied on the authentication process to achieve therequired threat resistance at each assurance level. At Levels 2 and above, theauthentication process shall provide sufficient information to the Verifier to uniquely
identify the appropriate registration information that was (i) provided by the Subscriber at
the time of registration, and (ii) verified by the RA in the issuance of the token andcredential. It is important to note that the requirements listed below will not protect theauthentication process if malicious code is introduced on the Claimant’s machine or at
the Verifier.
8.3.2.1. Level 1
Although there is no identity proofing requirement at this level, the authentication
mechanism provides some assurance that the same Claimant who participated in previoustransactions is accessing the protected transaction or data. It allows a wide range ofavailable authentication technologies to be employed and permits the use of any of thetoken methods of Levels 2, 3 or 4. Successful authentication requires that the Claimant
prove, through a secure authentication protocol, that he or she possesses and controls thetoken.
Plaintext passwords or secrets shall not be transmitted across a network at Level 1.
However this level does not require cryptographic methods that block offline analysis byeavesdroppers. For example, password challenge-response protocols that combine apassword with a challenge to generate an authentication reply satisfy this requirement
although an eavesdropper who intercepts the challenge and reply may be able to conduct
a successful off-line dictionary or password exhaustion attack and recover the password.
Since an eavesdropper who intercepts such a protocol exchange will often be able to findthe password with a straightforward dictionary attack, and this vulnerability is
independent of the strength of the operations, there is no requirement at this level to use
Approved cryptographic techniques. At Level 1, long-term shared authentication secrets
may be revealed to Verifiers.
A wide variety of technologies should be able to meet the requirements of Level 1. For
example, a Verifier might obtain a Subscriber password from a CSP and authenticate theClaimant by use of a challenge-response protocol. A password sent through a TLS
protocol session is another example. Other common protocols that meet Level 1
requirements include APOP [RFC 1939], S/KEY [RFC 1760], and password-based
versions of Kerberos [KERB].
8.3.2.2. Level 2
Level 2 allows a wide range of available authentication technologies to be employed andpermits the use of any of the token methods of Levels 2, 3 and 4. Successfulauthentication requires that the Claimant shall prove, through a secure authentication80

Electronic Authentication Guideline

protocol, that he or she controls the token. Session hijacking (when required based on the
FIPS 199 security category of the systems as described below), replay, and online
guessing attacks shall be resisted. Approved cryptography is required to resist
eavesdropping to capture authentication data. Protocols used at Level 2 and above shallbe at least weakly man-in-the-middle resistant, as described in the threat mitigation
strategies subsection.
Session data transmitted between the Claimant and the RP following a successful Level 2
authentication shall be protected as described in the NIST FISMA guidelines.
Specifically, all session data exchanged between information systems that are categorized
as FIPS 199 “Moderate” or “High” for confidentiality and integrity, shall be protected inaccordance with NIST SP 800-53 Control SC-8 (which requires transmission
confidentiality) and SC-9 (which requires transmission integrity).
A wide variety of technologies can meet the requirements of Level 2. For example, aVerifier might authenticate a Claimant who provides a password through a secure(encrypted) TLS protocol session (tunneling).
8.3.2.3. Level 3
Level 3 provides multi-factor remote network authentication. At least two authentication
factors are required. Level 3 authentication is based on proof of possession of the allowed
types of tokens through a cryptographic protocol. Level 3 also permits any of the token
methods of Level 4. Refer to Section 6 for requirements for single tokens and token
combinations that can achieve Level 3 authentication assurance. Additionally, At Level 3,strong cryptographic mechanisms shall be used to protect token secret(s) andauthenticator(s). Long-term shared authentication secrets, if used, shall never be revealed
to any party except the Claimant and CSP; however, session (temporary) shared secrets
may be provided to Verifiers by the CSP, possibly via the Claimant. Approved
cryptographic techniques shall be used for all operations including the transfer of session
data.
Level 3 assurance may be satisfied by client authenticated TLS (implemented in allmodern browsers), with Claimants who have public key certificates. Other protocols withsimilar properties may also be used.
Level 3 authentication assurance may also be met by tunneling the output of a MF OTP
Token, or the output of a SF OTP Token in combination with a Level 2 personal
password, through a TLS session.
8.3.2.4. Level 4
Level 4 is intended to provide the highest practical remote network authentication
assurance. Refer to Section 6 for single tokens and token combinations that are allowed
to be used to achieve Level 4 authentication assurance.

81Electronic Authentication Guideline

Level 4 requires strong cryptographic authentication of all parties, and all sensitive datatransfers between the parties. Either public key or symmetric key technology may be
used. The token secret shall be protected from compromise through the malicious code
threat as described in Section 8.2.1 above. Long-term shared authentication secrets, if
used, shall never be revealed to any party except the Claimant and CSP; however session(temporary) shared secrets may be provided to Verifiers or RPs by the CSP. Strong,
Approved cryptographic techniques shall be used for all operations including the transfer
of session data. All sensitive data transfers shall be cryptographically authenticated using
keys that are derived from the authentication process in such a way that MitM attacks arestrongly resisted.
Level 4 assurance may be satisfied by client authenticated TLS (implemented in allmodern browsers), with Claimants who have public key MF Hardware Cryptographic
Tokens. Other protocols with similar properties can also be used.
It should be noted that, in multi-token schemes, the token used to provide strong man-inthe-middle resistance need not be a hardware token. For example, if a software
cryptographic token is used to open a client-authenticated TLS session, and the output ofa multifactor OTP device is sent by the claimant in that session, then the resultant
protocol will still provide Level 4 assurance.

82Electronic Authentication Guideline

9.

Assertions
9.1. Overview

Assertions are statements from a Verifier to an RP that contain information about a
Subscriber. Assertions are used when the RP and the Verifier are not collocated (i.e., they
are connected through a shared network). The RP uses the information in the assertion to
identify the Claimant and make authorization decisions about his or her access toresources controlled by the RP. An assertion may include identification andauthentication statements regarding the Subscriber, and may additionally include attributestatements that further characterize the Subscriber and support the authorization decisionat the RP.
Assertion-based authentication of the Claimant serves several important goals. It supports
the process of Single-Sign-On for Claimants, allowing them to authenticate once to a
Verifier and subsequently obtain services from multiple RPs without being aware offurther authentication. Assertion mechanisms also support the implementation of afederated identity for a Subscriber, allowing the linkage of multiple identities/accounts
held by the Subscriber with different RPs through the use of a common “federated”
identifier. In this context, a federation is a group of entities (RPs, Verifiers and CSPs) thatare bound together through common agreed-upon business practices, policies, trust
mechanisms, profiles and protocols. Finally, assertion mechanisms can also facilitate
authentication schemes that are based on the attributes or characteristics of the Claimant
in lieu of (or in addition to) the identity of the Claimant. Attributes are often used indetermining access privileges for Attributes Based Access Control (ABAC) or Role
Based Access Control (RBAC).
It is important to note that assertion schemes are fairly complex multiparty protocols, andtherefore have fairly subtle security requirements which shall be satisfied. Whenevaluating a particular assertion scheme, it may be instructive to break it down into its
component interactions. Generally speaking, interactions between the
Claimant/Subscriber and the Verifier and between the Claimant/Subscriber and RP are
similar to the authentication mechanisms presented in Section 8, while interactions
between the Verifier and RP are similar to the token and credential verification services
presented in Section 7. Many of the requirements presented in this section will, therefore,
be similar to corresponding requirements in those two sections.
There are two basic models for assertion-based authentication. After successfulauthentication with the Verifier, the Subscriber is issued an assertion or an assertion
reference, which the Subscriber uses to authenticate to the RP.
•

The Direct Model – In the direct model, the Claimant uses his or her eauthentication token to authenticate to the Verifier. Following successfulauthentication of the Claimant, the Verifier creates an assertion, and sends itto the Subscriber to be forwarded to the RP. The assertion is used by the83Electronic Authentication Guideline

Claimant/Subscriber to authenticate to the RP. (This is usually handled
automatically by the Subscriber’s browser.) Figure 4 illustrates this model.

Figure 4 - Direct Assertion Model•

The Indirect Model – In the indirect model, the Claimant uses his or her token
to authenticate to the Verifier. Following successful authentication, theVerifier creates an assertion as well as an assertion reference (which identifies
the Verifier and includes a pointer to the full assertion held by the Verifier).
The assertion reference is sent to the Subscriber to be forwarded to the RP. In
this model, the assertion reference is used by the Claimant/Subscriber toauthenticate to the RP. The RP then uses the assertion reference to explicitly
request the assertion from the Verifier. Figure 5 illustrates this model.

Figure 5 - Indirect Assertion Model84

Electronic Authentication Guideline

As mentioned earlier, an assertion contains a set of claims or statements about anauthenticated Subscriber. Based on the statements contained within it, an authentication
assertion will fall into one of two categories (and either category can be used in both
direct and indirect models):•

Holder-of-Key Assertions – A holder-of-key assertion contains a reference to asymmetric key or a public key (corresponding to a private key) possessed by
the Subscriber. The RP may require the Subscriber to prove possession of thesecret that is referenced in the assertion. In proving possession of the
Subscriber’s secret, the Subscriber also proves with a certain degree of
assurance that he or she is the rightful owner of the assertion. It is therefore
difficult for an Attacker to use a holder-of-key assertion issued to a
Subscriber, since the former cannot prove possession of the secret referenced
within the assertion.

•

Bearer Assertions – A bearer assertion does not provide a mechanism for the
Claimant to prove that he or she is the rightful owner of the assertion. The RP
has to assume that the assertion was issued to the Subscriber who presents theassertion or the corresponding assertion reference to the RP. If a bearer
assertion (in the direct model) or assertion reference (in the indirect model)
belonging to a Subscriber is captured, copied, or manufactured by an Attacker,
the latter can impersonate the rightful Subscriber to obtain services from the
RP. Bearer assertions can be made secure only if some part of the assertion orassertion reference, sent to the Subscriber by the Verifier, is unpredictable toan Attacker and can reliably be kept secret.

There are cases in which the RP should be anonymous to the Verifier for the purpose ofprivacy. The direct model is more suitable for the “anonymous RP” scenario since there
is no requirement for the RP to authenticate to the Verifier as in the indirect model.
However, it is possible to devise authentication schemes (e.g., using key hierarchies
within a group or federation) that allow the use of the indirect model to support the“anonymous RP” scenario.
There are other cases where privacy concerns require that the Claimant’s identity/accountat the Verifier and RP not be linked through use of a common identifier/account name. In
such scenarios, pseudonymous identifiers are used within the assertions generated by the
Verifier for the RP.
It should be noted that the two models described above are abstractions. There may beother interactions between the three players preceding or interspersed with theinteractions described in the model. For example, the Claimant may initiate a connection
with an RP of his or her choice, at which point, the latter would redirect the Claimant toan appropriate Verifier to be authenticated using the direct model, resulting in an
assertion being sent to the RP. Alternately, the Claimant may first authenticate to a
Verifier of his or her choice and then select one or more RPs to obtain further services.

85

Electronic Authentication Guideline

The direct model is used to generate assertions for each of these RPs. Parallel scenarios
may be constructed for the indirect model as well.
There is one other basic assertion model.
•

The Proxy Model – In the proxy model, the Claimant uses his or
her e-authentication token to authenticate to the Verifier.
Following successful authentication of the Claimant, the Verifier
creates an assertion and includes it when interacting directly with
the RP, acting as an intermediary between the Claimant and theRP. Figure 6 illustrates this model.

Figure 6 – Proxy Model
The RP grants or denies the request based, at least in part, on theauthentication assertion made by the Verifier. There are several
common reasons for such proxies:
o Portals that provide users access to multiple RPs that
require user authenticationo Web caching mechanisms that are required to satisfy theRP’s access control policies, especially when clientauthenticated TLS with the Claimant is requiredo Network monitoring and/or filtering mechanisms thatterminate TLS in order to inspect and manipulate the traffic
It is good practice to protect communications between the Verifier
and the RP. Current commercial implementations tend to do thisby having the proxy use client-authenticated TLS with the Verifierand pass the authentication assertion in the HTTP header.
Note that the Verifier may have access to information that may be
useful to the RP in enforcing security policies, such as device
identity, location, system health checks, and configuration
management. If so, it may be a good idea to pass this information
along to the RP.
Three types of assertion technologies will be discussed within this section: Web browser
cookies, SAML (Security Assertion Markup Language) assertions, and Kerberos tickets.

86Electronic Authentication Guideline

Other assertion technologies may be used in an e-authentication environment as long as
they meet the requirements set forth in Section 9.3 below for the targeted assurance level.

9.1.1. Cookies
One type of assertion widely in use is Web cookie technology. Cookies are text files usedby a browser to store information provided by a particular web site. The contents of thecookie are sent back to the web site each time the browser requests a page from the same
web site. The web site uses the contents of the cookie to identify the user and prepare
customized Web pages for that user, or to authorize the user for certain transactions.
Cookies have two mandatory parameters:•

Name – This parameter states the name of the cookie.

•

Value – This parameter holds information that a cookie is storing. For
example, the value parameter could hold a user ID or session ID.

Cookies also have four optional parameters:•

Expiration date – This parameter determines how long the cookie stays valid.

•Path – This parameter sets the path over which the cookie is valid.

•

Domain – This parameter determines the domain in which the cookie is valid.

•

Secure – This parameter indicates the cookie requires that a secure connection
exist for the cookie to be used.

There are two types of cookies:•

Session cookies – A cookie that is erased when the user closes the webbrowser. The session cookie is stored in temporary memory and is notretained after the browser is closed.

•

Persistent cookies – A cookie that is stored on a user’s hard drive until it
expires (persistent cookies are set with expiration dates) or until the user
deletes the cookie.

Cookies are effective as assertions for Internet single-sign-on where the RP and Verifier
are part of the same Internet domain, and when the cookie contains authentication status
for that domain. They are not usable in scenarios where the RP and the Verifier are partof disparate domains.
Cookies are also often used by the Claimant to re-authenticate to a server. This may beconsidered to be a use of assertion technology. In this case, the server acts as a Verifier
when it sets the cookie in the Subscriber’s browser, and as an RP when it requests thecookie from a Claimant who wishes to re-authenticate to it. Often, the cookie contains arandom number, and the assertion data that it represents does not leave the server. Note

87

Electronic Authentication Guideline

that, if the cookie is used as an assertion reference in this way, no assertion needs to besent on an open network, and therefore, confidentiality and integrity requirements for
assertion data at Level 2 and below may be satisfied by access controls rather than bycryptographic methods. (The cookie itself, however, does need to be protected.) This is inline with the credential storage requirement presented in Section 7.

9.1.2. Security Assertion Markup Language (SAML) Assertions
SAML is an XML-based framework for creating and exchanging authentication andattribute information between trusted entities over the Internet. As of this writing, thelatest specification for [SAML] is SAML v2.0, issued 15 March 2005.
The building blocks of SAML include the Assertions XML schema which define thestructure of the assertion; the SAML Protocols which are used to request assertions andartifacts (that is, the assertion reference mentioned in Section 9.1); and the Bindings thatdefine the underlying communication protocols (such as HTTP or SOAP) and that can beused to transport the SAML assertions. The three components above define a SAML
profile that corresponds to a particular use case such as “Web Browser SSO”.
SAML Assertions are encoded in an XML schema and can carry up to three types ofstatements:•

Authentication statements – Include information about the assertion issuer, theauthenticated subject, validity period, and other authentication information.
For example, an Authentication Assertion would state the subject “John” wasauthenticated using a password at 10:32pm on 06-06-2004.

•

Attribute statements – Contain specific additional characteristics related to the
Subscriber. For example, subject “John” is associated with attribute “Role”
with value “Manager”.

•

Authorization statements – Identify the resources the Subscriber has
permission to access. These resources may include specific devices, files, andinformation on specific web servers. For example, subject “John” for action“Read” on “Webserver1002” given evidence “Role”.

Authorization statements are beyond the scope of this document and will not be
discussed.

9.1.3. Kerberos Tickets
The Kerberos Network Authentication Service [RFC 4120] was designed to provide
strong authentication for client/server applications using symmetric-key cryptography.
Extensions to Kerberos can support the use of public key cryptography for selected steps

88Electronic Authentication Guidelineof the protocol. Kerberos also supports confidentiality and integrity protection of session
data between the Subscriber and the RP.
Kerberos supports authentication of a Claimant over an untrusted, shared network using
two or more Verifiers. The Claimant implicitly authenticates to the Verifier bydemonstrating the ability to decrypt a random session key encrypted for the Subscriber by
the Verifier. (Some Kerberos variants also require the Subscriber to explicitly
authenticate to the Verifier, but this is not universal.) In addition to the encrypted session
key, the Verifier also generates another encrypted object called a Kerberos ticket. The
ticket contains the same session key, the identity of the Subscriber to whom the session
key was issued, and an expiration time after which the session key is no longer valid. The
ticket is confidentiality and integrity protected by a pre-established that is key shared
between the Verifier and the RP.
To authenticate using the session key, the Claimant sends the ticket to the RP along withencrypted data that proves that the Claimant possesses the session key embedded within
the Kerberos ticket. Session keys are either used to generate new tickets, or to encryptand authenticate communications between the Subscriber and the RP.
To begin the process, the Claimant sends an authentication request to the Authentication
Server (AS). The AS encrypts a session key for the Subscriber using the Subscriber’slong term credential. The long term credential may either be a secret key shared between
the AS and the Subscriber, or in the PKINIT variant of Kerberos, a public key certificate.
It should be noted that most variants of Kerberos based on a shared secret key between
the Subscriber and Verifier derive this key from a user generated password. As such, they
are vulnerable to offline dictionary attack by a passive eavesdropper.
In addition to delivering the session key to the subscriber, the AS also issues a ticket
using a key it shares with the Ticket Granting Server (TGS). This ticket is referred to as a
Ticket Granting Ticket (TGT), since the verifier uses the session key in the TGT to issue
tickets rather than to explicitly authenticate the Claimant. The TGS uses the session key
in the TGT to encrypt a new session key for the Subscriber and uses a key it shares with
the RP to generate a ticket corresponding to the new session key. The subscriber decrypts
the session key and uses the ticket and the new session key together to authenticate to theRP.

9.2. Assertion Threats
In this section, it is assumed that the two endpoints of the assertion transmission (namely,
the Verifier and the RP) are uncompromised. However, the Claimant is not assumed to be
entirely trustworthy as the Claimant may have an interest in modifying or replacing anassertion to obtain a greater level of access to a resource/service provided by the RP.
Other Attackers are assumed to lurk within the shared transmission medium (e.g.,
Internet) and may be interested in obtaining or modifying assertions and assertion
references to impersonate a Subscriber or access unauthorized data or services.

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Electronic Authentication Guideline

Furthermore, it is possible that two or more entities may be colluding to attack anotherparty. An Attacker may attempt to subvert assertion protocols by directly compromising
the integrity or confidentiality of the assertion data. For the purpose of this type of threat,authorized parties who attempt to exceed their privileges may be considered Attackers.
•

Assertion manufacture/modification – An Attacker may generate a bogus
assertion or modify the assertion content (such as the authentication or
attribute statements) of an existing assertion, causing the RP to grant
inappropriate access to the Subscriber. For example, an Attacker may modify
the assertion to extend the validity period; a Subscriber may modify theassertion to have access to information that they should not be able to view.

•

Assertion disclosure – Assertions may contain authentication and attributestatements that include sensitive Subscriber information. Disclosure of theassertion contents can make the Subscriber vulnerable to other types ofattacks.

•

Assertion repudiation by the Verifier – An assertion may be repudiated by a
Verifier if the proper mechanisms are not in place. For example, if a Verifier
does not digitally sign an assertion, the Verifier can claim that it was not
generated through the services of the Verifier.

•

Assertion repudiation by the Subscriber – Since it is possible for acompromised or malicious subscriber to issue assertions to the wrong party, a
subscriber can repudiate any transaction with the RP that was authenticatedusing only a bearer assertion.

•

Assertion redirect: An Attacker uses the assertion generated for one RP toobtain access to a second RP.

•

Assertion reuse – An Attacker attempts to use an assertion that has alreadybeen used once with the intended RP.

In addition to reliable and confidential transmission of assertion data from the Verifier tothe RP, assertion protocols have a further goal: in order for the Subscriber to berecognized by the RP, he or she shall be issued some secret information, the knowledgeof which distinguishes the Subscriber from Attackers who wish to impersonate theSubscriber. In the case of holder-of-key assertions, this secret is generally the
Subscriber’s long term token secret, which would already have been established with the
CSP prior to the initiation of the assertion protocol. 31
In other cases, however, the Verifier will generate a temporary secret and transmit it to
the authenticated Subscriber for this purpose. Since, when this secret is used toauthenticate to the RP, it generally replaces the token authenticator in the type of
31

The role of the Verifier in such protocols is not necessarily to issue new secrets. Rather, in a holder-ofkey-assertion, the Verifier communicates the information in the Subscriber’s credential (as well as any
supplementary information from the CSP such as revocation data) to the RP. The Verifier also vouches thatthe holder-of-key assertion represents current information from a trusted source (the CSP.)

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Electronic Authentication Guideline

protocols described in Section 8, this temporary secret will be referred to here as asecondary authenticator. Secondary authenticators include assertions in the direct model,
session keys in Kerberos, assertion references in the indirect model, and cookies used forauthentication. The threats to the secondary authenticator are as follows:•

Secondary authenticator manufacture – An Attacker may attempt to generate
a valid secondary authenticator and use it to impersonate a Subscriber.

•

Secondary authenticator capture – The Attacker may use a session hijacking
attack to capture the secondary authenticator when the Verifier transmits it to
the Subscriber after the primary authentication step, or the Attacker may use a
man-in-the-middle attack to obtain the secondary authenticator as it is being
used by the Subscriber to authenticate to the RP. If, as in the indirect model,
the RP needs to send the secondary authenticator back to the Verifier in orderto check its validity or obtain the corresponding assertion data, an Attacker
may similarly subvert the communication protocol between the Verifier andthe RP to capture a secondary authenticator. In any of the above scenarios, thesecondary authenticator can be used to impersonate the Subscriber.

Finally, in order for the Subscriber’s authentication to the RP to be useful, the binding
between the secret used to authenticate to the RP and the assertion data referring to the
Subscriber shall be strong.
•

Assertion substitution – A subscriber may attempt to impersonate a moreprivileged subscriber by subverting the communication channel between the
Verifier and RP, for example by reordering the messages, to convince the RPthat his or her secondary authenticator corresponds to assertion data sent onbehalf of the more privileged subscriber. This is primarily a threat to theindirect model, since in the direct model, assertion data is directly encoded in
the secondary authenticator.

9.2.1. Threat Mitigation Strategies
Mitigation techniques are described below for each of the threats described in the last
subsection.
Logically speaking, an assertion is issued by a Verifier and consumed by an RP – these
are the two end points of the session that needs to be secured to protect the assertion. In
the direct model, the session in which the assertion is passed traverses the Subscriber.
Furthermore, in the current web environment, the assertion may pass through two
separate secure sessions (one between the Verifier and the Subscriber, and the otherbetween the Subscriber and the RP), with a break in session security on the Subscriber’sbrowser. This is reflected in the mitigation strategies described below. In the indirectmodel, the assertion flows directly from the Verifier to the RP; this protocol session

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needs to be protected. All of the threat mitigation strategies in Section 8 apply to theprotocols used to request, retrieve and submit assertions and assertion references.
•

Assertion manufacture/modification: To mitigate this threat, one of thefollowing mechanisms may be used:
1. The assertion may be digitally signed by the Verifier. The RP should
check the digital signature to verify that it was issued by a legitimate
Verifier.
2. The assertion may be sent over a protected session such as TLS. In orderto protect the integrity of assertions from malicious attack, the Verifier
shall be authenticated.

•

Assertion disclosure – To mitigate this threat, one of the following
mechanisms may be implemented:
1. The assertion may be sent over a protected session to an authenticated RP.
Note that, in order to protect assertions against both disclosure andmanufacture/modification using a protected session, both the RP and the
Verifier need to be authenticated.
2. If assertions are signed by the Verifier, they may be encrypted for a
specific RP with no additional integrity protection. It should be noted thatany protocol that requires a series of messages between two parties to besigned by their source and encrypted for their recipient provides all thesame guarantees as a mutually authenticated protected session, and may
therefore be considered equivalent. The general requirement for protecting
against both assertion disclosure and assertion manufacture/modification
may therefore be described as a mutually authenticated protected session
or equivalent between Verifier and RP.

•

Assertion repudiation by the Verifier – To mitigate this threat, the assertion
may be digitally signed by the Verifier using a key that supports nonrepudiation. The RP should check the digital signature to verify that it wasissued by a legitimate Verifier.

•

Assertion repudiation by the Subscriber – To mitigate this threat, the Verifier
may issue holder of key, rather than bearer assertions. The Subscriber can thenprove possession of the asserted key to the RP. If the asserted key matches thesubscriber’s long term credential (as provided by the CSP) it will be clear to
all parties involved that it was the Subscriber who authenticated to the RP
rather than a compromised Verifier impersonating the Subscriber.

•

Assertion redirect – To mitigate this threat, the assertion may include theidentity of the RP for whom it was generated. The RP verifies that incoming
assertions include its identity as the recipient of the assertion.

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Electronic Authentication Guideline•

Assertion reuse – To mitigate this threat, the following mechanisms may beused:
1. The assertion includes a timestamp and has a short lifetime of validity.
The RP checks the timestamp and lifetime values to ensure that theassertion is currently valid. The lifetime value may either be in theassertion or set by the RP.
2. The RP keeps track of assertions that were consumed within a(configurable) time window to ensure that an assertion cannot be used
more than once within that time window.

•

Secondary authenticator manufacture – To mitigate this threat, one of thefollowing mechanisms may be implemented:
1. The secondary authenticator may contain sufficient entropy that an
Attacker without direct access to the Verifier’s random number generator
cannot guess the value of a valid secondary authenticator.
2. The secondary authenticator may contain timely assertion data that issigned by the Verifier or integrity protected using a key shared between
the Verifier and the RP.
3. The Subscriber may authenticate to the RP directly using his or her longterm token and avoid the need for a secondary authenticator altogether.

•

Secondary authenticator capture – To mitigate this threat, adequate
protections shall be in place throughout the lifetime of any secondary
authenticators used in the assertion protocol.
1. In order to protect the secondary authenticator while it is in transit
between the Verifier and the Subscriber, the secondary authenticator shallbe sent via a protected session established during the primaryauthentication of the Subscriber using his or her token. This requirement is
the same as the requirement in Section 8, regarding the Authentication
Process, to protect sensitive data (in this case the secondary authenticator)
from session hijacking attacks.
2. In order to protect the secondary authenticator from capture as it issubmitted to the RP, the secondary authenticator shall be used in anauthentication protocol which protects against eavesdropping and man-inthe-middle attacks as described in Section 8.
3. In order to protect the secondary authenticator after it has been used, it
shall never be transmitted on an unprotected session or to an
unauthenticated party while it is still valid. The secondary authenticator
may be sent in the clear only if the sending party has strong assurancesthat the secondary authenticator will not subsequently be accepted by anyother RP. This is possible if the secondary authenticator is specific to a93

Electronic Authentication Guideline

single RP, and if that RP will not accept secondary authenticators with thesame value until the maximum lifespan of the corresponding assertion has
passed.
•Assertion substitution – To mitigate this threat, one of the following
mechanisms may be implemented:
1. Responses to assertion requests, signed or integrity protected by the
Verifier, may contain the value of the assertion reference used in therequest or some other nonce that was cryptographically bound to therequest by the RP.
2. Responses to assertion requests may be bound to the corresponding
requests by message order, as in HTTP, provided that assertions andrequests are protected by a protocol such as TLS that can detect anddisallow malicious reordering of packets.

9.3. Assertion Assurance Levels
The stipulations for assertion assurance levels are described in the next sections.

9.3.1. Threat Resistance per Assurance Level
Table 12 lists the requirements for assertions (both in the direct and indirect models) andassertion references (in the indirect model) at each assurance level in terms of resistanceto the threats listed above.
Table 12 – Threat Resistance per Assurance Level
ThreatAssertion
manufacture/modification
Assertion disclosure
Assertion repudiation by Verifier
Assertion repudiation by
Subscriber
Assertion redirect
Assertion reuse
Secondary authenticator
manufacture
Secondary authenticator capture
Assertion substitution
32

Level 1

Level 2

Level 3

Level 4

Yes

Yes

Yes

Yes

No

Yes

Yes

Yes

No

No

Yes

32

Yes

32

No

No

No

Yes

32

No

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

No

Yes

Yes

Yes

No

Yes

Yes

Yes

Except for Kerberos.

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Electronic Authentication Guideline

9.3.2. Requirements per Assurance Level
The following sections summarize the requirements for assertions at each assurance level.
All assertions recognized within this guideline shall indicate the assurance level of theinitial authentication of the Claimant to the Verifier. The assurance level indication
within the assertion may be implicit (e.g., through the identity of the Verifier implicitly
indicating the resulting assurance level) or explicit (e.g., through an explicit field within
the assertion).
9.3.2.1. Level 1
At Level 1, it must be impractical for an Attacker to manufacture an assertion or assertion
reference that can be used to impersonate the Subscriber. If the direct model is used, theassertion which is used shall be signed by the Verifier or integrity protected using a secretkey shared by the Verifier and RP, and if the indirect model is used, the assertion
reference which is used shall have a minimum of 64 bits of entropy. Bearer assertions
shall be specific to a single transaction. 33 Also, if assertion references are used, they shallbe freshly generated whenever a new assertion is created by the Verifier. In other words,
bearer assertions and assertion references are generated for one-time use.
Furthermore, in order to protect assertions against modification in the indirect model, allassertions sent from the Verifier to the RP shall either be signed by the Verifier, ortransmitted from an authenticated Verifier via a protected session. In either case, a strong
mechanism must be in place which allows the RP to establish a binding between theassertion reference and its corresponding assertion, based on integrity protected (orsigned) communications with the authenticated Verifier.
To lessen the impact of captured assertions and assertion references, assertions that are
consumed by an RP which is not part of the same Internet domain as the Verifier shall
expire if they are not used within 5 minutes of their creation. Assertions intended for use
within a single Internet domain, including assertions contained in or referenced bycookies, however, may last as long as 12 hours without being used.
9.3.2.2. Level 2
If the underlying credential specifies that the subscriber name is a pseudonym, thisinformation must be conveyed in the assertion. Level 2 assertions shall be protectedagainst manufacture/modification, capture, redirect and reuse. Assertion references shallbe protected against manufacture, capture and reuse. Each assertion shall be targeted for a
33

For example, implementation of SSO requires a separate assertion each time a new session is started witha participating RP.

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Electronic Authentication Guideline

single RP and the RP shall validate that it is the intended recipient of the incoming
assertion.
All stipulations from Level 1 apply. Additionally, assertions, assertion references and any
session cookies used by the Verifier or RP for authentication purposes, shall betransmitted to the Subscriber through a protected session which is linked to the primaryauthentication process in such a way that session hijacking attacks are resisted (seeSection 8.2.2 for methods which may be used to protect against session hijacking
attacks). Assertions, assertion references and session cookies shall not be subsequently
transmitted over an unprotected session or to an unauthenticated party while they remain
valid. (To this end, any session cookies used for authentication purposes shall be flagged
as secure, and redirects used to forward secondary authenticators from the Subscriber tothe RP shall specify a secure protocol such as HTTPS.)
To protect assertions against manufacture, modification, and disclosure, assertions which
are sent from the Verifier to the RP, whether directly or through the Subscriber’s device,
shall either be sent via a mutually authenticated protected session between the Verifierand RP, or equivalently shall be signed by the Verifier and encrypted for the RP.
All assertion protocols used at Level 2 and above require the use of Approved
cryptographic techniques. As such, the use of Kerberos keys derived from user generated
passwords is not permitted at Level 2 or above.
9.3.2.3. Level 3
At Level 3, in addition to Level 2 requirements, assertions shall be protected against
repudiation by the Verifier; all assertions used at Level 3 shall be signed. Level 3
assertions shall specify verified names and not pseudonyms.
Kerberos uses symmetric key mechanisms to protect key management and session data,and it does not protect against assertion repudiation. However, based on the high degreeof vetting conducted on the Kerberos protocol and its wide deployment, Kerberos tickets
are acceptable for use as assertions at Level 3 as long as:•

All Verifiers (Kerberos Authentication Servers and Ticket Granting Servers) are
under the control of a single management authority that ensures the correct
operation of the Kerberos protocol;•

The Subscriber authenticates to the Verifier using a Level 3 token;•

All Level 3 requirements unrelated to non-repudiation are satisfied.

Also, at Level 3, single-domain assertions (e.g., Web browser cookies) shall expire if
they are not used within 30 minutes. Cross-domain assertions shall expire if not usedwithin 5 minutes.

96Electronic Authentication Guideline

However, in order to deliver the effect of single sign on, the Verifier may re-authenticate
the Subscriber prior to delivering assertions to new RPs, using a combination of longterm and short term single domain assertions provided that the following assurances aremet:•

The Subscriber has successfully authenticated to the Verifier within the last 12
hours.

•The Subscriber can demonstrate that he or she was the party that authenticatedto the Verifier. This could be demonstrated, for example, by the presence of a
cookie set by the Verifier in the Subscriber’s browser.

•

The Verifier can reliably determine whether the Subscriber has been in active
communication with an RP since the last assertion was delivered by the
Verifier. This means that the Verifier needs evidence that the Subscriber isactively using the services of the RP and has not been idle for more than 30
minutes. An authenticated assertion by the RP to this effect is consideredsufficient evidence for this purpose.
9.3.2.4. Level 4

At Level 4, bearer assertions (including cookies) shall not be used to establish the identityof the Claimant to the RP. Assertions made by the Verifier may however be used to bindkeys or other attributes to an identity. Holder-of-key assertions may be used, provided
that all three requirements below are met:•

The Claimant authenticates to the Verifier using a Level 4 token (as described in
Section 6) in a Level 4 authentication protocol (as described in Section 8).

•

The Verifier generates a holder-of-key assertion that references a key that is partof the Level 4 token (used to authenticate to the Verifier) or linked to it through achain of trust, and;•The RP verifies that the Subscriber possesses the key that is referenced in theholder-of-key assertion using a Level 4 protocol (where the RP plays the role
attributed to the Verifier by Section 8).

The RP should maintain records of the assertions it receives, so that if a suspicioustransaction occurs at the RP, the key asserted by the Verifier may be compared to thevalue registered with the CSP. This record keeping allows the RP to detect any attemptby the Verifier to impersonate the Subscriber using fraudulent assertions and may also be
useful for preventing the Subscriber from repudiating various aspects of theauthentication process.

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Electronic Authentication Guideline

Kerberos uses symmetric key mechanisms to protect key management and session data,and it does not protect against assertion repudiation by the Subscriber or the Verifier.
However, based on the high degree of vetting conducted on the Kerberos protocol and its
wide deployment, Kerberos tickets are acceptable for use as assertions at Level 4 as longas:•

All Verifiers (Kerberos Authentication Servers and Ticket Granting Servers) are
under the control of a single management authority that ensures the correct
operation of the Kerberos protocol;•

The Subscriber authenticates to the Verifier using a Level 4 token;•

All Level 4 requirements unrelated to non-repudiation are satisfied.

All Level 1-3 requirements for the protection of assertion data remain in force at Level 4.

98Electronic Authentication Guideline

10.

References

This section lists references that are current versions at the time of publication.
Subsequent versions of NIST publications (i.e., Federal Information Processing Standards
and Special Publications) are also acceptable.

10.1.
[DOJ 2000]

General References
Guide to Federal Agencies on Implementing Electronic Processes(November 2000), available at:
http://www.usdoj.gov/criminal/cybercrime/ecommerce.html[GSA ESIG]Use of Electronic Signatures in Federal Organization Transactions(2011), available at: http://www.gsa.gov/[FISMA]

Federal Information Security Management Act, available at:
http://csrc.nist.gov/drivers/documents/FISMA-final.pdf[OMB M-04-04]

OMB Memorandum M-04-04, E-Authentication Guidance forFederal agencies, December 16, 2003, available at:
http://www.whitehouse.gov/omb/memoranda/fy04/m04-04.pdf[OMB M-03-22]

OMB Memorandum M-03-22, OMB Guidance for Implementing thePrivacy Provisions of the E-Government Act of 2002, September 26,
2003 available at: http://www.whitehouse.gov/omb/memoranda/m0322.html.

[OMB M-11-11]

OMB Memorandum M-11-11, Continued Implementation ofHomeland Security Presidential Directive (HSPD) 12–Policy for a
Common Identification Standard for Federal Employees andContractors, Feb. 3, 2011 available at:
http://www.whitehouse.gov/sites/default/files/omb/memoranda/2011/
m11-11.pdf[KERB]

Neuman, C., and T. Ts’o, Kerberos: An Authentication Service for
Computer Networks, IEEE Communications, vol. 32, no.9, 1994.

[RFC 4120]

IETF, RFC 4120, The Kerberos Network Authentication Service(V5), July 2005, available at http://www.ietf.org/rfc/rfc4120.txt[RFC 1939]

IETF, RFC 1939, Post Office Protocol, Version 3, May 1996,
available at: http://www.ietf.org/rfc/rfc1939.txt[RFC 2246]

IETF, RFC 2246, The TLS Protocol, Version 1.0. January 1999,
available at: http://www.ietf.org/rfc/rfc2246.txt[RFC 5280]

IETF, RFC 5280, Internet X.509 Public Key Infrastructure Certificate
and CRL Profile, available at: http://www.ietf.org/rfc/rfc5280.txt
99Electronic Authentication Guideline[RFC 3546]

IETF, RFC 3546, Transport Layer Security (TLS) Extensions, June
2003, available at: http://www.ietf.org/rfc/rfc3546.txt[RFC 5246]

IETF, RFC 5246, The Transport Layer Security (TLS) Protocol,
Version 1.2, August 2008, available at
http://tools.ietf.org/html/rfc5246[RFC 1760]

IETF, RFC 1760, The S/KEY One-Time Password System, February
1995, available at: http://www.ietf.org/rfc/rfc1760.txt[ICAM]

National Security Systems and Identity, Credential and Access
Management Sub-Committee Focus Group, Federal CIO Council,
ICAM Lexicon, Version 0.5, March 2011.

[ISPKI]ITU-T Recommendation X.509 | ISO / IEC 9594-8: “Information
Technology - Open Systems Interconnection - The Directory: PublicKey and Attribute Certificate Frameworks.”[SAML]OASIS, SAML, “Security Assertion Markup Language 2.0,” v2.0,
March 2005, available at
http://www.oasis-open.org/standards#samlv2.0

10.2.
NIST Special Publications
NIST 800 Series Special Publications are available at:
http://csrc.nist.gov/publications/nistpubs/index.html. The following publications may beof particular interest to those implementing systems of applications requiring eauthentication.
[SP 800-30]

NIST Special Publication 800-30, Risk Management Guide forInformation Technology Systems, July 2002.

[SP 800-32]

NIST Special Publication, 800-32, Introduction to Public Key
Technology and the Federal PKI Infrastructure, February 2001.

[SP 800-33]

NIST Special Publication 800-33, Underlying Technical Models for
Information Technology Security, December 2001.

[SP 800-37]

NIST Special Publication 800-37, Revision1, Guide for Applying the
Risk Management Framework to Federal Information Systems,
February 2010.

[SP 800-40]

NIST Special Publication 800-40, Version 2.0, Creating a Patch andVulnerability Management Program,, November 2005.

[SP 800-41]

NIST Special Publication 800-41, Revision 1, Guidelines on
Firewalls and Firewall Policy, September 2009.

[SP 800-43]

NIST Special Publication 800-43, Guide to Securing Windows 2000
Professional, November 2002.

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Electronic Authentication Guideline[SP 800-44]

NIST Special Publication 800-44, Version 2,Guidelines on Securing
Public Web Servers, September 2007.

[SP 800-47]

NIST Special Publication 800-47, Security Guide for Interconnecting
Information Technology Systems, September 2002.

[SP 800-52]

NIST Special Publication 800-52, Guidelines for the Selection and
Use of Transport Layer Security Implementations, June 2005.

[SP 800-53]

NIST Special Publication 800-53, Revision 3, Recommended Security
Controls for Federal Information Systems and Organizations, August
2009 and Errata as of May 2010.

[SP 800-53A]

NIST Special Publication 800-53A, Revision 1, Guide for Assessing
the Security Controls in Federal Information Systems andOrganizations, Building Effective Security Assessment Plans, June2010.

[SP 800-57]

NIST Special Publication 800-57, Revision 2, Recommendation for
Key Management – Part 1: General, March 2007.

[SP 800-94]

NIST Special Publication, 800-94, Guide to Intrusion Detection andPrevention Systems (IDPS), February 2007.

[SP 800-115]

NIST Special Publication 800-115, Technical Guide to Information
Security Testing and Assessment, September 2008.

10.3.
Federal Information Processing Standards
FIPS can be found at: http://csrc.nist.gov/publications/fips/[FIPS 140-2]

Federal Information Processing Standard Publication 140-2, Security
Requirements for Cryptographic Modules, NIST, May 25, 2001.

[FIPS 180-2]

Federal Information Processing Standard Publication 180-2, Secure
Hash Standard (SHS), NIST, August 2002.

[FIPS186-2]

Federal Information Processing Standard Publication 186-2, Digital
Signature Standard (DSS), NIST, June 2000.

[FIPS 197]

Federal Information Processing Standard Publication 197, Advanced
Encryption Standard (AES), NIST, November 2001.

[FIPS 199]

Standards for Security Categorization of Federal Information andInformation Systems (February 2004), available at:
http://csrc.nist.gov/publications/fips/fips199/FIPS-PUB-199-final.pdf[FIPS 201]

Personal Identity Verification (PIV) of Federal Employees andContractors (March 2006), available at:
http://csrc.nist.gov/publications/fips/fips201-1/FIPS-201-1-chng1.pdf

101Electronic Authentication Guideline

10.4.
Certificate Policies
These certificate policies can be found at: http://www.cio.gov/fpkipa/[FBCA1]

X.509 Certificate Policy For The Federal Bridge Certification
Authority (FBCA), version 2.1 January 12, 2006. Available at:
http://www.cio.gov/fpkipa/documents/FBCA_CP_RFC3647.pdf[FBCA2]

Citizen & Commerce Certificate Policy, Version 1.0 December 3,
2002. Available at:
http://www.cio.gov/fpkipa/documents/citizen_commerce_cpv1.pdf[FBCA3]

X.509 Certificate Policy for the Common Policy Framework, Version
2.4 February 15, 2006. Available at:
http://www.cio.gov/fpkipa/documents/CommonPolicy.pdf

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Appendix A: Estimating Entropy and Strength
Password Entropy
Passwords represent a very popular implementation of memorized secret tokens. In thiscase impersonation of an identity requires only that the impersonator obtain the
password. Moreover, the ability of humans to remember long, arbitrary passwords islimited, so passwords are often vulnerable to a variety of attacks including guessing, useof dictionaries of common passwords, and brute force attacks of all possible password
combinations. There are a wide variety of password authentication protocols that differ
significantly in their vulnerabilities, and many password mechanisms are vulnerable topassive and active network attacks. While some cryptographic password protocols resist
nearly all direct network attacks, these techniques are not at present widely used and all
password authentication mechanisms are vulnerable to keyboard loggers and observationof the password when it is entered. Experience also shows that users are vulnerable to“social engineering” attacks where they are persuaded to reveal their passwords tounknown parties, who are basically “confidence men.”
Claude Shannon coined the use of the term “entropy 34” in information theory. The
concept has many applications to information theory and communications and Shannon
also applied it to express the amount of actual information in English text. Shannon says,“The entropy is a statistical parameter which measures in a certain sense, how muchinformation is produced on the average for each letter of a text in the language. If thelanguage is translated into binary digits (0 or 1) in the most efficient way, the entropy H
is the average number of binary digits required per letter of the original language.” 35
Entropy in this sense is at most only loosely related to the use of the term inthermodynamics. A mathematical definition of entropy in terms of the probability
distribution function is:
H ( X ) := −∑ P( X = x) log 2 P( X = x)xwhere P(X=x) is the probability that the variable X has the value x.
Shannon was interested in strings of ordinary English text and how many bits it would
take to code them in the most efficient way possible. Since Shannon coined the term,“entropy” has been used in cryptography as a measure of the difficulty in guessing or
determining a password or a key. Clearly the strongest key or password of a particularsize is a truly random selection, and clearly, on average such a selection cannot becompressed. However it is far from clear that compression is the best measure for thestrength of keys and passwords, and cryptographers have derived a number of alternative
34

C. E. Shannon, “A mathematical Theory of Communication,” Bell System Technical Journal, v. 27, pp.
379-423, 623-656, July, October 1948, see http://cm.bell-labs.com/cm/ms/what/shannonday/paper.html
35
C. E. Shannon, “Prediction and Entropy of Printed English”, Bell System Technical Journal, v.30, n. 1,
1951, pp. 50-64.

103Electronic Authentication Guideline

forms or definitions of entropy, including “guessing entropy” and “min-entropy.” Asapplied to a distribution of passwords the guessing entropy is, roughly speaking, anestimate of the average amount of work required to guess the password of a selected user,
and the min-entropy is a measure of the difficulty of guessing the easiest single password
to guess in the population.
If we had a good knowledge of the frequency distribution of passwords chosen under aparticular set of rules, then it would be straightforward to determine either the guessing
entropy or the min-entropy of any password. An Attacker who knew the password
distribution would find the password of a chosen user by first trying the most probablepassword for that chosen username, then the second most probable password for thatusername and so on in decreasing order of probability until the Attacker found thepassword that worked with the chosen username. The average for all passwords would be
the guessing entropy. The Attacker who is content to find the password of any user would
follow a somewhat different strategy, he would try the most probable password withevery username, then the second most probable password with every username, until hefound the first “hit.” This corresponds to the min-entropy.
Unfortunately, we do not have much data on the passwords users choose under particular
rules, and much of what we do know is found empirically by “cracking” passwords, thatis by system administrators applying massive dictionary attacks to the files of hashedpasswords (in most systems no plaintext copy of the password is kept) on their systems.
NIST would like to obtain more data on the passwords users actually choose, but, where
they have the data, system administrators are understandably reluctant to reveal password
data to others. Empirical and anecdotal data suggest that many users choose very easily
guessed passwords, where the system will allow them to do so.
A.1

Randomly Selected Passwords

As we use the term here, “entropy” denotes the uncertainty in the value of a password.
Entropy of passwords is conventionally expressed in bits. If a password of k bits ischosen at random there are 2k possible values and the password is said to have k bits ofentropy. If a password of length l characters is chosen at random from an alphabet of b
characters (for example the 94 printable ISO characters on a typical keyboard) then theentropy of the password is bl (for example if a password composed of 8 characters fromthe alphabet of 94 printable ISO characters the entropy is 948 ≈ 6.09 x 1015 – this is about
252, so such a password is said to have about 52 bits of entropy). For randomly chosenpasswords, guessing entropy, min-entropy, and Shannon entropy are all the same value.
The general formula for entropy, H is given by:
H = log2 (bl)Table A.1 gives the entropy versus length for a randomly generated password chosenfrom the standard 94 keyboard characters (not including the space). Calculation of
randomly selected passwords from other alphabets is straightforward.

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Electronic Authentication Guideline

A.2

User Selected Passwords

It is much more difficult to estimate the entropy in passwords that users choose forthemselves, because they are not chosen at random and they will not have a uniform
random distribution. Passwords chosen by users probably roughly reflect the patterns andcharacter frequency distributions of ordinary English text, and are chosen by users so thatthey can remember them. Experience teaches us that many users, left to choose their own
passwords will choose passwords that are easily guessed and even fairly shortdictionaries of a few thousand commonly chosen passwords, when they are compared toactual user chosen passwords, succeed in “cracking” a large share of those passwords.
A.2.1 Guessing Entropy Estimate
Guessing entropy is arguably the most critical measure of the strength of a password
system, since it largely determines the resistance to targeted, online password guessing
attacks.
In these guidelines, we have chosen to use Shannon’s estimate of the entropy in ordinary
English text as the starting point to estimate the entropy of user-selected passwords. It is a
big assumption that passwords are quite similar to other English text, and it would bebetter if we had a large body of actual user selected passwords, selected under different
composition rules, to work from, but we have no such resource, and it is at least plausible
to use Shannon’s work for a “ballpark” estimate. Readers are cautioned againstinterpreting the following rules as anything more than a very rough rule of thumb method
to be used for the purposes of e-authentication.
Shannon conducted experiments where he gave people strings of English text and asked
them to guess the next character in the string. From this he estimated the entropy of eachsuccessive character. He used a 27-character alphabet, the ordinary English lower caseletters plus the space.
In the following discussion we assume that passwords are user selected from the normal
keyboard alphabet of 94 printable characters, and are at least 6-characters long. Since
Shannon used a 27 character alphabet it may seem that the entropy of user selectedpasswords would be much larger, however the assumption here is that users will choose
passwords that are almost entirely lower case letters, unless forced to do otherwise, andthat rules that force them to include capital letters or non-alphabetic characters will
generally be satisfied in the simplest and most predictable manner, often by putting acapital letter at the start (as we do in ordinary English) and punctuation or special
characters at the end, or by some simple substitution, such as $ for the letter “s.”Moreover rules that force passwords to appear to be highly random will becounterproductive because they will make the passwords hard to remember. Users willthen write the passwords down and keep them in a convenient (that is insecure) place,such as pasted on their monitor. Therefore it is reasonable to start from estimates of theentropy of simple English text, assuming only a 27-symbol alphabet.

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Electronic Authentication Guideline

Shannon observed that, although there is a non-uniform probability distribution of letters,
it is comparatively hard to predict the first letter of an English text string, but, given thefirst letter, it is much easier to guess the second and given the first two the third is easier
still, and so on. He estimated the entropy of the first symbol at 4.6 to 4.7 bits, declining toon the order of about 1.5 bits after 8 characters. Very long English strings (for example
the collected works of Shakespeare) have been estimated to have as little as .4 bits ofentropy per character. 36 Similarly, in a string of words, it is harder to predict the first
letter of a word than the following letters, and the first letter carries about 6 times more
information than the fifth or later letters 37.
An Attacker attempting to find a password will try the most likely chosen passwords first.
Very extensive dictionaries of passwords have been created for this purpose. Because
users often choose common words or very simple passwords systems commonly impose
rules on password selection in an attempt to prevent the choice of “bad” passwords andimprove the resistance of user chosen passwords to such dictionary or rule driven
password guessing attacks. For the purposes of these guidelines, we break those rules into
two categories:
1. Dictionary tests that test prospective passwords against an “extensive dictionary
test” of common words and commonly used passwords, then disallow passwords
found in the dictionary. We do not precisely define a dictionary test, since it must
be tailored to the password length and rules, but it should prevent selection ofpasswords that are simple transformations of any one word found in an
unabridged English dictionary, and should include at least 50,000 words. There isno intention to prevent selection of long passwords (16 characters or more basedon phrases) and no need to impose a dictionary test on such long passwords of 16
characters or more.
2. Composition rules that typically require users to select passwords that include
lower case letters, upper case letters, and non-alphabetic symbols (e.g.;:“~!@#$%^&*()_-+={}[]|\:;’<,>.?/1234567890”).
Either dictionary tests or composition rules eliminate some passwords and reduce thespace that an adversary must test to find a password in a guessing or exhaustion attack.
However they can eliminate many obvious choices and therefore we believe that theygenerally improve the “practical entropy” of passwords, although they reduce the workrequired for a truly exhaustive attack. The dictionary check requires a dictionary of atleast 50,000 legal passwords chosen to exclude commonly selected passwords. Upper
case letters in candidate passwords should be converted to lower case before comparison.
Table A.1 provides a rough estimate of the average entropy of user chosen passwords as afunction of password length. Estimates are given for user selected passwords drawn fromthe normal keyboard alphabet that are not subject to further rules, passwords subject to adictionary check to prevent the use of common words or commonly chosen passwords
36

Thomas Schurmann and Peter Grassberger, “Entropy estimation of symbol sequences,”
http://arxiv.org/ftp/cond-mat/papers/0203/0203436.pdf
37
ibid.

106

Electronic Authentication Guideline

and passwords subject to both composition rules and a dictionary test. In addition an
estimate is provided for passwords or PINs with a ten-digit alphabet. The table also
shows the calculated entropy of randomly selected passwords and PINs. The values ofTable A.1 should not be taken as accurate estimates of absolute entropy, but they doprovide a rough relative estimate of the likely entropy of user chosen passwords, andsome basis for setting a standard for password strength.
The logic of the Table A.1 is as follows for user-selected passwords drawn from the full
keyboard alphabet:•

The entropy of the first character is taken to be 4 bits;•

The entropy of the next 7 characters are 2 bits per character; this is roughly
consistent with Shannon’s estimate that “when statistical effects extending
over not more than 8 letters are considered the entropy is roughly 2.3 bits per
character;”•

For the 9th through the 20th character the entropy is taken to be 1.5 bits per
character;•

For characters 21 and above the entropy is taken to be 1 bit per character;•A “bonus” of 6 bits of entropy is assigned for a composition rule that requires
both upper case and non-alphabetic characters. This forces the use of thesecharacters, but in many cases these characters will occur only at the beginningor the end of the password, and it reduces the total search space somewhat, so
the benefit is probably modest and nearly independent of the length of thepassword;•

A bonus of up to 6 bits of entropy is added for an extensive dictionary check.
If the Attacker knows the dictionary, he can avoid testing those passwords,
and will in any event, be able to guess much of the dictionary, which will,
however, be the most likely selected passwords in the absence of a dictionary
rule. The assumption is that most of the guessing entropy benefits for adictionary test accrue to relatively short passwords, because any long
password that can be remembered must necessarily be a “pass-phrase”
composed of dictionary words, so the bonus declines to zero at 20 characters.

For user selected PINs the assumption of Table A.1 is that such pins are subjected at least
to a rule that prevents selection of all the same digit, or runs of digits (e.g., “1234” or“76543”). This column of Table A.1 is at best a very crude estimate, and experience withpassword crackers suggests, for example, that users will often preferentially select simple
number patterns and recent dates, for example their year of birth.
A.2.2 Min-Entropy Estimates
Experience suggests that a significant share of users will choose passwords that are very
easily guessed (“password” may be the most commonly selected password, where it isallowed). Suppose, for example, that one user in 1,000 chooses one of the 2 most107

Electronic Authentication Guidelinecommon passwords, in a system that allows a user 3 tries before locking a password. An
Attacker with a list of user names, who knows the two most commonly chosen passwords
can use an automated attack to try those 2 passwords with each user name, and can
expect to find at least one password about half the time by trying 700 usernames withthose two passwords. Clearly this is a practical attack if the only goal is to get access tothe system, rather than to impersonate a single selected user. This is usually too
dangerous a possibility to ignore.
We know of no accurate general way to estimate the actual min-entropy of user chosenpasswords, without examining in detail the passwords that users actually select under therules of the password system, however it is reasonable to argue that testing user chosenpasswords against a sizable dictionary of otherwise commonly chosen legal passwords,
and disallowing matches, will raise the min-entropy of a password. A dictionary test is
specified here that is intended to ensure at least 10 bits of min-entropy. That test is:•Upper case letters in passwords are converted to entirely lower case andcompared to a dictionary of at least 50,000 commonly selected otherwise legal
passwords and rejected if they match any dictionary entry, and•Passwords that are detectable permutations of the username are not allowed.

This is estimated to ensure at least 10 bits of min-entropy. Other means may besubstituted to ensure at least 10 bits of min-entropy. User chosen passwords of at least 15
characters are assumed to have at least 10 bits of min-entropy. For example a user might
be given a short randomly chosen string (two randomly chosen characters from a 94-bit
alphabet have about 13 bits of entropy). A password, for example, might combine shortsystem selected random elements, to ensure 10 bits of min-entropy, with a longer userchosen password.

A.3

Other Types of Passwords

Some password systems require a user to memorize a number of images, such as faces.
Users are then typically presented with successive fields of several images (typically 9 at
a time), each of which contains one of the memorized images. Each selection represents
approximately 3.17 bits of entropy. If such a system used five rounds of memorized
images, then the entropy of system would be approximately 16 bits. Since this is
randomly selected password the guessing entropy and min-entropy are both the samevalue.
It is possible to combine randomly chosen and user chosen elements into a single
composite password. For example a user might be given a short randomly selected value
to ensure min-entropy to use in combination with a user chosen password string. The
random component might be images or a character string.

108Electronic Authentication Guideline

Table A.1 – Estimated Password Guessing Entropy in bits vs. Password Length
User Chosen
94 Character Alphabet
LengthChar.

No Checks

1
2
3
4
5
6
7
8
10
12
14
16
18
20
22
24
30
40

4
6
8
10
12
14
16
18
21
24
27
30
33
36
38
40
46
56

Dictionary Dict. &RuleComposition
Rule
14
16
17
20
20
23
22
27
24
30
26
32
28
34
30
36
32
38
34
40
36
42
38
44
40
46
46
52
56
62

109

Randomly Chosen
10 char. alphabet
94 charalphabet

3
5
7
9
10
11
12
13
15
17
19
21
23
25
27
29
35
45

3.3
6.7
10.0
13.3
16.7
20.0
23.3
26.6
33.3
40.0
46.6
53.3
59.9
66.6
73.3
79.9
99.9
133.2

6.6
13.2
19.8
26.3
32.9
39.5
46.1
52.7
65.9
79.0
92.2
105.4
118.5
131.7
144.7
158.0
197.2
263.4

Estimated bits of entropy

Electronic Authentication Guideline

60

50
40

no checks

30

dictionary rule

20
10

0

comp & dictionary
0

10
20
Password length in characters

30

Figure A.1 - Estimated User Selected Password Entropy vs. Length110

Electronic Authentication Guideline

Appendix B: Mapping of Federal PKI Certificate Policies and PIV
Credentials to E-authentication Assurance Levels
Agencies are, in general, issuing certificates under the policies specified in the Common
Policy Framework [FBCA3] to satisfy FIPS 201. Organizations outside the US
Government have begun issuing credentials under a parallel set of policies andrequirements known collectively as PIV Interoperability specifications (PIV-I). Agencies
that were early adopters of PKI technology, and organizations outside the Federal
government, issue PKI certificates under organization specific policies instead of theCommon Policy Framework. The primary mechanism for evaluating the assurance
provided by public key certificates issued under organization specific policies is thepolicy mapping of the Federal Policy Authority to the Federal Bridge CA policies. These
policies include the Rudimentary, Basic, Medium, Medium-HW, and High assurance
policies specified in [FBCA1] and the Citizen and Commerce class policy specified in[FBCA2].
These policies incorporate all aspects of the credential lifecycle, often in greater detailthan specified here. These policies also include security controls (e.g., multi-party
control and system auditing for CSPs) that are outside the scope of this document.
However, the FPKI policies are based on work that largely predates this specification,and the security requirements are not always strictly aligned with those specified here.
As a result, this appendix provides an overall mapping between FPKI certificate policies
and the e-authentication Levels instead of a strict evaluation of compliance. There are
known discrepancies, such as FIPS 201’s allowance for pseudonyms on credentials
issued to personnel in dangerous jobs, or the ability to issue PIV credentials based on asingle federal government issued identity credential. While these discrepancies arerecognized, the overall level of assurance provided by these policies is deemed to meetthe requirements based on the additional controls.
The table below summarizes how certificates issued under the Common Policy
Framework correspond to the e-authentication assurance levels. Note that the Common
Device policy is not listed; this policy supports authentication of a system rather than aperson. In addition, table B.1 summarizes how organization specific certificate policies
correspond to e-authentication assurance levels. At Level 2 agencies may use certificatesissued under policies that have not been mapped by the Federal policy authority, but are
determined to meet the Level 2 identify proofing, token and status reporting
requirements. (For this evaluation, a strict compliance mapping should be used, rather
than the rough mapping used for the FPKI policies.) For Levels 3 and 4, agencies shalldepend upon the mappings provided by the Federal PKI.
The Federal PKI has also added two policies, Medium Commercial Best Practices(Medium-CBP) and Medium Hardware Commercial Best Practices (MediumHW-CBP)
to support recognition of non-Federal PKIs. In terms of e-authentication levels, theMedium CBP and MediumHW-CBP are equivalent to Medium and Medium-HW,
respectively.

111

Electronic Authentication Guideline

Agencies are required [OMB M-11-11] to broadly enable PIV credentials
for user authentication to Federal information systems. PIV credentials
are PKI certificates issued under policies that typically meet level 4, as
specified in Table B.1. Most Federal employees and many Federal
contractors possess PIV credentials and agencies are expected to enable
the use of PIV credentials for authentication at levels 1 through 4.

Table B.1 – Certificate Policies and the E-authentication Assurance Levels

Certificate Policy
Common-Auth
PIVI-Auth
39
SHA1-Auth
Common-SW
Common-HWPIVI-HW
39
SHA1-HW
Common-High40
FBCA Basic40
FBCA Medium
40
FBCA Medium-HW
40
FBCA HighCommon-cardAuth
PIVI-cardAuth
39
SHA1-cardAuth

Selected Policy Components
Token and
Identity
Credential
38
Proofing
Token
Management
Meets Level 4
Meets Level 4
Meets Level 4

Overall
Equivalence
Meets Level 4

Meets Level 4
Meets Level 4

Meets Level 3
Meets Level 4

Meets Level 4
Meets Level 4

Meets Level 3
Meets Level 4

Meets Level 4
Meets Level 3
Meets Level 4
Meets Level 4
Meets Level 4
Meets Level 4

Meets Level 4
Meets Level 3
Meets Level 3
Meets Level 4
Meets Level 4
Meets Level 2

Meets Level 4
Meets Level 3
Meets Level 4
Meets Level 4
Meets Level 4
Meets Level 4

Meets Level 4
Meets Level 3
Meets Level 3
Meets Level 4
Meets Level 4
Meets Level 2

38

The key component in token and credential management is the credential status mechanism.
The SHA1 policies have been deprecated and will not be acceptable after December 31, 2013.
40
These policies are not asserted in the user certificates, but equivalence is established through policy
mapping at the Federal Bridge CA.
39112