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NIST Special Publication 800-63 Version 1.0.2

Electronic Authentication Guideline

April 2006

December 2011

SP 800-63 Version 1.0.2 is superseded in its entirety by the publication of

SP 800-63-1 (December 2011).

NIST Special Publication 800-63-1

Electronic Authentication Guideline

William E. Burr, Donna F. Dodson, Elaine M. Newton, Ray A. Perlner,

W. Timothy Polk, Sarbari Gupta, Emad A. Nabbus

December 2011

http://dx.doi.org/10.6028/NIST.SP.800-63-1

Computer Security Division (Information Technology Lab)

SP 800-63-2 (as of August 6, 2015)

http://csrc.nist.gov/groups/ST/eauthentication/

N/A
 NIST Special Publication 800-63 

 Version 1.0.2 Electronic Authentication 

Guideline 

Recommendations of the 

National Institute of 

Standards and Technology 

William E. Burr 

Donna F. Dodson 

W. Timothy Polk 

I N F O R M A T I O N S E C U R I T Y

Computer Security Division 

Information Technology Laboratory 

National Institute of Standards and Technology 

Gaithersburg, MD 20899-8930 

April 2006

U.S. Department of Commerce 

Donald L. Evans, Secretary 

Technology Administration 

Robert Cresanti, Under Secretary of Commerce for Technology 

National Institute of Standards and Technology 

William Jeffrey, Director 
Special Publication 800-63 Electronic Authentication Guideline

Reports on Computer Systems Technology 

The Information Technology Laboratory (ITL) at the National Institute of Standards and 

Technology (NIST) promotes the U.S. economy and public welfare by providing technical 

leadership for the nation’s measurement and standards infrastructure. ITL develops tests, test 

methods, reference data, proof of concept implementations, and technical analysis to advance the 

development and productive use of information technology. ITL’s responsibilities include the 

development of technical, physical, administrative, and management standards and guidelines for 

the cost-effective security and privacy of sensitive unclassified information in Federal computer 

systems. This Special Publication 800-series reports on ITL’s research, guidance, and outreach 

efforts in computer security and its collaborative activities with industry, government, and 

academic organizations. 

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Special Publication 800-63 Electronic Authentication Guideline

Authority 

This document has been developed by the National Institute of Standards and Technology 

(NIST) in furtherance of its statutory responsibilities under the Federal Information 

Security Management Act (FISMA) of 2002, Public Law 107-347. 

NIST is responsible for developing standards and guidelines, including minimum 

requirements, for providing adequate information security for all agency operations and 

assets, but such standards and guidelines shall not apply to national security systems. 

This guideline is consistent with the requirements of the Office of Management and 

Budget (OMB) Circular A-130, Section 8b(3), Securing Agency Information Systems, as 

analyzed in A-130, Appendix IV: Analysis of Key Sections. Supplemental information is 

provided A-130, Appendix III. 

This guideline has been prepared for use by Federal agencies. It may also be used by 

nongovernmental organizations on a voluntary basis and is not subject to copyright. 

(Attribution would be appreciated by NIST.) 

Nothing in this document should be taken to contradict standards and guidelines made 

mandatory 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. 

National Institute of Standards and Technology Special Publication 800-63, 64 pages 

(April 2006) 

Certain commercial entities, equipment, or material may be identified in the document 

in order to describe an experimental procedure or concept adequately. Such 

identification is not intended to imply recommendation or endorsement by the National 

Institute of Standards and Technology, nor is it intended to imply that these entities, 

materials, or equipment are necessarily the best available for the purpose.

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Special Publication 800-63 Electronic Authentication Guideline

Abstract 

This recommendation provides technical guidance to Federal agencies implementing 

electronic authentication. The recommendation covers remote authentication of users 

over open networks. It defines technical requirements for each of four levels of 

assurance in the areas of identity proofing, registration, tokens, authentication protocols 

and related assertions. 

KEY WORDS: Authentication, Authentication Assurance, Credentials Service Provider, 

Cryptography, Electronic Authentication, Electronic Credentials, Electronic Transactions, 

Electronic Government, Identity Proofing, Passwords, PKI, Public Key Infrastructure, 

Tokens. 

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Special Publication 800-63 Electronic Authentication Guideline

Acknowledgments 

The authors, Bill Burr, Tim Polk and Donna Dodson of the National Institute of 

Standards and Technology (NIST), wish to thank their colleagues who reviewed drafts of 

this document and contributed to its development. The authors also gratefully 

acknowledge and appreciate the many contributions from the public and private sectors 

whose thoughtful and constructive comments improved the quality and usefulness of this 

publication. 

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Special Publication 800-63 Electronic Authentication Guideline

Executive Summary 

Electronic authentication (E-authentication) is the process of establishing confidence in 

user identities electronically presented to an information system. E-authentication 

presents a technical challenge when this process involves the remote authentication of 

individual people over a network, for the purpose of electronic government and 

commerce. This recommendation provides technical guidance to agencies to allow an 

individual person to remotely authenticate his/her identity to a Federal IT system. This 

guidance addresses only traditional, widely implemented methods for remote 

authentication based on secrets. With these methods, the individual to be authenticated 

proves that he or she knows or possesses some secret information. NIST expects to 

explore other means of remote authentication (for example using biometrics, or by 

extensive knowledge of private, but not truly secret, personal information) and may 

develop additional guidance on the use of these methods for remote authentication. 

This technical guidance supplements OMB guidance, E-Authentication Guidance for 

Federal Agencies, [OMB 04-04] that defines four levels of authentication Levels 1 to 4, 

in terms of the consequences of the authentication errors and misuse of credentials. 

Level 1 is the lowest assurance and Level 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 the consequences of an authentication error become more 

serious, the required level of assurance increases. The OMB guidance provides agencies 

with the criteria for determining the level of e-authentication assurance required for 

specific applications and transactions, based on the risks and their likelihood of 

occurrence of each application or transaction. 

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 the 

technical requirements for the required level of assurance. In particular, the document 

states specific technical requirements for each of the four levels of assurance in the 

following areas: 

• Tokens (typically a cryptographic key or password) for proving identity, 

• Identity proofing, registration and the delivery of credentials which bind an 

identity to a token, 

• Remote authentication mechanisms, that is the combination of credentials, 

tokens and authentication protocols used to establish that a claimant is in fact 

the subscriber he or she claims to be, 

• Assertion mechanisms used to communicate the results of a remote 

authentication to other parties. 

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, the 

authentication mechanism provides some assurance that the same claimant is accessing 

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Special Publication 800-63 Electronic Authentication Guideline

the protected transaction or data. It allows a wide range of available authentication 

technologies to be employed and allows 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 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 by an 

eavesdropper. 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. 

Assertions issued about claimants as a result of a successful authentication are either 

cryptographically authenticated by relying parties (using Approved methods), or are 

obtained directly from a trusted party via a secure authentication protocol. 

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 be 

employed at Level 2. It allows any of the token methods of Levels 3 or 4, as well as 

passwords and PINs. Successful authentication requires that the claimant prove through 

a secure authentication protocol that he or she controls the token. Eavesdropper, replay, 

and on-line guessing attacks are prevented. 

Long-term shared authentication secrets, if used, are never revealed to any party except 

the claimant and verifiers operated by the Credentials Service Provider (CSP); however, 

session (temporary) shared secrets may be provided to independent verifiers by the CSP. 

Approved cryptographic techniques are required. Assertions issued about claimants as a 

result of a successful authentication are either cryptographically authenticated by relying 

parties (using Approved methods), or are obtained directly from a trusted party via a 

secure authentication protocol. 

Level 3- Level 3 provides multi-factor remote network authentication. At this level, 

identity proofing procedures require verification of identifying materials and information. 

Level 3 authentication is based on proof of possession of a key or a one-time password 

through a cryptographic protocol. Level 3 authentication requires cryptographic strength 

mechanisms that protect the primary authentication token (secret key, private key or onetime password) against compromise by the protocol threats including: eavesdropper, 

replay, on-line guessing, verifier impersonation and man-in-the-middle attacks. A 

minimum of two authentication factors is required. Three kinds of tokens may be used: 

“soft” cryptographic tokens, “hard” cryptographic tokens and “one-time password” 

device tokens. 

Authentication requires that the claimant prove through a secure authentication protocol 

that he or she controls the token, and must first unlock the token with a password or 

biometric, or must also use a password in a secure authentication protocol, to establish 

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Special Publication 800-63 Electronic Authentication Guideline

two factor authentication. Long-term shared authentication secrets, if used, are never 

revealed to any party except the claimant and verifiers operated directly by the 

Credentials Service Provider (CSP), however session (temporary) shared secrets may be 

provided to independent verifiers by the CSP. Approved cryptographic techniques are 

used for all operations. Assertions issued about claimants as a result of a successful 

authentication are either cryptographically authenticated by relying parties (using 

Approved methods), or are obtained directly from a trusted party via a secure 

authentication protocol. 

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. Level 4 is similar to Level 3 except that only “hard” 

cryptographic tokens are allowed, FIPS 140-2 cryptographic module validation 

requirements are strengthened, and subsequent critical data transfers must be 

authenticated via a key bound to the authentication process. The token shall be a 

hardware cryptographic module validated at FIPS 140-2 Level 2 or higher overall with at 

least FIPS 140-2 Level 3 physical security. By requiring a physical token, which cannot 

readily be copied and since FIPS 140-2 requires operator authentication at Level 2 and 

higher, this level ensures good, two factor remote authentication. 

Level 4 requires strong cryptographic authentication of all parties and all sensitive data 

transfers between the parties. Either public key or symmetric key technology may be 

used. Authentication requires that the claimant prove through a secure authentication 

protocol that he or she controls the token. The protocol threats including: eavesdropper, 

replay, on-line guessing, verifier impersonation and man-in-the-middle attacks are 

prevented. Long-term shared authentication secrets, if used, are never revealed to any 

party except the claimant and verifiers operated directly by the Credentials Service 

Provider (CSP), however session (temporary) shared secrets may be provided to 

independent verifiers by the CSP. Strong Approved cryptographic techniques are used 

for all operations. All sensitive data transfers are cryptographically authenticated using 

keys bound to the authentication process. 

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Special Publication 800-63 Electronic Authentication Guideline

Table of Contents 

1. Purpose......................................................................................................................... 1 

2. Authority...................................................................................................................... 1 

3. Introduction.................................................................................................................. 1 

4. Definitions and Abbreviations ..................................................................................... 4 

5. E-Authentication Model............................................................................................... 9 

5.1. Subscribers, RAs and CSPs ................................................................................ 10 

5.2. Tokens................................................................................................................. 11 

5.3. Electronic Credentials......................................................................................... 12 

5.4. Verifiers .............................................................................................................. 13 

5.5. Assertions............................................................................................................ 13 

5.6. Relying Parties.................................................................................................... 14 

6. Tokens........................................................................................................................ 15 

6.1. Token Threats ..................................................................................................... 16 

6.2. Token Levels....................................................................................................... 16 

7. Registration and Identity Proofing............................................................................. 19 

7.1. Registration Threats............................................................................................ 19 

7.1.1. Threat Model................................................................................................ 19 

7.1.2. Resistance to Registration Threats............................................................... 20 

7.2. Registration Levels ............................................................................................. 20 

7.2.1. Registration and Identity Proofing Requirements........................................ 21 

7.2.2. Records Retention Requirements................................................................. 25 

7.3. Mapping FPKI Certificate Policies to Registration Levels................................. 25 

8. Authentication Protocols............................................................................................ 26 

8.1. Authentication Threats........................................................................................ 26 

8.1.1. Authentication Protocol Threats .................................................................. 26 

8.1.2. Resistance to Protocol Threats..................................................................... 27 

8.1.3. Other Threats ............................................................................................... 29 

8.2. Authentication Mechanism Requirements.......................................................... 30 

8.2.1. Level 1 ......................................................................................................... 31 

8.2.2. Level 2 ......................................................................................................... 32 

8.2.3. Level 3 ......................................................................................................... 34 

8.2.4. Level 4 ......................................................................................................... 37 

9. Summary of Technical Requirements by level.......................................................... 38 

9.1.1. Relationship of PKI Policies to E-authentication Assurance Levels........... 41 

10. References................................................................................................................ 43 

10.1. General References ........................................................................................... 43 

10.2. NIST ITL Bulletins........................................................................................... 43 

10.3. NIST Special Publications ................................................................................ 44 

10.4. Federal Information Processing Standards ....................................................... 45 

10.5. Certificate Policies............................................................................................ 45 

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Special Publication 800-63 Electronic Authentication Guideline

Appendix A: Estimating Password Entropy and Strength................................................ 46 

A.1 Randomly Selected Passwords............................................................................ 47 

A.2 User Selected Passwords..................................................................................... 47 

A.2 Other Types of Passwords................................................................................... 51 

A.3 Examples............................................................................................................. 51 

Appendix B: Errata ........................................................................................................... 54 

Appendix B.1: Errata for Version 1.0.1........................................................................ 54 

Appendix B.2: Errata for Version 1.0.2........................................................................ 54 

x
Special Publication 800-63 Electronic Authentication Guideline 

1. Purpose 

This recommendation provides technical guidance to agencies in the implementation of 

electronic authentication (e-authentication). 

2. Authority 

This document has been developed by the National Institute of Standards and Technology 

(NIST) in furtherance of its statutory responsibilities under the Federal Information 

Security Management Act (FISMA) of 2002, Public Law 107-347. 

NIST is responsible for developing standards and guidelines, including minimum 

requirements, for providing adequate information security for all agency operations and 

assets, but such standards and guidelines shall not apply to national security systems. 

This guideline is consistent with the requirements of the Office of Management and 

Budget (OMB) Circular A-130, Section 8b(3), Securing Agency Information Systems, as 

analyzed in A-130, Appendix IV: Analysis of Key Sections. Supplemental information is 

provided A-130, Appendix III. 

This guideline has been prepared for use by Federal agencies. It may be used by 

nongovernmental organizations on a voluntary basis and is not subject to copyright. 

(Attribution would be appreciated by NIST.) 

Nothing in this document should be taken to contradict standards and guidelines made 

mandatory 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. 

3. Introduction 

Electronic authentication (e-authentication) is the process of establishing confidence in 

user identities electronically presented to an information system. E-authentication 

presents a technical challenge when this process involves the remote authentication of 

individual people over a network. This recommendation provides technical guidance to 

agencies to allow an individual person to remotely authenticate his/her identity to a 

Federal IT system. 

This technical guidance supplements OMB guidance, E-Authentication Guidance for 

Federal Agencies, [OMB 04-04] that defines four levels of assurance Levels 1 to 4, in 

terms of the consequences of the authentication errors and misuse of credentials. Level 1 

is the lowest assurance and Level 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 an authentication error become more serious, the required 

level of assurance increases. The OMB guidance provides agencies with criteria for 

determining the level of e-authentication assurance required for specific electronic 

transactions and systems, based on the risks and their likelihood of occurrence. 

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Special Publication 800-63 Electronic Authentication Guideline 

This document states specific technical requirements for each of the four levels of 

assurance in the following areas: 

• Tokens (typically a cryptographic key or password) for proving identity, 

• Identity proofing, registration and the delivery of credentials which bind an 

identity to a token, 

• Remote authentication mechanisms, that is the combination of credentials, 

tokens and authentication protocols used to establish that a claimant is in fact 

the subscriber he or she claims to be, 

• Assertion mechanisms used to communicate the results of a remote 

authentication to other parties. 

The overall authentication assurance level is determined by the lowest assurance level 

achieved in any of the four areas listed above. 

This technical guidance covers remote electronic authentication of human users to 

Federal agency IT systems over a network. It does not address the authentication of a 

person who is physically present, for example for access to buildings, although some 

credentials and tokens that are used remotely may also be used for local authentication. 

While this technical guidance does, in many cases, establish requirements that Federal IT 

systems and service providers participating in authentication protocols be authenticated to 

subscribers, it does not specifically address machine-to-machine (such as router-torouter) authentication, nor does this guidance establish specific requirements for issuing 

authentication credentials and tokens to machines and servers when they are used in eauthentication protocols with people. 

The paradigm of this document is that individuals are enrolled and undergo an identity 

proofing process in which their identity is bound to an authentication secret, called a 

token. Thereafter, the individuals are remotely authenticated to systems and applications 

over an open network, using the token in an authentication protocol. The authentication 

protocol allows an individual to demonstrate to a verifier that he has or knows the secret 

token, in a manner that protects the secret from compromise by different kinds of attacks. 

Higher authentication assurance levels require use of stronger tokens (harder to guess 

secrets) and better protection of the token from attacks. This document covers only 

authentication mechanisms that work by making the individual demonstrate possession 

and control of a secret. 

It may also be practical to achieve authentication by testing the personal knowledge of 

the individual (referred to as knowledge based authentication). As this information is 

private but not actually secret, confidence in the identity of an individual can be hard to 

achieve. In addition, the complexity and interdependencies of knowledge based 

authentication systems are difficult to quantify. However, knowledge based 

authentication techniques are included as part of registration in this document. 

Biometric methods are widely used to authenticate individuals who are physically present 

at the authentication point, for example for entry into buildings. Biometrics do not 

constitute secrets suitable for use in the conventional remote authentication protocols 

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Special Publication 800-63 Electronic Authentication Guideline 

addressed in this document. In the local authentication case, where the claimant is 

observed and uses a capture device controlled by the verifier, authentication does not 

require that biometrics be kept secret. The use of biometrics to “unlock” conventional 

authentication tokens and to prevent repudiation of registration is identified in this 

document. 

NIST is continuing to study both the topics of knowledge based authentication and 

biometrics and may issue additional guidance on their uses for remote authentication of 

individuals across a network. 

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 

may lead 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 the 

E-Government Act of 2002 [OMB 03-22]. See the Guide to Federal Agencies on 

Implementing Electronic Processes for additional information on legal risks, especially 

those that related to the need to satisfy legal standards of proof and prevent repudiation 

[DOJ 2000]. 

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Special Publication 800-63 Electronic Authentication Guideline 

4. Definitions and Abbreviations 

Active Attack An attack on the authentication protocol where the attacker transmits 

data to the claimant or verifier. Examples of active attacks include a 

man-in-the-middle, impersonation, and session hijacking. 

Address of Record The official location where an individual can be found. The address of 

record always includes the residential street address of an individual 

and 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 another 

contact individual can be used when a residential street address for the 

individual is not available. 

Attack An attempt to obtain a subscriber’s token or to fool a verifier into 

believing that an unauthorized individual possess a claimant’s token. 

Attacker A party who is not the claimant or verifier but wishes to successfully 

execute the authentication protocol as a claimant. 

Approved 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. Approved cryptographic 

algorithms must be implemented in a crypto module validated under 

FIPS 140-2. For more information on validation and alist of validated 

FIPS 140-2 validated crypto modules see http://csrc.nist.gov/cryptval/. 

Assertion A statement from a verifier to a relying party that contains identity 

information about a subscriber. Assertions may also contain verified 

attributes. Assertions may be digitally signed objects or they may be 

obtained from a trusted source by a secure protocol. 

Asymmetric keys Two related keys, a public key and a private key that are used to 

perform complementary operations, such as encryption and decryption 

or signature generation and signature verification. 

Authentication The process of establishing confidence in user identities. 

Authentication 

protocol 

A well specified message exchange process that verifies possession of a 

token to remotely authenticate a claimant. Some authentication 

protocols also generate cryptographic keys that are used to protect an 

entire session, so that the data transferred in the session is 

cryptographically protected. 

Authenticity The property that data originated from its purported source. 

Bit A binary digit: 0 or 1. 

Biometric An image or template of a physiological attribute (e.g., a fingerprint) 

that may be used to identify an individual. In this document, biometrics 

may be used to unlock authentication tokens and prevent repudiation of 

registration. 

Certification Authority 

(CA) 

A trusted entity that issues and revokes public key certificates. 

Certificate Revocation A list of revoked public key certificates created and digitally signed by 

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Special Publication 800-63 Electronic Authentication Guideline 

List (CRL) a Certification Authority. See [RFC 3280] 

Challenge-response 

protocol 

An authentication protocol where the verifier sends the claimant a 

challenge (usually a random value or a nonce) that the claimant 

combines with a shared secret (often by hashing the challenge and 

secret together) to generate a response that is sent to the verifier. The 

verifier knows the shared secret and can independently compute the 

response and compare it with the response generated by the claimant. 

If the two are the same, the claimant is considered to have successfully 

authenticated himself. When the shared secret is a cryptographic key, 

such protocols are generally secure against eavesdroppers. When the 

shared secret is a password, an eavesdropper does not directly intercept 

the password itself, but the eavesdropper may be able to find the 

password with an off-line password guessing attack. 

Claimant A party whose identity is to be verified using an authentication 

protocol. 

Credential An object that authoritatively binds an identity (and optionally, 

additional attributes) to a token possessed and controlled by a person. 

Credentials Service 

Provider (CSP) 

A trusted entity that issues or registers subscriber tokens and issues 

electronic credentials to subscribers. The CSP may encompass 

Registration Authorities and verifiers that it operates. A CSP may be 

an independent third party, or may issue credentials for its own use. 

Cryptographic key A value used to control cryptographic operations, such as decryption, 

encryption, signature generation or signature verification. For the 

purposes of this document, keys must provide at least 80-bits of 

protection. This means that it must be as hard to find an unknown key 

or decrypt a message, given the information exposed to an 

eavesdropper by an authentication, as to guess an 80-bit random 

number. 

See also Asymmetric keys, Symmetric key. 

Cryptographic strength A measure of the expected number of operations required to defeat a 

cryptographic mechanism. For the purposes of this document, this term 

is defined to mean that breaking or reversing an operation is at least as 

difficult computationally as finding the key of an 80-bit block cipher by 

key exhaustion, that is it requires at least on the order of 279 operations. 

Cryptographic token A token where the secret is a cryptographic key. 

Data integrity The property that data has not been altered by an unauthorized entity. 

Digital Signature An asymmetric key operation where the private key is used to digitally 

sign an electronic document and the public key is used to verify the 

signature. Digital signatures provide authentication and integrity 

protection. 

Electronic Credentials Digital documents used in authentication that bind an identity or an 

attribute to a subscriber’s token. Note that this document distinguishes 

between credentials, and tokens (see below) while other documents 

may interchange these terms. 

Entropy A measure of the amount of uncertainty that an attacker faces to 

determine the value of a secret. Entropy is usually stated in bits. See 

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Special Publication 800-63 Electronic Authentication Guideline 

Appendix A. 

FIPS Federal Information Processing Standard. 

Guessing entropy 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 n-bit 

random quantity. The attacker is assumed to know the actual password 

frequency distribution. See Appendix A. 

Hash function 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 that 

maps to any pre-specified output, and 

2. (Collision resistant) It is computationally infeasible to find any two 

distinct inputs that map to the same output. 

HMAC Hash-based Message Authentication Code: a symmetric key 

authentication method using hash functions. 

Identity A unique name of an individual person. Since the legal names of 

persons are not necessarily unique, the identity of a person must 

include sufficient additional information (for example an address, or 

some unique identifier such as an employee or account number) to 

make the complete name unique. 

Identity proofing The process by which a CSP and an RA validate sufficient information 

to uniquely identify a person. 

Kerberos 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 is known to be vulnerable to off-line 

dictionary attacks by eavesdroppers who capture the initial user-toKDC exchange. 

Man-in-the-middle 

attack (MitM) 

An attack on the authentication protocol run in which the attacker 

positions himself in between the claimant and verifier so that he can 

intercept and alter data traveling between them. 

Message A cryptographic checksum on data that uses a symmetric key to detect 

Authentication Code both accidental and intentional modifications of the data. 

(MAC) 

Min-entropy A measure of the difficulty that an attacker has to guess the most 

commonly 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 that 

password 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. 

Network An open communications medium, typically the Internet, that is used to 

transport messages between the claimant and other parties. Unless 

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Special Publication 800-63 Electronic Authentication Guideline 

otherwise stated no assumptions are made about the security of the 

network; it is assumed to be open and subject to active (e.g., 

impersonation, man-in-the-middle, session hijacking...) and passive 

(e.g., eavesdropping) attack at any point between the parties (claimant, 

verifier, CSP or relying party). 

Nonce A value used in security protocols that is never repeated with the same 

key. For example, challenges used in challenge-response 

authentication protocols generally must not be repeated until 

authentication keys are changed, or there is a possibility of a replay 

attack. Using a nonce as a challenge is a different requirement than a 

random challenge, because a nonce is not necessarily unpredictable. 

Off-line attack An attack where the attacker obtains some data (typically by 

eavesdropping on an authentication protocol run, or by penetrating a 

system and stealing security files) that he/she is able to analyze in a 

system of his/her own choosing. 

On-line attack 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. The goal of the attack may be to gain 

authenticated access or learn authentication secrets. 

On-Line Certificate 

Status Protocol 

(OCSP) 

An on-line protocol used to determine the status of a public key 

certificate. See [RFC 2560]. 

Passive attack An attack against an authentication protocol where the attacker 

intercepts data traveling along the network between the claimant and 

verifier, but does not alter the data (i.e. eavesdropping). 

Password A secret that a claimant memorizes and uses to authenticate his or her 

identity. Passwords are typically character strings. 

Possession and control 

of a token 

The ability to activate and use the token in an authentication protocol. 

Personal Identification 

Number (PIN) 

A password consisting only of decimal digits. 

Practice Statement A formal statement of the practices followed by an authentication entity 

(e.g., RA, CSP, or verifier); typically the specific steps taken to register 

and verify identities, issue credentials and authenticate claimants. 

Private key The secret part of an asymmetric key pair that is typically used to 

digitally sign or decrypt data. 

Proof of Possession 

(PoP) protocol 

A protocol where a claimant proves to a verifier that he/she possesses 

and controls a token (e.g., a key or password) 

Protocol run An instance of the exchange of messages between a claimant and a 

verifier in a defined authentication protocol that results in the 

authentication (or authentication failure) of the claimant. 

Public key The public part of an asymmetric key pair that is typically used to 

verify signatures or encrypt data. 

Public key certificate A digital document issued and digitally signed by the private key of a 

Certification Authority that binds the name of a subscriber to a public 

key. The certificate indicates that the subscriber identified in the 

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certificate has sole control and access to the private key. See also [RFC 

3280] 

Pseudonym A subscriber name that has been chosen by the subscriber that is not 

verified as meaningful by identity proofing. 

Registration The process through which a party applies to become a subscriber of a 

CSP and an RA validates the identity of that party on behalf of the 

CSP. 

Registration Authority 

(RA) 

A trusted entity that establishes and vouches for the identity 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). 

Relying party An entity that relies upon the subscriber’s credentials, typically to 

process a transaction or grant access to information or a system. 

Salt A non-secret value that is used in a cryptographic process, usually to 

ensure that the results of computations for one instance cannot be 

reused by an attacker. 

Security Assertion 

Markup Language 

(SAML) 

A specification for encoding security assertions in the XML markup 

language. See: http://www.oasisopen.org/committees/tc_home.php?wg_abbrev=security 

Shared secret A secret used in authentication that is known to the claimant and the 

verifier. 

Subject The person whose identity is bound in a particular credential. 

Subscriber A party who receives a credential or token from a CSP and becomes a 

claimant in an authentication protocol. 

Symmetric key A cryptographic key that is used to perform both the cryptographic 

operation and its inverse, for example to encrypt and decrypt, or create 

a message authentication code and to verify the code. 

Token Something that the claimant possesses and controls (typically a key or 

password) used to authenticate the claimant’s identity. 

Transport Layer 

Security (TLS) 

An authentication and security protocol widely implemented in 

browsers and web servers. TLS is defined by [RFC 2246] and [RFC 

3546]. TLS is similar to the older Secure Socket Layer (SSL) protocol 

and is effectively SSL version 3.1. 

Tunneled password 

protocol 

A protocol where a password is sent through a protected channel. For 

example, the TLS protocol is often used with a verifier’s public key 

certificate to (1) authenticate the verifier to the claimant, (2) establish 

an encrypted session between the verifier and claimant, and (3) transmit 

the claimant’s password to the verifier. The encrypted TLS session 

protects the claimant’s password from eavesdroppers. 

Verified Name A subscriber name that has been verified by identity proofing. 

Verifier An entity that verifies the claimant’s identity by verifying the 

claimant’s possession 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. 

Verifier impersonation 

attack 

An attack where the attacker impersonates the verifier in an 

authentication protocol, usually to learn a password. 

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5. E-Authentication Model 

In accordance with [OMB 04-04] e-authentication is the process of establishing 

confidence 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 place 

across 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. 

E-authentication begins with registration. An applicant applies to a Registration 

Authority (RA) to become a subscriber of a Credential Service Provider (CSP) and, as a 

subscriber, is issued or registers a secret, called a token, and a credential that binds the 

token to a name and possibly other attributes that the RA has verified. The token and 

credential may be used in subsequent authentication events. 

The subscriber’s name may either be a verified name or a pseudonym. A verified name is 

associated with the identity of a real person and before an applicant can receive 

credentials or register a token associated with a verified name, he or she must 

demonstrate that the identity is a real identity, and that he or she is the person who is 

entitled to use that identity. This process is called identity proofing, and is performed by 

an RA that registers subscribers with the CSP. At Level 1, since names are not verified, 

names are always assumed to be pseudonyms. Level 2 credentials and assertions must 

specify whether the name is a verified name or a pseudonym. This information assists 

relying parties, that is parties who rely on the name or other authenticated attributes, in 

making access control or authorization decisions. Only verified names are allowed at 

Levels 3 and 4. 

In this guidance, 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 in an on-line authentication to a verifier through an authentication 

protocol, the verifier can verify that the clamant is the subscriber. The verifier passes on 

an assertion about the identity of the subscriber to the relying party. That assertion 

includes identity information about a subscriber, such as 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 relying party, the assertion may be implicit. In addition, the 

subscriber’s identifying information may be incorporated in credentials (e.g., public key 

certificates) made available by the claimant. The relying party can use the authenticated 

information provided by the verifier/CSP to make access control or authorization 

decisions. 

Authentication simply establishes identity, or in some cases verified 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 or organization), not what that identity 

is authorized to do or what access privileges he or she has; this is a separate decision. 

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Relying parties, typically government agencies, will use a subscriber’s authenticated 

identity and other factors to make access control or authorization decisions. In many 

cases, the authentication process and services will be shared by many applications and 

agencies, but the individual agency or application is the relying party that must make the 

decision to grant access or process a transaction based on the specific application 

requirements. This guidance provides technical recommendations for the process of 

authentication, not authorization. 

In summary, an individual applicant applies first to an RA. The RA identity proofs that 

applicant. As the result of successful identity proofing, the applicant becomes a 

subscriber of a CSP associated with the RA, with a credential and a secret token 

registered to the subscriber. When the subscriber needs to authenticate to perform a 

transaction, he or she becomes a claimant to a verifier. The claimant proves to the 

verifier that he or she controls the token, using an authentication protocol. If the verifier 

is separate from the relying party (application), the verifier provides an assertion about 

the claimant to the relying party, which uses the information in the assertion to make an 

access control or authorization decision. If the transaction is significant, the relying party 

may log the subscriber identity and credential(s) used in the authentication along with 

relevant transaction data. 

5.1. Subscribers, RAs and CSPs 

In the conceptual e-authentication model, a claimant in an authentication protocol is a 

subscriber to some CSP. At some point, an applicant registers with an RA, which verifies 

the identity of the applicant, typically through the presentation of paper credentials and 

by records in databases. This process is called identity proofing. The RA, in turn, vouches 

for the identity of the applicant (and possibly other verified attributes) to a CSP. The 

applicant then becomes a subscriber of the CSP. 

The CSP establishes a mechanism to uniquely identify each subscriber and the associated 

tokens and credentials issued to that subscriber. The CSP registers or gives the 

subscriber a token to be used in an authentication protocol and issues credentials as 

needed to bind that token to the identity, or to bind the identity to some other useful 

verified attribute. The subscriber may be given electronic credentials to go with the token 

at the time of registration, or credentials may be generated later as needed. Subscribers 

have a duty to maintain control of their tokens and comply with the responsibilities to the 

CSP. The CSP maintains registration records for each subscriber to allow recovery of 

registration records. 

There is always a relationship between the RA and CSP. In the simplest and perhaps the 

most common case, the RA/CSP are separate functions of the same entity. However, an 

RA might be part of a company or organization that registers subscribers with an 

independent 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 have 

relationships with different CSPs as well. 

Section 7 provides recommendations for the identity proofing and registration process. 

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5.2. Tokens 

Tokens generically are something the claimant possesses and controls that may be used to 

authenticate the claimant’s identity. In e-authentication, the claimant authenticates to a 

system or application over a network. Therefore, a token used for e-authentication is a 

secret and the token must be protected. The token may, for example, be a cryptographic 

key, that is protected by encrypting it under a password. An impostor must steal the 

encrypted key and learn the password to use the token. 

Authentication systems are often categorized by the number of factors that they 

incorporate. The three factors often considered as the cornerstone of authentication are: 

• 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 voice print or other biometric) 

Authentication systems that incorporate all three factors are stronger than systems that 

only incorporate one or two of the factors. 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 be presented to the verifier. For example, consider a hardware device that 

holds a cryptographic key. The key might be activated by a password or the hardware 

device might include a biometric capture device and uses a biometric to activate the key. 

Such a device is considered to effectively provide two factor authentication, although the 

actual authentication protocol between the verifier and the claimant simply proves 

possession of the key. 

The secrets are often 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 is used by the claimant as a token. A verifier, knowing the claimant’s public key 

through some credential (typically a public key certificate), can use an authentication 

protocol to verify the claimant’s identity, by proving that the claimant has control of the 

associated private key token (proof of possession). 

Shared secrets are either symmetric keys or passwords. In a protocol sense, all shared 

secrets are similar, and can be used in similar authentication protocols; however, 

passwords, since they are often committed to memory, are something the claimant 

knows, rather than something he has. Passwords, because they are committed to 

memory, 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 or “shoulder surfing” attacks. Therefore 

keys and passwords demonstrate somewhat separate authentication properties (something 

you know rather than something you have). Passwords often have lesser resistance to 

network attacks. However, when using either public key pairs or shared secrets, the 

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subscriber has a duty to maintain exclusive control of his token, since possession and 

control of the token is used to authenticate the subscriber’s identity. 

Biometrics are unique personal attributes that can be used to identify a person. They 

include facial pictures, fingerprints, DNA, iris and retina scans, voiceprints and many 

other things. In this document, biometrics are used in the registration process to be able 

to later prevent a subscriber who in fact registered from repudiating the registration, to 

help identify those who commit registration fraud, and to unlock tokens. Biometrics are 

not used directly as tokens in this document. 

As defined in Section 6, this guidance recognizes four kinds of claimant tokens: hard 

tokens, soft tokens, one-time password device tokens and password tokens. 

5.3. Electronic Credentials 

Paper credentials are documents that attest to the identity or other attributes of an 

individual or entity called the subject of the credentials. Some common paper credentials 

include passports, birth certificates, driver’s licenses, and employee identity cards. The 

credentials themselves are authenticated in a variety of ways: traditionally perhaps by a 

signature or a seal, special papers and inks, high quality engraving, and today by more 

complex mechanisms, such as holograms, that make the credentials recognizable and 

difficult to copy or forge. In some cases, simple possession of the credentials is 

sufficient to establish that the physical holder of the credentials is indeed the subject of 

the credentials. More commonly, the credentials contain biometric information such as 

the subject’s description, a picture of the subject or the handwritten signature of the 

subject that can be used to authenticate that the holder of the credentials is indeed the 

subject of the credentials. When these paper credentials are presented in-person, 

authentication biometrics contained in those credentials can be checked to confirm that 

the physical holder of the credential is the subject. 

Electronic identity credentials bind a name and perhaps other attributes to a token. This 

recommendation does not prescribe particular kinds of electronic credentials. There are a 

variety of electronic credential types in use today, and new types of credentials are 

constantly being created. At a minimum, credentials include identifying information that 

permits recovery of the records of the registration associated with the credentials and a 

name that is associated with the subscriber. In every case, given the issuer and the 

identifying information in the credential, it must be possible to recover the registration 

records upon which the credentials are based. Electronic credentials may be generalpurpose credentials or targeted to a particular verifier. Some common types of credentials 

are: 

• X.509 public key identity certificates that bind an identity to a public key; 

• X.509 attribute certificates that bind an identity or a public key with some 

attribute; 

• Kerberos tickets that are encrypted messages binding the holder with some 

attribute or privilege. 

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Electronic credentials may be stored as data in a directory or database. These credentials 

may be digitally signed objects (e.g., X.509 certificates), in which case their integrity 

may be verified. In this case, the directory or database may be an untrusted entity, since 

the data it supplies is self-authenticating. Alternatively, the directory or database server 

may be a trusted entity that authenticates itself to the relying party or verifier. When the 

directory or database server is trusted, unsigned credentials may simply be stored as 

unsigned data. 

5.4. Verifiers 

In any authenticated on-line transaction, the verifier must verify that the claimant has 

possession and control of the token that verifies his or her identity. A claimant 

authenticates his or her identity to a verifier by the use of a token and an authentication 

protocol. This is called Proof of Possession (PoP). Many PoP protocols are designed so 

that a verifier, with no knowledge of the token before the authentication protocol run, 

learns nothing about the token from the run. The verifier and CSP may be the same 

entity, the verifier and relying party may be the same entity or they may all three be 

separate entities. It is undesirable for verifiers to learn shared secrets unless they are a 

part of the same entity as the CSP that registered the tokens. Where the verifier and the 

relying party are separate entities, the verifier must convey the result of the authentication 

protocol to the relying party. The object created by the verifier to convey this result is 

called an assertion. 

5.5. Assertions 

Assertions can be used to pass information about the claimant or the e-authentication 

process from the verifier to a relying party. Assertions contain, at a minimum, the name 

of the claimant, as well as identifying information that permits recovery of registration 

records. A relying party trusts an assertion based on the source, the time of creation, and 

attributes associated with the claimant. 

Examples of assertions include: 

• SAML assertions, specified using a mark up language intended for describing 

security assertions, can be used by a verifier to make a statement to a relying party 

about the identity of a claimant. SAML assertions may optionally be digitally 

signed. 

• Cookies, character strings placed in a web browser’s memory, are available 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 be assertions or may 

contain pointers to assertions.1

Assertions may be stored as directory or database objects. Where assertions are digitally 

signed objects (e.g., signed SAML assertions), their integrity may be verified. 

 

1

 There are specific requirements that agencies must follow when implementing cookies. See OMB 

Memorandum 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. 

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Alternatively, the directory or database server may be a trusted, authenticated entity. 

When the server is trusted, unsigned assertions may be accepted based on the source. 

5.6. Relying Parties 

A relying party relies on results of an on-line authentication to establish the identity or 

attribute of a subscriber for the purpose of some transaction. The verifier and the relying 

party may be the same entity, or they may be separate entities. If they are separate 

entities, the relying party normally receives an assertion from the verifier. The relying 

party ensures that the assertion came from a verifier trusted by the relying party. The 

relying party also processes any additional information in the assertion, such as personal 

attributes or expiration times. 

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6. Tokens 

This guidance recognizes four kinds of claimant tokens for e-authentication. Each type 

of token incorporates one or more of the authentication factors (something you know, 

something you have, and something you are.) Tokens that provide a higher level of 

assurance incorporate two or more factors. The four kinds of tokens are: 

• Hard token – a hardware device that contains a protected cryptographic key. 

Authentication is accomplished by proving possession of the device and control 

of the key. Hard tokens shall: 

o require the entry of a password or a biometric to activate the 

authentication key; 

o not be able to export authentication keys; 

o be FIPS 140-2 validated: 

 overall validation at Level 2 or higher, 

 physical security at Level 3 or higher. 

• Soft token – a cryptographic key that is typically stored on disk or some other 

media. Authentication is accomplished by proving possession and control of the 

key. The soft token key shall be encrypted under a key derived from some 

activation data. Typically, this activation data will be a password known only to 

the user, so a password is required to activate the token. For soft tokens, the 

cryptographic module shall be validated at FIPS 140-2 Level 1 or higher, and may 

be either a hardware device or a software module. Each authentication shall 

require entry of the password or other activation data and the unencrypted copy of 

the authentication key shall be erased after each authentication. 

Some “mobility solutions” also allow keys to be stored on servers and 

downloaded to subscriber systems as needed. Other mobility solutions employ 

key components generated from passwords with key components stored on 

servers for use in split signing schemes. Such solutions may provide satisfactory 

soft tokens, provided that a subscriber password or other activation data is 

required to download and activate the key, that the protocol for downloading the 

keys block eavesdroppers and man-in-the-middle attacks, and the authentication 

process produces Approved digital signatures or message authentication codes. 

These mobility solutions usually present what appear to relying parties to be 

ordinary PKI digital signatures, and may be acceptable under this 

recommendation provided they meet the PKI cross certification requirements. 

This cross certification will require a detailed analysis of the implementation of 

the specific mobility scheme. 

• One-time password device token - a personal hardware device that generates “one 

time” passwords for use in authentication. The device may or may not have some 

kind of integral entry pad, an integral biometric (e.g., fingerprint) reader or a 

direct computer interface (e.g., USB port). The passwords shall be generated by 

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using an Approved block cipher or hash algorithm to combine a symmetric key 

stored on a personal hardware device with a nonce to generate a one-time 

password. The nonce may be a date and time, a counter generated on the device, 

or a challenge from the verifier (if the device has an entry capability). The onetime password typically is displayed on the device and manually input to the 

verifier as a password (direct electronic input from the device to a computer is 

also allowed). The one-time password must have a limited lifetime, on the order 

of minutes, although the shorter the better. 

• Password token – a secret that a claimant memorizes and uses to authenticate his 

or her identity. Passwords are typically character strings, however some systems 

use a number of images that the subscriber memorizes and must identify when 

presented along with other similar images.

6.1. Token Threats 

If an attacker can gain control of a token, they will be able to masquerade as the token’s 

owner. Threats to tokens can be categorized into attacks on the three factors: 

• Something you have may be 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 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 install 

malicious software (e.g., a keyboard logger) to capture this information. Finally, 

an attacker may determine the secret through off-line attacks on network traffic 

from an authentication attempt. 

• Something you are may be replicated. An attacker may obtain a copy of the token 

owner’s fingerprint and construct a replica. 

There are several complementary strategies to mitigate these threats: 

• Multiple factors raise the threshold for successful attacks. If an attacker needs to 

steal a cryptographic token and guess a password, the work factor 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. 

• Complex passwords may reduce the likelihood of a successful guessing attack. 

By requiring use of long passwords that don’t appear in common dictionaries, 

attackers may be forced 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. 

6.2. Token Levels 

Password authentication is easy to implement and familiar to users, so many systems rely 

only on a password for authentication. In this case impersonation of an identity requires 

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only that the impersonator obtain the password. Moreover, the ability of humans to 

remember long, arbitrary passwords is limited, so password tokens are often vulnerable to 

a variety of attacks including guessing, dictionaries of commonly used passwords, and 

simple exhaustion of all possibilities. There are a wide variety of password 

authentication protocols that differ significantly in their vulnerabilities, and many 

password mechanisms are vulnerable to passive 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 observation of 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 to unknown parties, who are basically 

“confidence men.” 

Impersonation of an identity using a hard or soft token requires that the impersonator 

obtain two separate things: either the key (token) and a password, or the token and the 

ability to enter a biometric into the token. Therefore both hard and soft tokens provide 

more assurance than passwords by themselves normally provide. Moreover, a hard 

token is a physical object and its theft is likely to be noticed by its owner, while a soft 

token can sometimes be copied without the owner being aware. Therefore a hard token 

offers more assurance than a soft token. 

One-time password device tokens are similar to hard tokens. They can be used in 

conjunction with a password or activated by a password or a biometric to provide 

multifactor authentication, however one-time password devices do not result in the 

generation of a shared session authentication key derived from the authentication. 

This recommendation requires multifactor authentication for authentication assurance 

Levels 3 and 4 and assigns tokens to the four levels corresponding to the OMB guidance 

as follows: 

• Password tokens can satisfy the assurance requirements for Levels 1 and 2. 

• Soft cryptographic tokens may be used at authentication assurance Levels 1 to 3, 

but must be combined with a password or biometric to achieve Level 3. 

• One-time password devices are considered to satisfy the assurance requirements 

for Levels 1 through 3, and must be used with a password or biometric to achieve 

Level 3. 

• Hard tokens that are activated by a password or biometric can satisfy assurance 

requirements for Levels 1 through 4. 

The above list is a general summary of the assurance levels for tokens. Specific 

requirements, however, vary with respect to the details of the authentication protocols. 

Levels 3 and 4 require two-factor authentication. Typically this means that for Level 3 or 

4 a password or biometric is used to activate a key. Alternatively, a password protocol 

may be used in conjunction with a soft token, hard token, or one-time password token to 

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achieve two-factor authentication. Detailed level by level token requirements are 

described in conjunction with protocol requirements in Section 8. 

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7. Registration and Identity Proofing 

In the registration process an applicant undergoes identity proofing by a trusted 

registration authority (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. 

 The RA may be a part of the CSP, or the RA may be a separate and independent entity; 

however a trusted relationship always exists between the RA and CSP. Either the RA or 

CSP must maintain records of the registration. The RA and CSP may 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 a result. 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 employee or student.) Where 

the RA provides services to the public, the identity proofing process is generally limited 

to confirming publicly available information and previously issued credentials. 

 

The registration and identity proofing process is designed, to a greater or lesser degree 

depending on the assurance level, to ensure that the RA/CSP knows the true identity of 

the applicant. Specifically, the requirements include measures to ensure that: 

1. A person with the applicant’s claimed attributes exists, and those attributes are 

sufficient to uniquely identity a single person; 

2. The applicant whose token is registered is in fact the person who is entitled to the 

identity; 

3. The applicant cannot later repudiate the registration; therefore, if there is a dispute 

about a later authentication using the subscriber’s token, the subscriber cannot 

successfully deny he or she registered that 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. 

7.1. Registration Threats 

There are two general categories of threats to the registration process, impersonation and 

either 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, etc.) and are outside the scope of this document. 

7.1.1. Threat Model 

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While some impostors may attempt to register as any subscriber in the system and other 

impostors may wish to register as a specific subscriber, registration threats can be 

categorized as follows: 

• Impersonation of a claimed identity – An applicant claims an incorrect identity, 

supporting the claim with a specific set of attributes created over time or by 

presenting false credentials. 

• Repudiation of registration – A subscriber denies registration, claiming that 

he/she did not register that token. 

7.1.2. Resistance to Registration Threats 

Registration fraud can be deterred by making it more difficult to accomplish or increasing 

the likelihood of detection. This recommendation deals primarily with methods for 

making impersonation more difficult, however it does prescribe certain methods and 

procedures that may help to prove who carried out an impersonation. At each level, 

methods are employed to determine that a person with the claimed identity exists, the 

applicant is the person who is entitled to that identity and the applicant cannot later 

repudiate the registration. As the level of assurance increases, the methods employed 

provide increasing resistance to casual, systematic and insider impersonation. 

7.2. Registration Levels 

The following sections list the NIST recommendations for registration and identity 

proofing for the four levels corresponding to the OMB guidance. As noted in the OMB 

guidance, Levels 1 and 2 recognize the use of anonymous credentials. When anonymous 

credentials are used to imply membership in a group, the level of proofing should be 

consistent with the requirements for the identity credential of that level. Explicit 

requirements for registration processes for anonymous credentials are not specified, as 

they are unique to the membership criteria for each specific group. 

At Level 2 and higher, 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 of 

each individual whose identity has been verified, and the steps taken to verify his/her 

identity, including the evidence required in the sections below. The CSP shall be 

prepared to provide records of identity proofing to relying parties as necessary. The 

identity proofing and registration process shall be performed according to a written policy 

or practice statement that specifies the particular steps taken to verify identities. 

If the RA and CSP are remotely located, and communicate over a network, the entire 

registration transaction between RA and CSP shall be cryptographically authenticated 

using an authentication protocol that meets the requirements for the assurance level of the 

registration, and any secrets transmitted shall be encrypted using an Approved encryption 

method. 

The CSP shall be able to uniquely identify each subscriber and the associated tokens and 

the credentials issued to that subscriber. The CSP shall be capable of conveying this 

information to verifiers and relying parties. At Level 1, the name associated with the 

subscriber is provided by the applicant and accepted without verification. At Level 2, the 

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name associated with the subscriber may be pseudonymous but the RA or CSP must 

know the actual identity of the subscriber. In addition, pseudonymous Level 2 

credentials must be distinguishable from Level 2 credentials that contain meaningful 

names. At Level 3 and above, the name associated with the subscriber must be 

meaningful. At all levels, personal identifying information collected as part of the 

registration process must be protected from unauthorized disclosure or modification. 

The following subsection, Section 7.2.1, establishes registration and identity proofing 

requirements specific to each level. Records retention requirements for each level are 

specified in Section 7.2.2. 

7.2.1. Registration and Identity Proofing Requirements 

The following text establishes registration requirements specific to each level. There are 

no level-specific requirements at Level 1. Both in-person and remote registration are 

permitted for Levels 2 and 3. Explicit requirements are specified for each scenario in 

Levels 2 and 3. Only in-person registration is permitted at Level 4. 

At Level 2 and higher, the applicant supplies his or her full legal name, an address of 

record, and date of birth, and may, subject to the policy of the RA or CSP, also supply 

other individual identifying information. Detailed level-by-level identity proofing 

requirements are stated in Table 1 below. 

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 Table 1. Identity Proofing Requirements by Assurance Level 

In-Person Remote 

Level 2 

Basis for 

issuing 

credentials 

Possession of a valid current primary 

Government Picture ID that contains 

applicant’s picture, and either address of 

record or nationality (e.g. driver’s 

license or passport) 

Possession of a valid Government ID 

(e.g. a driver’s license or passport) 

number and a financial account number 

(e.g., checking account, savings 

account, loan or credit card) with 

confirmation via records of either 

number. 

RA actions Inspects photo-ID, compare picture to 

applicant, record ID number, address 

and DoB. If ID appears valid and photo 

matches applicant then: 

a) If ID confirms address of record, 

authorize or issue credentials and 

send notice to address of record, 

or; 

b) If ID does not confirm address of 

record, issue credentials in a 

manner that confirms address of 

record. 

• Inspects both ID number and 

account number supplied by 

applicant. Verifies information 

provided by applicant including ID 

number or account number through 

record checks either with the 

applicable agency or institution or 

through credit bureaus or similar 

databases, and confirms that: name, 

DoB, address other personal 

information in records are on 

balance consistent with the 

application and sufficient to 

identify a unique individual. 

• Address confirmation and 

notification: 

a) Sends notice to an address 

of record confirmed in the 

records check or; 

b) Issues credentials in a 

manner that confirms the 

address of record supplied 

by the applicant; or 

c) Issues credentials in a 

manner that confirms the 

ability of the applicant to 

receive telephone 

communications or e-mail at 

number or e-mail address 

associated with the applicant 

in records. 

Level 3 

Basis for 

issuing 

credentials 

Possession of verified current primary 

Government Picture ID that contains 

applicant’s picture and either address of 

Possession of a valid Government ID 

(e.g. a driver’s license or passport) 

number and a financial account number 

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Special Publication 800-63 Electronic Authentication Guideline 

In-Person Remote 

record or nationality (e.g. driver’s 

license or passport) 

(e.g., checking account, savings 

account, loan or credit card) with 

confirmation via records of both 

numbers. 

RA actions Inspects Photo-ID and verify via the 

issuing government agency or through 

credit bureaus or similar databases. 

Confirms that: name, DoB, address and 

other personal information in record are 

consistent with the application. 

Compare picture to applicant, record ID 

number, address and DoB. If ID is valid 

and photo matches applicant then: 

a) If ID confirms address of record, 

authorize or issue credentials and 

send notice to address of record, 

or; 

b) If ID does not confirm address of 

record, issue credentials in a 

manner that confirms address of 

record 

• Verifies information provided by 

applicant including ID number and 

account number through record 

checks either with the applicable 

agency or institution or through 

credit bureaus or similar databases, 

and confirms that: name, DoB, 

address and other personal 

information in records are 

consistent with the application and 

sufficient to identify a unique 

individual. 

• Address confirmation: 

a) Issue credentials in a 

manner that confirms the 

address of record supplied 

by the applicant; or 

b) Issue credentials in a 

manner that confirms the 

ability of the applicant to 

receive telephone 

communications at a 

number associated with the 

applicant in records, while 

recording the applicant’s 

voice. 

Level 4 

Basis for 

issuing 

credentials 

In-person appearance and verification of 

two independent ID documents or 

accounts, meeting the requirements of 

Level 3 (in-person and remote), one of 

which must be current primary 

Government Picture ID that contains 

applicant’s picture, and either address of 

record or nationality (e.g. driver’s 

license or passport), and a new 

recording of a biometric of the applicant 

at the time of application 

Not Applicable 

RA actions • Primary Photo ID:

Inspects Photo-ID and verify via the 

issuing government agency, 

Not applicable 

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Special Publication 800-63 Electronic Authentication Guideline 

In-Person Remote 

compare picture to applicant, record 

ID number, address and DoB. 

• Secondary Government ID or 

financial account

a) Inspects Photo-ID and if apparently 

valid, compare picture to applicant, 

record ID number, address and DoB, 

or; 

b) Verifies financial account number 

supplied by applicant through record 

checks or through credit bureaus or 

similar databases, and confirms that: 

name, DoB, address other personal 

information in records are on 

balance consistent with the 

application and sufficient to identify 

a unique individual. 

• Record Current Biometric 

Record a current biometric (e.g. 

photograph or fingerprints to ensure 

that applicant cannot repudiate 

application. 

• Confirm Address 

Issue credentials in a manner that 

confirms address of record. 

At Level 2, employers and educational instructors who verify the identity of their 

employees or students by means comparable to those stated above for Level 2 may elect 

to 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 on-line processes, 

where notification is via the distribution channels normally used for sensitive, personal 

communications. 

At Level 2, financial institutions subject to the supervision of the Department of 

Treasury’s Office of Comptroller of the Currency may issue credentials to their 

customers via the mechanisms normally used for on-line banking credentials and may use 

on-line banking credentials and tokens as Level 2 credentials provided they meet the 

provisions of Section 8. 

In some contexts, agencies may choose to use additional knowledge-based authentication 

methods to increase their confidence in the registration process. For example, an 

applicant could be asked to supply non-public information on his or her past dealing with 

the agency that could help confirm the applicant’s identity. 

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Special Publication 800-63 Electronic Authentication Guideline 

7.2.2. Records Retention Requirements 

A record of the facts of registration (including revocation) shall be maintained by the 

CSP or its representative. The minimum record retention period for registration data for 

Level 2 credentials is seven years and six months beyond the expiration or revocation 

(whichever is later) of the credential. CSPs operated by or on behalf of executive branch 

agencies must also follow either the General Records Schedule established 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 for registration data is: 

• For Levels 2, and 3, seven years and six months beyond the expiration, 

and 

• For Level 4, ten years and six months beyond the expiration. 

7.3. Mapping FPKI Certificate Policies to Registration Levels 

The identity proofing and certificate issuance processes specified in the Federal PKI 

Certificate Policies [FCBA1, FBCA2, FBCA3] may be mapped to the Registration levels 

specified in the preceding section. These mappings are as follows: 

• The identity proofing and certificate issuance processes of Certification Authorities 

cross-certified with the Federal Bridge CA under policies mapped to the Citizen and 

Commerce Class policies [FBCA2] are deemed to meet the identity proofing 

provisions of Level 2. 

• The identity proofing and certificate issuance processes of Certification Authorities 

cross-certified with the Federal Bridge CA under policies mapped to the Basic 

Certificate Policy [FBCA1] are deemed to meet the identity proofing provisions of 

Levels 2 and 3. 

• The identity proofing and certificate issuance processes of Certification Authorities 

cross-certified with the Federal Bridge CA under policies mapped to the Medium, 

Medium-HW, or High Assurance Certificate policies in [FBCA1] or Common-Auth, 

Common-SW, Common-HW, and Common-High Certificate Policies in [FBCA3] are 

deemed to meet the identity proofing provisions of Levels 2, 3, and 4. 

However, agencies are not limited to relying upon only those certificates by CAs crosscertified with the Federal Bridge CA at Levels 1 and 2. At these levels, agencies may 

choose to rely on any CA that has been determined to meet the identity proofing and 

registration requirements stated in the General Requirements, Section 7.2.1. At Levels 3 

and 4, PKI credentials must be issued by a CA cross-certified2

 with the Federal Bridge 

CA under one of the certificate policies identified above, or a policy mapped to one of 

those policies. 

 

2

 Note that bi-directional cross-certification is not required; it is sufficient that a valid certificate path exist 

from the Bridge CA to the issuing CA. The reverse certificate path need not exist. 

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Special Publication 800-63 Electronic Authentication Guideline 

8. Authentication Protocols 

An authentication protocol is a defined sequence of messages between a claimant and a 

verifier that enables the verifier to verify that the claimant has control of a valid token to 

establish his/her identity. An exchange of messages between a claimant and a verifier 

that results in the authentication (or authentication failure) of the claimant is a protocol 

run. 

8.1. Authentication Threats 

Threats can be divided into those threats that involve attacks against the actual 

authentication protocol itself, and other attacks that may reveal either token values, or 

compromise confidential information. In general, attacks that reveal the token value are 

worse than attacks that simply compromise some information, because the attacker can 

then use the token to assume a subscriber’s identity. 

8.1.1. Authentication Protocol Threats 

Registration Authorities, CSPs, verifiers and relying parties are ordinarily trustworthy (in 

the sense of 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, while RAs, CSPs and verifiers are normally trustworthy, they are not 

invulnerable, or could become corrupted. Therefore, protocols that expose long-term 

authentication secrets more than is absolutely required, even to trusted entities, should be 

avoided. 

Protocol threats include: 

• Eavesdroppers observing authentication protocol runs for later analysis. In some 

cases the eavesdropper may intercept messages between a CSP and a verifier, or 

other parties rather than between the claimant and the verifier. Eavesdroppers 

generally attempt to obtain tokens to pose as claimants; 

• Impostors: 

o impostor claimants posing as subscribers to verifiers to test guessed tokens or 

obtain other information about a specific subscriber; 

o impostor verifiers posing as verifiers to legitimate subscriber claimants to 

obtain tokens that can then be used to impersonate subscribers to legitimate 

verifiers; 

o impostor relying parties posing as the Federal IT system to verifiers to obtain 

sensitive user information; 

• Hijackers who take over an already authenticated session to then: 

o pose as subscribers to relying parties to learn sensitive information or input 

invalid information; 

o pose as relying parties to verifiers to learn sensitive information or output 

invalid information. 

Eavesdroppers are assumed to be physically able to intercept authentication protocol 

runs; however, the protocol may be designed to render the intercepted messages 

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unintelligible, or to resist analysis that would allow the eavesdropper to obtain 

information useful to impersonate the claimant. Subscriber impostors need only normal 

communications access to verifiers or relying parties. Impostor verifiers may have 

special network capabilities to divert, insert or delete packets, but, in many cases, such 

attacks can be mounted simply by tricking subscribers with incorrect links in e-mails or 

on web pages, or by using domain names similar to those of relying parties or verifiers, 

and therefore the impostors need not necessarily have any unusual network capabilities. 

Because of their ubiquitous use, and the way they are implemented, users of web browser 

clients are particularly vulnerable to impostor verifiers in password protocols. Hijackers 

must be able to divert communications sessions, but this capability may be comparatively 

easy to achieve today when many subscribers use wireless network access. 

Specific attack mechanisms on authentication protocols include: 

• Eavesdroppers who listen passively to the authentication protocol exchange, and 

then attempt to learn secrets, such as passwords or keys. 

• Active on-line attacks against authentication mechanisms including: 

o In-band attacks where the attacker assumes the role of a claimant with a 

genuine verifier. These include: 

 Password guessing attacks, where an impostor attempts to guess a 

password in repeated logon trials and succeeds when he/she is able 

to log onto a system. A targeted guessing attack is an attack 

against the password of a selected user whose name is known. 

 Replay attacks, where an attacker records and replays some part of 

a previous good protocol run to the verifier. 

o Out-of-band attacks where the attacker alters the authentication channel in 

some way such as: 

 Hijacking sessions after authentication is complete; 

 Verifier impersonation attacks where the attacker impersonates the 

verifier and induces the claimant to reveal his secret token. 

Because of the functional complexity of web browsers, the 

complexity of their user interfaces, and the control they give 

servers over what users see, users of web browsers are likely to be 

vulnerable to password verifier impersonation attacks, even when 

using or “apparently using” secure protocols (e.g. TLS) that 

authenticate verifiers; 

 Man-in-the middle attacks where the attacker inserts himself in the 

path of an authentication exchange, to obtain secret tokens. 

Because of the functional complexity of web browsers, the 

complexity of their user interfaces, and the control they give 

servers over what users see, users of web browsers are likely to be 

vulnerable to man-in-the-middle attacks on passwords, even when 

using or “apparently using” secure protocols (e.g. TLS) that are 

intended to block such attacks; 

8.1.2. Resistance to Protocol Threats 

This section defines the meaning of resistance to specific protocol threats. 

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Special Publication 800-63 Electronic Authentication Guideline 

• Eavesdropping resistance: An authentication protocol is resistant to 

eavesdropping attacks if an eavesdropper who records all the messages passing 

between a claimant and a verifier or relying party finds that it is impractical to 

learn the private key, secret key or password or to otherwise obtain information 

that would allow the eavesdropper to impersonate the claimant. Eavesdropping 

resistant protocols make it impractical3 for an attacker to carry out an off-line 

attack where he/she records an authentication protocol run then analyses it on 

his/her own system for an extended period, for example by systematically 

attempting to try every password in a large dictionary, or by brute force 

exhaustion. 

• Password guessing resistance: An authentication protocol is resistant to password 

guessing attacks if it is impractical for the attacker, with no a priori knowledge of 

the password, to find the password by repeated authentication attempts with 

guessed passwords. Both the entropy of the password and the protocol itself 

contribute to this property. Password authentication systems can make targeted 

password guessing impractical by requiring use of high-entropy passwords (see 

Appendix A) and limiting the number of unsuccessful authentication attempts, or 

by controlling the rate at which attempts can be carried out. To resist untargeted 

password attacks, a verifier may supplement these controls with network security 

controls. 

• Replay resistance: An authentication protocol resists replay attacks if it is 

impractical to achieve a successful authentication by recording and replaying a 

previous authentication message. 

• Hijacking resistance: A property of both the authentication protocol and the 

subsequent session protocol used to transfer data. An authentication and transfer 

protocol in combination is resistant to hijacking if the authentication is bound to 

the transfer in a manner that prevents an adversary capable of inserting, deleting, 

or rerouting messages from altering the contents of any information sent between 

the claimant and the relying party without being detected. This is usually 

accomplished by generating a per-session shared secret during the authentication 

process that is subsequently used by the claimant and the relying party to 

authenticate the transfer of all sensitive information. 

• Verifier impersonation resistance: In a verifier impersonation attack, the attacker 

poses as a legitimate verifier. It may be comparatively easy to impersonate a 

verifier by “name spoofing,” or some more advanced network attack may be 

required (wireless LAN access today makes these “advanced” network attacks 

relatively easy for attackers in many circumstances). An authentication protocol 

is resistant to verifier impersonation if the impersonator does not learn the value 

of any token when acting as the verifier. However, even secure protocols can 

sometimes be bypassed by fooling the claimant into using another protocol or 

 

3

 “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 is 

very small compared to the number of possible key or password values. 

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Special Publication 800-63 Electronic Authentication Guideline 

overriding security controls (for example by accepting unverified server 

certificates). 

• Man-in-the-middle resistance: In a man-in-the-middle attack on an authentication 

protocol, the attacker interposes himself between the claimant and verifier, posing 

as the verifier to the claimant, and as the claimant to the verifier. The attacker 

thereby learns the value of the authentication token. Authentication protocols are 

resistant to a man-in-the-middle attack when both parties (e.g., claimant and 

verifier) are authenticated to the other in a manner that prevents the undetected 

participation of a third party. However, even secure protocols can sometimes be 

bypassed by fooling the claimant into using another protocol or overriding 

security controls (for example by accepting unverified server certificates). 

8.1.3. Other Threats 

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

• Malicious code attacks that may compromise authentication tokens; 

• Intrusion attacks that obtain credentials or tokens by penetrating the 

subscriber/claimant, CSP or verifier system; 

• Insider threats that may compromise authentication tokens; 

• Out–of-band attacks that obtain tokens in some other manner, such as social 

engineering to get a subscriber to reveal his password to the attacker, or 

“shoulder-surfing;” 

• Attacks that fool claimants into using an insecure protocol, when they think that 

they are using a secure protocol, or trick them into overriding security controls 

(for example, by accepting server certificates that cannot be validated); 

• Intentional repudiation by subscribers who deliberately compromise their tokens. 

Malicious code could be introduced into the claimant’s computer system for the purpose 

of compromising the claimant’s authentication 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 of malicious 

code on claimant systems. General good practice to mitigate malicious code threats is 

outside the scope of this document. Hardware tokens prevent malicious software from 

extracting and copying the authentication secret token from the token. However, 

malicious code may still misuse the token, particularly if activation data is presented to 

the token via the computer. Similarly, the cryptographic tokens at least make it difficult 

to trick a user into verbally giving away his authentication secret, making social 

engineering more difficult, while many kinds of passwords are readily expressed over the 

telephone. 

Insider threats are a major concern in many IT systems; however, good security, 

personnel, and auditing practices may mitigate these risks. General good practice to 

mitigate insider threats is outside the scope of this document. 

From a protocol perspective, shared secrets must be closely held and carefully protected 

by CSPs. In general, at assurance Levels 2, 3 and 4 independent verifiers must not be 

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Special Publication 800-63 Electronic Authentication Guideline 

given long-term shared secrets by CSPs, as this increases exposure to insider attacks. 

Independent verifiers may be given one time challenge-response information, provided 

that the shared secret is a cryptographic key4

. If the shared secret is a password, 

challenge-response mechanisms are vulnerable to insider or penetration attacks. 

Network intrusion attacks are similar in many ways to insider threats, and are a risk for 

all on-line IT systems. Much information is available on the use of preventive measures 

such as firewalls, system configuration, and intrusion detection to mitigate the risks of 

network intrusion attacks (see sections 10.2 and 10.3 for some helpful references). Note 

that subscriber/claimant systems are also subject to network intrusion attacks, but 

appropriate authentication mechanisms are one defense against such attacks. 

The most serious consequence of a network intrusion attack is that it might allow an 

attacker to gain possession or control of tokens used in authentication protocols. A 

general treatment of methods for mitigating intrusion attacks is outside the scope of this 

document. However, as with insider threats, some elements of the design of an 

authentication service can increase or mitigate penetration risks to the authentication 

service itself. Hardware tokens and cryptographic modules provide protection for keys 

and passwords against penetration attacks, due to the constrained environment that holds 

the keys. Other authentication mechanisms may be vulnerable to an attacker who has 

access to or can penetrate the claimant’s system. However, shared secret mechanisms are 

potentially subject to penetration attacks against the verifier or CSP as well, where the 

attacker may find files of many shared secrets. Public key mechanisms are usually less 

vulnerable to attacks against verifiers or CSPs. Encryption of files containing long-term 

shared secrets reduces the risks of a successful penetration attack. 

Subscribers may intentionally compromise tokens to repudiate authentication. A full 

discussion of repudiation is outside the scope of this document; typically, however, 

safeguarding the authentication protocol against other threats will also help to restrict 

repudiation. A variety of measures will reduce the risk of repudiation, including periodic 

confirmations that a user has complied with security requirements, confirmations of 

transactions through a separate channel (such as electronic mail), and reminders to users 

that delegation of tokens is prohibited. Additional discussion appears in DOJ 2000. 

8.2. Authentication Mechanism Requirements 

This section covers the mechanical authentication process of a claimant who already has 

registered a token. Identity proofing and registration are dealt with separately in Section 

7. The authentication process shall provide sufficient information to the relying party to 

 

4

 Cell phone systems commonly employ such shared secret challenge-response authentication mechanisms. 

A shared secret key is maintained on the cell phone and at the home service provider’s “home location 

register.” When a user roams and registers with a base station of another host provider, the home service 

provider generates a challenge and a reply and sends it to the host service provider to be used to 

authenticate the roaming user. If the shared secret keys have sufficient entropy, insider offline attacks at the 

host service provider are impractical. 

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uniquely identify the registration information provided by the subscriber and verified by 

the RA in the issuance of the credential. 

Four assurance levels are defined, numbered 1 to 4. Level 4 provides the highest level of 

authentication assurance, while Level 1 provides the least assurance. The technical 

requirements for authentication mechanisms (tokens, protocols and security protections) 

are stated in this section. 

8.2.1. Level 1 

Although there is no identity proofing requirement at this level, the authentication 

mechanism provides some assurance that the same claimant 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 token methods of Levels 2, 3 or 4. Successful 

authentication requires that the claimant shall prove, through a secure authentication 

protocol, that he/she controls the token. 

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 by 

eavesdroppers. For example, password challenge-response protocols that combine a 

password 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. 

Common protocols that meet Level 1 requirements include APOP [RFC 1939], S/KEY 

[SKEY], and Kerberos [KERB]. Since an eavesdropper who intercepts such a protocol 

exchange will often be able to find the 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. 

8.2.1.1.Credential Lifetime, Status or Revocation 

There are no stipulations about the revocation or lifetime of credentials at Level 1. 

8.2.1.2.Assertions 

Relying parties may accept assertions that are: 

• digitally signed by a trusted entity (e.g., the verifier); or 

• obtained directly from a trusted entity (e.g. a repository or the verifier) using a 

protocol where the trusted entity authenticates to the relying party using a secure 

protocol (e.g. TLS) that cryptographically authenticates the verifier and protects 

the assertion; 

8.2.1.3.Protection of Long-Term Shared Secrets 

Files of shared secrets used by verifiers at Level 1 authentication shall be protected by 

discretionary access controls that limit access to administrators and only those 

applications that require access. Such shared secret files shall not contain the plaintext 

passwords; typically they contain a one-way hash or “inversion” of the password. In 

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Special Publication 800-63 Electronic Authentication Guideline 

addition, any method allowed for the protection of long-term shared secrets at Levels 2, 3 

or 4 may be used at Level 1. 

8.2.1.4.Password Strength 

For password (or PIN) based Level 1 authentication systems, the probability of success of 

a targeted on-line password guessing attack by an attacker who has no a priori

knowledge of the password, but knows the user name of the target, shall not exceed 2-10

(1 in 1024), over the life of the password. There are no min-entropy requirements for 

Level 1. Appendix A contains information about estimating the entropy of passwords. 

8.2.1.5.Example Implementations 

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 the 

claimant by use of a challenge-response protocol. 

8.2.2. Level 2 

Level 2 allows a wide range of available authentication technologies to be employed and 

permits the use of any of the token methods of Levels 3 or 4, as well as passwords. 

Successful authentication requires that the claimant shall prove, through a secure 

authentication protocol, that he/she controls the token. Eavesdropper, replay, and on-line 

guessing attacks shall be prevented. Approved cryptography is required to prevent 

eavesdroppers. 

8.2.2.1.Credential and Token Lifetime, Status or Revocation 

CSPs shall provide a secure mechanism, such as a digitally signed revocation list or a 

status responder, to allow verifiers or relying parties to ensure that the credentials are still 

valid. Verifiers or relying parties shall check to ensure that the credentials they use are 

valid. Shared secret based authentication systems may simply remove revoked 

subscribers from the verification database. 

CSPs shall revoke credentials and tokens within 72 hours after being notified that a 

credential 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 fresh certificates 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 deregistration of the password can be accomplished in no more than 72 hours and that the 

use of that password in authentication shall fail. 

CAs cross-certified with the Federal Bridge CA at the Basic, Medium, High, Citizen and 

Commerce Class, or Common Certificate Policy levels are considered to meet credential 

status and revocation provisions of this level. 

8.2.2.2.Assertions 

Relying parties may accept assertions that are: 

• digitally signed by a trusted entity (e.g., the verifier); or 

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• obtained directly from a trusted entity (e.g. a repository or the verifier) using a 

protocol where the trusted entity authenticates to the relying party using a secure 

protocol (e.g. TLS) that cryptographically authenticates the verifier and protects 

the assertion; 

Assertions generated by a verifier shall expire after 12 hours and should not be accepted 

thereafter by the relying party. 

8.2.2.3.Protection of Long-term Shared Secrets 

Long term shared authentication secrets, if used, shall never be revealed to any party 

except the subscriber and CSP (including verifiers operated as a part of the CSP), 

however session (temporary) shared secrets may be provided by the CSP to independent 

verifiers. 

Files of shared secrets used by CSPs at Level 2 shall be protected by discretionary access 

controls that limit access to administrators and only those applications that require access. 

Such shared secret files shall not contain the plaintext passwords or secret; two 

alternative methods may be used to protect the shared secret: 

1. Passwords may be concatenated to a salt and/or username and then hashed with a 

Approved algorithm so that the computations used to conduct a dictionary or 

exhaustion attack on a stolen password file are not useful to attack other similar 

password files. The hashed passwords are then stored in the password file. 

2. Store shared secrets in encrypted form using Approved encryption algorithms and 

modes and decrypt the needed secret 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. 

8.2.2.4.Password Strength 

For password based Level 2 authentication systems, the probability of success of an online password guessing attack by an attacker who has no a priori knowledge of the 

password, but knows the user name of the target, shall not exceed 2-14 (1 in 16,384), over 

the life of the password. Level 2 passwords shall have at least 10 bits of min-entropy. 

Appendix A contains information about estimating the entropy of passwords. 

8.2.2.5.Example Implementations 

A wide variety of technologies can meet the requirements of Level 2. For example, a 

verifier might authenticate a claimant who provides a password through a secure 

(encrypted) TLS protocol session (tunneling). This prevents eavesdropper attacks, but 

generally does not adequately block not man-in-the middle attacks or verification 

impersonation attacks because common web browser clients offer many avenues to fool 

or trick users. After a successful authentication, the verifier then puts a security assertion 

for the claimant in a secure server, and sends a “handle” for that assertion to a relying 

party in an HTTP referral. 

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8.2.3. Level 3 

Level 3 authentication is based on proof of possession of a cryptographic key using a 

cryptographic protocol. Level 3 authentication assurance requires cryptographic strength 

mechanisms that protect the primary authentication token (a secret key or a private key) 

against compromise by the following protocol threats defined in section 8.1.1: 

eavesdropper, replay, on-line guessing, verifier impersonation and man-in-the-middle 

attacks. Level 3 also requires two factor authentication; in addition to the key, the user 

must employ a password or biometric to activate the key. 

Three kinds of tokens described below may be used to meet Level 3 requirements: 

• Soft cryptographic token: a cryptographic key stored on a general-purpose 

computer. Hardware tokens validated at FIPS 140-2 Level 1 or higher may also be 

used to hold the key and perform cryptographic operations. The claimant shall be 

required to activate the key before using it with a password or biometric, or, 

alternatively shall use a password as well as the key in an authentication protocol 

with the verifier. If a password is employed to unlock the soft token key, the key 

shall be kept encrypted under a key derived from a password meeting the 

requirements for Level 2 authentication, and decrypted only for actual use in 

authentication. Alternatively, if a password protocol is employed with the verifier, 

the use of the password shall meet the requirements for Level 2 authentication 

assurance. 

• Hard token: a cryptographic key stored on a special hardware device. Tokens 

must be validated at FIPS 140-2 Level 1 or higher overall. The claimant shall be 

required to activate the key before using it with a password or biometric, or, 

alternatively, shall use a password as well as the key in an authentication protocol 

with the verifier. The authentication mechanism used to authenticate the claimant 

to unlock token shall be validated as meeting the operator authentication 

requirements for FIPS 140-2 Level 2. Alternatively, if a password protocol is 

employed with a verifier, the use of the password shall meet the requirements for 

Level 1 authentication assurance. 

• One-time password device tokens: the authentication depends on a symmetric key 

stored on a personal hardware device that is a cryptographic module validated at 

FIPS 140-2 Level 1 or higher overall. The device combines a nonce with a 

cryptographic key to produce an output that is sent to the verifier as a password. 

The password shall be used only once and is cryptographically generated; 

therefore it needs no additional eavesdropper protection. The one-time password 

output by the device shall have at least 106 possible values. The verifier must be 

authenticated cryptographically to the claimant, for example using a TLS server. 

To protect against the use of a stolen token, one of the following measures shall 

be used: 

• The authentication mechanism used to authenticate the claimant to the token 

shall be validated as meeting the operator authentication requirements for 

FIPS 140-2 Level 2. 

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Special Publication 800-63 Electronic Authentication Guideline 

• The claimant sends the verifier a personal password meeting the requirements 

for (E-authentication) Level 1 with the one-time password. 

Authentication requires that the claimant shall prove through a secure authentication 

protocol that he or she controls the token. 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. Approved 

cryptographic techniques shall be used for all operations. 

Each of the three token types has somewhat different utility and security properties. Soft 

token solutions are easily realized in “thin clients” with TLS and client certificates. 

Moreover this solution allows not only initial authentication of claimants, but also allows 

the entire session, or as much of it as is security critical, to be cryptographically 

authenticated by a key created during the authentication process. Hard token solutions 

provide the additional assurance of a physical token, and users should know if their token 

has been stolen. Like soft tokens, hard tokens allow not only initial authentication of 

claimants, but also allows the entire session, or as much of it as is security critical, to be 

cryptographically authenticated by a key created during the authentication process. Onetime password device token systems are commercially available, portable and work easily 

with any browser client. Like hard tokens, one-time password device tokens have the 

security advantage that the token is a tangible, physical object. Subscribers should know 

if their token is stolen, and the key is not vulnerable to network, shoulder-surfing or 

keyboard sniffer attacks. Unlike soft tokens or hard tokens, a session key is not created 

from the authentication process to authenticate subsequent data transfers. 

All three token types present the eavesdroppers with similar strong cryptographic 

protection. Each has its advantages and disadvantages against various types of attacks. 

All three offer considerably greater strength than Level 2 solutions. Application 

implementers with specific Level 3 authentication requirements, who need to select a 

particular technology should chose the one that best suits the functional needs and risks 

of their application. 

8.2.3.1. Credential/Token Lifetime, Status or Revocation 

CSPs shall provide a secure mechanism to allow verifiers or relying parties to ensure that 

the credentials are valid. Such mechanisms may include: revocation lists, on-line 

validation servers, and the use of credentials with short life-times or the involvement of 

CSP servers that have access to status records in authentication transactions. Shared 

secret based authentication systems may simply remove revoked subscribers from the 

verification database. Verifiers shall check to ensure that the credentials they use are 

valid. 

CSPs shall have a procedure to revoke credentials and tokens within 24 hours. The 

certificate status provisions of CAs cross-certified with the Federal Bridge CA at the 

Basic, Medium, High or Common Certificate Policy levels are considered to meet 

credential status and revocation provisions of this level. 

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Special Publication 800-63 Electronic Authentication Guideline 

Verifiers shall ensure that the tokens they rely upon are either freshly issued (within 24 

hours) or still valid. 

8.2.3.2. Assertions 

Relying parties may accept assertions that are: 

• digitally signed by a trusted entity (e.g., the verifier); or 

• obtained directly from a trusted entity (e.g. a repository or the verifier) using a 

protocol where the trusted entity authenticates to the relying party using a secure 

protocol (e.g. TLS) that cryptographically authenticates the verifier and protects 

the assertion; 

Assertions generated by a verifier shall expire after 2 hours and should not be accepted 

thereafter by the relying party. 

8.2.3.3.Protection of Long-term Shared Secrets 

Files of long-term shared secrets used by CSPs or verifiers at Level 3 shall be protected 

by discretionary access controls that limit access to administrators and only those 

applications that require access. Such shared secret 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 and decrypted 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. 

3. Shared secrets are split by a cryptographic secret sharing method between m

separate verifier systems, so that the cooperation of n (where 2 ≤ n ≤ m) systems 

in a secure protocol is required to perform the authentication and an attacker who 

learns n-1 of the secret shares, learns nothing about the secret (except, perhaps, its 

size). 

Temporary session authentication keys may be generated from long-term shared secret 

keys by CSPs and distributed to third party verifiers, in an appropriate protocol, but longterm shared secrets shall not be shared with any third parties, including third party 

verifiers. Session authentication keys are typically created by cryptographically 

combining the long 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 protocols are permitted 

at this level provided that all keys preserve at least 80-bits of entropy and approved 

cryptographic algorithms (e.g., AES, SHA-1, SHA256, HMAC) are used for all 

operations. 

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Special Publication 800-63 Electronic Authentication Guideline 

8.2.3.4.Example Implementations 

Level 3 assurance can be satisfied by client authenticated TLS (implemented in all 

modern browsers), with claimants who have public key certificates. Other protocols with 

similar properties can also be used. Level 3 authentication assurance can also be met by 

tunneling the output of a one-time password device and a Level 1 personal password 

through a TLS session. 

8.2.4. 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. Level 4 is similar to Level 3 except that only “hard” 

cryptographic tokens are allowed, FIPS 140-2 cryptographic module validation 

requirements are strengthened, and subsequent critical data transfers must be 

authenticated via a key bound to the authentication process. The token shall be a 

hardware cryptographic module validated at FIPS 140-2 Level 2 or higher overall with at 

least FIPS 140-2 Level 3 physical security. By requiring a physical token, which cannot 

readily be copied and since FIPS 140-2 requires operator authentication at Level 2 and 

higher, this level ensures good, two factor remote authentication. 

Level 4 requires strong cryptographic authentication of all parties and all sensitive data 

transfers between the parties. Either public key or symmetric key technology may be 

used. Authentication requires that the claimant shall prove through a secure 

authentication protocol that he or she controls the token. The protocol threats defined in 

section 8.1.1 above (eavesdropper, replay, on-line guessing, verifier impersonation and 

man-in-the-middle attacks) shall be prevented. In addition, the token shall protect the 

secret from compromise by the malicious code threat as described in section 8.1.3 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 relying parties by the CSP. Strong, Approved cryptographic 

techniques shall be used for all operations. All sensitive data transfers shall be 

cryptographically authenticated using keys derived in the authentication process. 

8.2.4.1.Credential/Token Lifetime, Status or Revocation 

CSPs shall provide a secure mechanism to allow verifiers or relying parties to ensure that 

the credentials are valid. Such mechanisms may include: revocation lists, on-line 

validation servers, and the use of credentials with short life-times or the involvement of 

CSP servers that have access to status records in authentication transactions. Shared 

secret based authentication systems may simply remove revoked subscribers from the 

verification database. Verifiers shall check to ensure that the credentials they use are 

either freshly issued or still valid. 

CSPs shall have a procedure to revoke credentials within 24 hours. Verifiers or relying 

parties shall ensure that the credentials they rely upon are either freshly issued (within 24 

hours) or still valid. The certificate status provisions of CAs cross-certified with the 

Federal Bridge CA at the High and Common Certificate Policies shall be considered to 

meet credential status provisions of Level 4. [FBCA1]. 

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Special Publication 800-63 Electronic Authentication Guideline 

At this level 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. 

8.2.4.2.Protection of Long-term Shared Secrets 

Files of long-term shared secrets used by CSPs or verifiers at Level 4 shall be protected 

in the same manner as long-term shared secrets for Level 3 (specified in section 8.2.3.3

above.) 

8.2.4.3.Example Implementations 

Level 4 assurance can be satisfied by client authenticated TLS (implemented in all 

modern browsers), with claimants who have public key hard tokens. Other protocols 

with similar properties can also be used. 

9. Summary of Technical Requirements by level 

This section summarizes the technical requirements for each level in tabular form. Table 

2 shows the types of tokens that may be used at each authentication assurance level. 

Table 3 identifies the protections that are required at each level. Protections are defined 

in section 8.1.2 above. Table 4 summarizes the requirements for the resistance of 

passwords to on-line password guessing attacks. Table 5 identifies the types of 

authentication protocols that are applicable to each assurance level. Table 6 identifies 

additional required protocol and system properties at each level. 

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Special Publication 800-63 Electronic Authentication Guideline 

Table 2. Token Types Allowed at Each Assurance Level 

Token type Level 1 Level 2 Level 3 Level 4 

Hard crypto token √ √ √ √

One-time password device √ √ √

Soft crypto token √ √ √

Passwords & PINs √ √

Table 3. Required Protections 

Protect against Level 1 Level 2 Level 3 Level 4

On-line guessing √ √ √ √

Replay √ √ √ √

Eavesdropper √ √ √

Verifier impersonation √ √

Man-in-the-middle √ √

Session hijacking √

Table 4. Minimum Online Password Guessing Resistance 

Attack Type Level 1 Level 2

Targeted Attack: Maximum chance of an 

attacker guessing the password of a 

selected user over the life of the password 

with no a priori knowledge other than the 

username 

one in 210

(1/1024) 

one in 214

(1/16384) 

Untargeted Attack: min-entropy - 10-bits 

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Special Publication 800-63 Electronic Authentication Guideline 

Table 5. Authentication Protocol Types 

Protocol Type Level 1 Level 2 Level 3 Level 4 

Private key PoP √ √ √ √

Symmetric key PoP √ √ √ √

Tunneled or Zero knowledge 

password 

√ √

Challenge-response password √

Table 6. Additional Required Properties 

Required Property Level 1 Level 2 Level 3 Level 4 

Shared secrets not revealed to third 

parties by verifiers or CSPs 

√ √ √

Multi-factor authentication √ √

Sensitive data transfer authenticated √

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Special Publication 800-63 Electronic Authentication Guideline 

9.1.1. Relationship of PKI Policies 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. Table 7 summarizes how certificates 

issued under these policies correspond to the E-authentication assurance levels. Note that 

the Card Authentication and Common Device policies are not listed; these policies 

support authentication of a system or a cryptographic module rather than a person. 

Table 7. E-authentication Assurance Levels and the Common Policy Framework 

E-auth Selected Policy Components 

Level 

Overall 

Identity Equivalence 

Proofing 

Token Status 

Reporting 

Level 2 Common-Auth, Common-Auth, 

Common-SW, 

Common-HW, 

and CommonHigh Certificate 

Policies 

Common-Auth, 

Common-SW, 

Common-HW, 

and CommonHigh Certificate 

Policies 

Common-SW, 

Common-HW, 

and CommonHigh Certificate 

Policies 

Common-Auth, 

Common-SW, 

Common-HW, 

and CommonHigh Certificate 

Policies 

Level 3 Common-Auth, 

Common-SW, 

Common-HW, 

and CommonHigh Certificate 

Policies 

Common-Auth, 

Common-SW, 

Common-HW, 

and CommonHigh Certificate 

Policies 

Common-Auth, 

Common-SW, 

Common-HW, 

and CommonHigh Certificate 

Policies 

Common-Auth, 

Common-SW, 

Common-HW, 

and CommonHigh Certificate 

Policies 

Level 4 Common-Auth, 

Common-SW, 

Common-HW, 

and CommonHigh Certificate 

Policies 

Common-Auth, 

Common-HW, 

and CommonHigh Certificate 

Policies 

Common-Auth, 

Common-SW, 

Common-HW, 

and CommonHigh Certificate 

Policies 

Common-Auth, 

Common-HW, 

and CommonHigh Certificate 

Policies 

Agencies that were early adopters of PKI technology, and organizations outside the 

Federal government, issue PKI certificates under organization specific policies instead of 

the Common Policy Framework. The primary mechanism for evaluating the assurance 

provided by public key certificates issued under organization specific policies is the 

policy 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]. Table 8 below summarizes how these certificate policies 

correspond to E-authentication assurance levels. At Level 2 agencies may use certificates 

issued under policies that have not been mapped by the Federal policy authority, but are 

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Special Publication 800-63 Electronic Authentication Guideline 

determined to meet the Level 2 identify proofing, token and status reporting 

requirements. 

Table 8. E-authentication Assurance Levels and PKI Certificate Policy Mappings 

E-auth Selected Policy Components 

Level Identity 

Proofing 

Token 

Overall 

Status Equivalence 

Reporting 

Level 2 Basic, Citizen 

and Commerce 

Class, Medium, 

Medium-HW, or 

High Certificate 

Policy or other 

policies that 

meet level 2 ID 

proofing 

requirements 

Rudimentary, 

Basic, Citizen and 

Commerce Class, 

Medium, 

Medium-HW, or 

High Certificate 

Policy, any cert 

with at least 1024-

bit RSA key & 

SHA1 or 

equivalent. 

Basic, Citizen 

and Commerce 

Class, Medium, 

Medium-HW or 

High Certificate 

Policy or certs. 

issued by other 

CAs with a 72 

hour or smaller 

CRL or 

revocation cycle 

Basic, Citizen 

and Commerce 

Class, Medium, 

Medium-HW, or 

High Certificate 

Policy or other 

policies that meet 

all level 2 

requirements 

Level 3 Basic, Medium, 

Medium-HW, or 

High Certificate 

Policy 

Rudimentary, 

Basic, Citizen and 

Commerce Class, 

Medium, 

Medium-HW, or 

High Certificate 

Policy 

Basic, Medium, 

Medium-HW, or 

High Certificate 

Policy 

Basic, Medium, 

Medium-HW, or 

High Certificate 

Policy 

Level 4 Medium, 

Medium-HW, 

or High 

Certificate 

Policy 

Medium-HW or 

High Certificate 

Policy 

Medium, 

Medium-HW, or 

High Certificate 

Policy 

Medium-HW or 

High Certificate 

Policy 

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, the 

Medium CBP and MediumHW-CBP are equivalent to Medium and Medium-HW, 

respectively. 

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Special Publication 800-63 Electronic Authentication Guideline 

10. References 

10.1. General References 

[DOJ 2000] Guide to Federal Agencies on Implementing Electronic Processes 

(November 2000), available at: 

http://www.usdoj.gov/criminal/cybercrime/ecommerce.html

[OCC] Customer Identification Programs for Banks, Savings Associations, Credit 

Unions and Certain Non-Federally Regulated Banks. Office of the 

Comptroller of the Currency, 12 CFR Part 21. May 2003. Available at: 

http://www.fdic.gov/regulations/laws/federal/03joint326.pdf

[OMB 04-04] OMB Memorandum M-04-04, E-Authentication Guidance for Federal 

agencies, December 16, 2003, available at: 

http://www.whitehouse.gov/omb/memoranda/fy04/m04-04.pdf

[OMB 03-22] OMB Memorandum M-03-22, OMB Guidance for Implementing the 

Privacy Provisions of the E-Government Act of 2002, September 26, 2003 

available at: http://www.whitehouse.gov/omb/memoranda/m03-22.html. 

[KERB] Neuman, C., and T. Ts’o, Kerberos: An Authentication Service for 

Computer Networks, IEEE Communications, vol. 32, no.9, 1994. 

[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 

[RFC2560] IETF, RFC 2560, X.509 Internet Public Key Infrastructure Online 

Certificate Status Protocol – OCSP, available at: 

http://www.ietf.org/rfc/rfc2560.txt 

[RFC 3280] IETF, RFC 3280, Internet X.509 Public Key Infrastructure Certificate and 

CRL Profile, available at: http://www.ietf.org/rfc/rfc3280.txt 

[RFC 3546] IETF, RFC 3546, Transport Layer Security (TLS) Extensions, June 2003, 

available at: http://www.ietf.org/rfc/rfc3546.txt 

[SKEY] IETF, RFC 1760, The S/KEY One Time Password System, February 

1995, available at: http://www.ietf.org/rfc/rfc1760.txt 

10.2. NIST ITL Bulletins 

NIST ITL Bulletins are available at: http://csrc.nist.gov/publications/nistbul/index.html. 

The following bulletins may be of particular interest to those implementing systems of 

applications requiring e-authentication. 

[ITL Dec02] ITL Bulletin, Security of Public Webservers, Dec. 2002 

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Special Publication 800-63 Electronic Authentication Guideline 

[ITL July02] ITL Bulletin, Overview: The Government Smartcard Interoperability 

Specification, July 2002 

[ITL Jan02] ITL Bulletin, Guideline on Firewalls and Firewall Policy, January 2002 

[ITL Feb00] ITL Bulletin, Guideline for Implementing Cryptography in the Federal 

Government, February 2000 

[ITL Dec99] ITL Bulletin, Operating System Security: Adding to the Arsenal of 

Security Techniques, December 1999 

[ITL Nov99] ITL Bulleting, Acquiring and Deploying Intrusion Detection Systems, 

November 1999 

[ITL Sep99] ITL Bulletin, Securing Web Servers, September 1999 

[ITL May99] ITL Bulletin, Computer Attacks: What They Are and How to Defend 

Against Them, May 1999 

10.3. NIST Special Publications 

NIST 800 Series Special Publications are available at: 

http://csrc.nist.gov/publications/nistpubs/index.html. The following publications may be 

of particular interest to those implementing systems of applications requiring eauthentication. 

[SP 800-31] NIST Special Publication, 800-31, Intrusion Detection Systems (IDS), 

November 2001 

[SP 800-32] NIST Special Publication, 800-32, Introduction to Public Key Technology 

and the Federal PKI Infrastructure, February 2001 

[SP 8000-33] NIST Special Publication 800-33, Underlying Technical Models for 

Information Technology Security, December 2001 

[SP 800-40] NIST Special Publication 800-40, Procedures for Handling Security 

Patches, September 2002 

[SP 800-41] NIST Special Publication 800-41, Guidelines on Firewalls and Firewall 

Policy, January 2002 

[SP 800- 42] NIST Special Publication 800-42, Guideline on Network Security Testing, 

draft 

[SP 800-43] NIST Special Publication 800-43, Guide to Securing Windows 2000 

Professional, November 2002 

[SP 800-44] NIST Special Publication 800-44, Guidelines on Securing Public Web 

Servers, September 2002 

[SP 800- 47] NIST Special Publication 800-47, Security Guide for Interconnecting 

Information Technology Systems, September 2002 

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Special Publication 800-63 Electronic Authentication Guideline 

[SP 800- 52] NIST Special Publication 800-52, Guidelines for the Selection and Use of 

Transport Layer Security Implementations, draft. 

10.4. Federal Information Processing Standards 

FIPS can be found at: http://csrc.nist.gov/publications/fips/ 

[FIPS 46-3] Federal Information Processing Standard Publication 46-3, Data 

Encryption Standard (DES), NIST, October 25, 1999 

[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 Publication197, Advanced 

Encryption Standard (AES), NIST, November 2001. 

[FIPS 198] Federal Information Processing Standard Publication 198, Keyed-Hash 

Message Authentication Code (HMAC), NIST, March 2002. 

10.5. Certificate Policies 

These certificate policies can be found at: http://www.cio.gov/fpkipa/policies.htm. 

[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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Special Publication 800-63 Electronic Authentication Guideline 

Appendix A: Estimating Password Entropy and Strength 

Claude Shannon coined the use of the term “entropy5” 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 much 

information is produced on the average for each letter of a text in the language. If the 

language 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.”6 

Entropy in this sense is at most only loosely related to the use of the term in 

thermodynamics. A mathematical definition of entropy in terms of the probability 

distribution function is: 

∑ =−= = x

xXPxXPXH )(log)(:)( 2

where 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 particular 

size is a truly random selection, and clearly, on average such a selection cannot be 

compressed. However it is far from clear that compression is the best measure for the 

strength of keys and passwords, and cryptographers have derived a number of alternative 

forms or definitions of entropy, including “guessing entropy” and “min-entropy.” As 

applied to a distribution of passwords the guessing entropy is, roughly speaking, an 

estimate 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 a 

particular 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 probable 

password for that chosen username, then the second most probable password for that 

username and so on in decreasing order of probability until the attacker found the 

password 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 

with every username, then the second most probable password with every username, until 

he found the first “hit.” This corresponds to the min-entropy. 

 

5

 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 6

 C. E. Shannon, “Prediction and Entropy of Printed English”, Bell System Technical Journal, v.30, n. 1, 

1951, pp. 50-64. 

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Special Publication 800-63 Electronic Authentication Guideline 

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, that 

is by system administrators applying massive dictionary attacks to the files of hashed 

passwords (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 is 

chosen at random there are 2k

 possible values and the password is said to have k bits of 

entropy. 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 the 

entropy of the password is bl

(for example if a password composed of 8 characters from 

the 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 chosen 

passwords, 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 chosen 

from the standard 94 keyboard characters (not including the space). Calculation of 

randomly selected passwords from other alphabets is straightforward. 

A.2 User Selected Passwords 

It is much more difficult to estimate the entropy in passwords that users choose for 

themselves, 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 

and character frequency distributions of ordinary English text, and are chosen by users so 

that they 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 short 

dictionaries of a few thousand commonly chosen passwords, when they are compared to 

actual 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, in band password guessing 

attacks. 

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Special Publication 800-63 Electronic Authentication Guideline 

In this guidance, 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 be 

better 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 against 

interpreting 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 each 

successive character. He used a 27-character alphabet, the ordinary English lower case 

letters 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 selected 

passwords 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, and 

that 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 a 

capital 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 be 

counterproductive because they will make the passwords hard to remember. Users will 

then 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 the 

entropy of simple English text, assuming only a 27-symbol alphabet. 

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 the 

first 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 

to on 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 of entropy per character.7

 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 5th or later letters8

. 

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 and 

 

7

 Thomas Schurmann and Peter Grassberger, “Entropy estimation of symbol sequences,” 

http://arxiv.org/ftp/cond-mat/papers/0203/0203436.pdf 

8 ibid.

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Special Publication 800-63 Electronic Authentication Guideline 

improve the resistance of user chosen passwords to such dictionary or rule driven 

password guessing attacks. For the purposes of this guidance 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 of 

passwords that are simple transformations of any one word found in an 

unabridged English dictionary, and should include at least 50,000 words. There is 

no intention to prevent selection of long passwords (16 characters or more based 

on 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 the 

space 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 they 

generally improve the “practical entropy” of passwords, although they reduce the work 

required for a truly exhaustive attack. The dictionary check requires a dictionary of at 

least 50,000 legal passwords chosen to exclude commonly selected passwords. Upper 

case letters in candidate passwords converted to lower case before comparison. 

Table A.1 provides a rough estimate of the average entropy of user chosen passwords as a 

function of password length. Estimates are given for user selected passwords drawn from 

the normal keyboard alphabet that are not subject to further rules, passwords subject to a 

dictionary check to prevent the use of common words or commonly chosen passwords 

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 of 

Table A.1 should not be taken as accurate estimates of absolute entropy, but they do 

provide a rough relative estimate of the likely entropy of user chosen passwords, and 

some 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; 

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Special Publication 800-63 Electronic Authentication Guideline 

• 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 these 

characters, but in many cases thee characters will occur only at the beginning or 

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 the password; 

• 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 a dictionary 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 with 

password 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 is 

allowed). Suppose, for example, that one user in 1,000 chooses one of the 2 most 

common 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 with 

those two passwords. Clearly this is a practical attack if the only goal is to get access to 

the 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 chosen 

passwords, without examining in detail the passwords that users actually select under the 

rules of the password system, however it is reasonable to argue that testing user chosen 

passwords 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 and compared 

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. 

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Special Publication 800-63 Electronic Authentication Guideline 

This is estimated to ensure at least 10-bits of min entropy. Other means may be 

substituted 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 to character randomly chosen string (two randomly chosen 

characters from a 94-bit alphabet have about 13 bits of entropy). A password, for 

example might combine short system selected random elements, to ensure 10-bits of minentropy, with a longer user-chosen password. 

A.2 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 same 

value. 

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. 

A.3 Examples 

The intent of this guidance is to allow designers and implementers flexibility in designing 

password authentication systems. System designers can trade off password length, rules 

and measures imposed to limit the number of guesses an adversary can attempt. 

The approach of this recommendation to password strength is that it is a measure of the 

probability that an attacker, who knows nothing but a user’s name, can discover the 

user’s password by means of “in-band” password guessing attack. That is the attacker 

attempts to try different passwords until he/she authenticates successfully. At each level 

given below, the maximum probability that, over the life of the password, an attacker 

with no a priori knowledge of the password will succeed in an in-band password 

guessing attack is: 

1. Level 1- 2-10 (1 in 1024) 

2. Level 2 - 2-14 (1 in 16,384) 

Consider a system that assigns subscribers 6 character passwords, randomly selected 

from an alphabet of 94 printable keyboard characters. From Table A.1 we see that such a 

password is considered to have 39.5 bits of entropy. If the authentication system limits 

the number of possible unsuccessful authentication trials to 239.5/214 = 225.5 trials, the 

password strength requirements of Level 2 are satisfied. The authentication system 

could, for example, simply maintain a counter that locked the password after 225.5 (about 

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Special Publication 800-63 Electronic Authentication Guideline 

forty-five million) total unsuccessful trials. An alternative scheme would be to lock out 

the claimant for a minute after three successive failed authentication attempts. Such a 

lock out would suffice to limit automated attacks to 3 trials a minute and it would take 

about 90 years to carryout 225.5 trials. If the system required that password authentication 

attempts be locked for one minute after three unsuccessful trials and that passwords be 

changed every ten years, then the targeted password guessing attack requirements of 

Level 2 would be comfortably satisfied. Because the min-entropy of a randomly chosen 

password is the same as the guessing entropy, the min-entropy requirements of level two 

are met. 

Consider a system that used: 

• a minimum of 8 character passwords, selected by subscribers from an alphabet of 

94 printable characters, 

• required subscribers to include at least one upper case letter, one lower case letter, 

one number and one special character, and; 

• Used a dictionary to prevent subscribers from including common words and 

prevented permutations of the username as a password. 

Such a password would meet the composition and dictionary rules for user-selected 

passwords in Appendix A, and from Table A.1 we estimate guessing entropy at 30 bits. 

Any system that limited a subscriber to less than 216 (about 65,000) failed authentication 

attempts over the life of the password would satisfy the targeted guessing attack 

requirements of Level 2. For example, consider a system that required passwords to be 

changed every two years and limited trials by locking an account for 24 hours after 6 

successive failed authentication attempts. An attacker could get 2 × 365 × 6 = 4,380 

attempts during the life of the password and this would easily meet the targeted attack 

requirements of Level 2. Because of the dictionary test, this would also meet the minentropy rules for Level 2. 

It will be very hard to impose dictionary rules on longer passwords, and many people 

may prefer to memorize a relatively long “pass-phrases” of words, rather than a shorter, 

more arbitrary password. An example might be: “IamtheCapitanofthePina4”. 

As an alternative to imposing some arbitrary specific set of rules, an authentication 

system might grade user passwords, using the rules stated above, and accept any that 

meet some minimum entropy standard. For example, suppose passwords with at least 24-

bits of entropy were required. We can calculate the entropy estimate of 

“IamtheCapitanofthePina4” by observing that the string has 23 characters and would 

satisfy a composition rule requiring upper case and non-alphabetic characters. Table A.1 

estimates 45 bits of guessing entropy for this password. 

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Special Publication 800-63 Electronic Authentication Guideline 

Table A.1 – Estimated Password Guessing Entropy in bits vs. Password Length 

User Chosen Randomly Chosen 

94 Character Alphabet 94 char 

alphabet

Length 

Char. 

No Checks Dictionary 

Rule

Dict. & 

Comp. Rule

10 char. alphabet

1 4 - - 3 3.3 6.6

2 6 - - 5 6.7 13.2

3 8 - - 7 10.0 19.8

4 10 14 16 9 13.3 26.3

5 12 17 20 10 16.7 32.9

6 14 20 23 11 20.0 39.5

7 16 22 27 12 23.3 46.1

8 18 24 30 13 26.6 52.7

10 21 26 32 15 33.3 65.9

12 24 28 34 17 40.0 79.0

14 27 30 36 19 46.6 92.2

16 30 32 38 21 53.3 105.4

18 33 34 40 23 59.9 118.5

20 36 36 42 25 66.6 131.7

22 38 38 44 27 73.3 144.7

24 40 40 46 29 79.9 158.0

30 46 46 52 35 99.9 197.2

40 56 56 62 45 133.2 263.4

0

10

20

30

40

50

60

0 10 20 30

Password length in characters

estimated bits of entropy

no checks

dictionary rule

comp & dictionary

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

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Special Publication 800-63 Electronic Authentication Guideline 

Appendix B: Errata 

Appendix B.1: Errata for Version 1.0.1 

1. Cover page: Changed Version number from 1.0 to 1.0.1. 

2. Cover page: Changed date from June 2004 to September 2004. 

3. Page vii: Clarified text to indicate Level 3 authentication may be supported using 

one-time passwords, but not reusable passwords. 

4. Definition of “Approved” revised to include FIPS 140-2 validation of 

cryptographic modules and include a URL pointing to the list of validated 

modules. 

5. Page 26: Clarified meaning of “cross-certification” with the Federal Bridge CA 

by adding a footnote to explicitly state that cross-certification with the Federal 

Bridge CA need not be bi-directional for the purposes of this guideline. 

6. Page 40, Table 2: Clarified that PINS, a form of passwords, are allowed at Levels 

1 and 2. 

In addition, minor editorial changes (e.g., capitalization, spelling, and punctuation) have 

been made throughout, and some links have been fixed. 

Appendix B.2: Errata for Version 1.0.2 

1. Cover page: Changed Version number from 1.0.1 to 1.0.2. 

2. Cover page: Changed date from September 2004 to April 2006. 

3. Cover page: Specified William Jeffrey as NIST Director and Robert Cresanti as 

Department of Commerce Under Secretary for Technology 

4. Page 25: Updated mapping of FPKI Certificate Policies to 800-63 registration 

levels to reflect changes in FBCA Basic Assurance Level and incorporate three 

new FPKI certificate policies (FBCA Medium Hardware policy, Common 

Authentication, and Common-High). FBCA Basic was upgraded by the Federal 

PKI Policy Authority to meet Level 3 registration requirements; the new policies 

satisfy Level 4 Registration requirements. 

5. Page 41: A new Table 7 was inserted to clarify the relationship of the certificate 

policies in the Common Policy Framework with the E-Authentication Assurance 

Levels. New introductory text associated with this table explains that the Card 

Authentication and Common Device policies are excluded from consideration, 

since these policies support authentication of devices. 

6. Page 42: Table 8 (Table 7 in version 1.0.1) was updated to reflect the 

modifications in the Basic certificate policy, which now satisfies Level 3. The 

new Medium-HW certificate policy, which satisfies Level 4, was also added to 

this table. Certificate policies from the Common Policy Framework were deleted 

from this table, since they are specified in the new Table 7. 

7. Page 45: URLs for FPKI certificate policies have been updated. 

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