NIST Special Publication 800-57 Part 3

Revision 1

Recommendation for

Key Management

Part 3: Application-Specific Key 

Management Guidance

Elaine Barker

Quynh Dang

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.SP.800-57pt3r1

C O M P U T E R S E C U R I T Y
NIST Special Publication 800-57 Part 3

Revision 1

Recommendation for 

Key Management

Part 3: Application-Specific Key 

Management Guidance

Elaine Barker

Quynh Dang

Computer Security Division

Information Technology Laboratory

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.SP.800-57pt3r1

January 2015

U.S. Department of Commerce

Penny Pritzker, Secretary

National Institute of Standards and Technology

Willie May, Acting Under Secretary of Commerce for Standards and Technology and Acting Director
ii

Authority

This publication has been developed by NIST to further its statutory responsibilities under the 

Federal Information Security Management Act (FISMA), Public Law (P.L.) 107-347. NIST is 

responsible for developing information security standards and guidelines, including minimum 

requirements for Federal information systems, but such standards and guidelines shall not apply 

to national security systems without the express approval of appropriate Federal officials 

exercising policy authority over such systems. This guideline is consistent with the requirements 

of the Office of Management and Budget (OMB) Circular A-130, Section 8b(3), Securing Agency 

Information Systems, as analyzed in Circular A-130, Appendix IV: Analysis of Key Sections. 

Supplemental information is provided in Circular A-130, Appendix III, Security of Federal 

Automated Information Resources.

Nothing in this publication should be taken to contradict the standards and guidelines 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. 

This publication may be used by nongovernmental organizations on a voluntary basis and is not 

subject to copyright in the United States. Attribution would, however, be appreciated by NIST.

National Institute of Standards and Technology Special Publication 800-57 Part 3, 

Revision 1

Natl. Inst. Stand. Technol. Spec. Publ. 800-57 Part 3, Revision 1, 102 pages (January 2015)

CODEN: NSPUE2

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.SP.800-57pt3r1

National Institute of Standards and Technology

Attn: Computer Security Division, Information Technology Laboratory

100 Bureau Drive (Mail Stop 8930) Gaithersburg, MD 20899-8930

Email: SP80057Part3@nist.gov

Certain commercial entities, equipment, or materials may be identified in this document in order to 

describe an experimental procedure or concept adequately. Such identification is not intended to imply 

recommendation or endorsement by NIST, nor is it intended to imply that the entities, materials, or 

equipment are necessarily the best available for the purpose.

There may be references in this publication to other publications currently under development by NIST 

in accordance with its assigned statutory responsibilities. The information in this publication, including 

concepts and methodologies, may be used by Federal agencies even before the completion of such 

companion publications. Thus, until each publication is completed, current requirements, guidelines, 

and procedures, where they exist, remain operative. For planning and transition purposes, Federal 

agencies may wish to closely follow the development of these new publications by NIST.

Organizations are encouraged to review all draft publications during public comment periods and 

provide feedback to NIST. All NIST Computer Security Division publications, other than the ones 

noted above, are available at http://csrc.nist.gov/publications.
iii

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 analyses to advance the development and productive use of information 

technology. ITL’s responsibilities include the development of management, 

administrative, technical, and physical standards and guidelines for the cost-effective 

security and privacy of other than national security-related information in Federal 

information systems. The Special Publication 800-series reports on ITL’s research, 

guidelines, and outreach efforts in information system security, and its collaborative 

activities with industry, government, and academic organizations.

Abstract

NIST Special Publication 800-57 provides cryptographic key management guidance. It 

consists of three parts. Part 1 provides general guidance and best practices for the 

management of cryptographic keying material. Part 2 provides guidance on policy and 

security planning requirements for U.S. government agencies. Finally, Part 3 provides 

guidance when using the cryptographic features of current systems.

Keywords

accreditation; assurances; authentication; authorization; availability; backup; 

certification; compromise; confidentiality; cryptanalysis; cryptographic key; 

cryptographic module; digital signature; key management; key management policy; key 

recovery; private key; public key; public key infrastructure; security plan; trust anchor; 

validation.
iv

Acknowledgements

The co-authors of this version of SP 800-57, Part 3 greatly appreciate the contributions of 

previous co-authors of this document, namely William Burr, Alicia Jones, Timothy Polk, 

Scott Rose and Miles Smid. We also appreciate contributions by Katrin Reitsma, Sheila 

Frankel, David Cooper, Judith Spencer, the National Security Agency (NSA) and many 

others in the public and private sectors whose thoughtful and constructive comments 

improved the quality and usefulness of this publication.
v

Table of Contents

1 Introduction ........................................................................................................ 1

1.1 Purpose.........................................................................................................................2

1.2 Requirement Terms......................................................................................................3

1.3 General Protocol Considerations .................................................................................3

1.3.1 Mandatory-to-Implement versus Optional-to-Implement .....................................4

1.3.2 Cryptographic Negotiation ....................................................................................4

1.3.3 Single or Multi-Use Keys......................................................................................6

1.3.4 Algorithm and Key Size Transition.......................................................................6

2 Public Key Infrastructure (PKI) ......................................................................... 8

2.1 Description ...................................................................................................................8

2.2 Security and Compliance Issues.................................................................................11

2.2.1 Recommended Key Sizes and Algorithms..........................................................11

2.3 Procurement Guidance ...............................................................................................14

2.3.1 CA/RA Software and Hardware:.........................................................................14

2.3.2 OCSP Responders: ..............................................................................................15

2.3.3 Cryptographic Modules.......................................................................................16

2.3.4 Key Recovery Servers.........................................................................................16

2.3.5 Relying Party Software........................................................................................16

2.3.6 Client Software....................................................................................................17

2.4 Recommendations for System Installers/Administrators...........................................17

2.4.1 Certificate Issuance .............................................................................................17

2.4.2 Certificate Revocation Requests..........................................................................18

2.4.3 Certificate Revocation List Generation ...............................................................19

2.4.4 PKI Repositories for the Distribution of Certificates and CRLs.........................19

2.4.5 OCSP Responders ...............................................................................................20

2.4.6 Backup and Archive ............................................................................................20

2.4.7 Relying Party Integration and Configuration ......................................................20

2.5 User Guidance (Subscribers)......................................................................................21

3 Internet Protocol Security (IPsec)................................................................... 23

3.1 Description .................................................................................................................23

3.2 Security and Compliance Issues.................................................................................25

3.2.1 Cryptographic Algorithms...................................................................................25

3.2.2 Additional Recommendations.............................................................................29

3.3 Procurement Guidance ...............................................................................................29

3.4 Recommendations for System Installers....................................................................30

3.5 Recommendations for System Administrators ..........................................................30

3.6 Recommendations for End Users...............................................................................30

4 Transport Layer Security (TLS)....................................................................... 30

5 Secure/Multipart Internet Mail Extensions (S/MIME) ..................................... 31

5.1 Description .................................................................................................................31

5.2 Security and Compliance Issues.................................................................................31

5.3 Procurement Guidance ...............................................................................................34
vi

5.4 Recommendations for System Installers....................................................................34

5.5 Recommendations for System Administrators ..........................................................35

5.6 Recommendations for End Users...............................................................................35

6 Kerberos............................................................................................................ 36

6.1 Description .................................................................................................................36

6.2 Security and Compliance Issues.................................................................................39

6.3 Procurement Guidance ...............................................................................................40

6.4 Recommendations for System Installers....................................................................40

6.5 Recommendations for System Administrators ..........................................................41

6.6 Recommendations for End Users...............................................................................42

7 Over-The-Air Rekeying (OTAR) Key Management Messages (KMMs)......... 43

7.1 Description .................................................................................................................43

7.2 Security and Compliance Issues.................................................................................44

7.2.1 Cryptographic Algorithms...................................................................................44

7.2.2 Message Authentication and Cryptoperiods........................................................44

7.2.3 Key Usage ...........................................................................................................44

7.2.4 Backup.................................................................................................................45

7.2.5 Rekeying..............................................................................................................45

7.2.6 Random bit generators.........................................................................................45

7.3 Procurement Guidance ...............................................................................................45

7.4 Recommendations for System Installers....................................................................46

7.5 Recommendations for System Administrators ..........................................................46

7.6 Recommendations for End Users...............................................................................47

8 Domain Name System Security Extensions (DNSSEC) ................................ 48

8.1 Description .................................................................................................................48

8.1.1 DNS Data Authentication....................................................................................49

8.1.2 DNS Transaction Authentication ........................................................................49

8.1.3 DNS Cryptographic Algorithms/Schemes, Modes and Combinations ...............50

8.1.4 Special Considerations for Key Sizes..................................................................51

8.1.5 Special Considerations for NSEC3 .....................................................................52

8.2 Security/Compliance Issues.......................................................................................52

8.3 Procurement Guidance ...............................................................................................53

8.4 Recommendations for System Installers....................................................................53

8.4.1 Recommendations for System Installers (Authoritative Servers) .......................53

8.4.2 Recommendations for System Installers (Caching Recursive Servers) ..............54

8.4.3 Recommendations for System Installers (Client Systems) .................................54

8.5 Recommendations for System Administrators ..........................................................54

8.5.1 Recommendations for System Administrators (Authoritative Server) ...............54

8.5.2 Recommendations for System Administrators (Caching Recursive Servers).....54

8.5.3 Recommendations for System Administrators (Client Systems) ........................55

8.6 Recommendations for End Users...............................................................................55

9 Encrypted File Systems (EFS)......................................................................... 56

9.1 Description .................................................................................................................56

9.1.1 Number of Keys Required...................................................................................56
vii

9.1.2 Access to Symmetric Keys used in File Encryption ...........................................58

9.2 Security and Compliance Issues.................................................................................60

9.3 Recommendations for Procurement Officials............................................................60

9.4 Recommendations for System Installers....................................................................61

9.5 Recommendations for System Administrators ..........................................................61

9.6 Recommendations for End Users...............................................................................61

10 The Secure Shell (SSH) .................................................................................. 62

10.1 Description ................................................................................................................62

10.1.1 Transport Layer Protocol (SSH-TLP) .................................................................62

10.1.2 The User Authentication Protocol (UAP) ..........................................................62

10.1.3 Connection Protocol (CP) ..................................................................................63

10.2 Security and Compliance Issues................................................................................63

10.2.1 TLP Issues.........................................................................................................63

10.2.2 UAP Issues........................................................................................................67

10.3 Procurement Guidance ..............................................................................................68

10.4 Recommendations for System Installers...................................................................68

10.5 Recommendations for System Administrators..........................................................69

10.6 Recommendations for End Users..............................................................................69

Appendix A: Glossary .......................................................................................... 70

Appendix B: Acronyms ........................................................................................ 77

Appendix C: A Word to Novice End Users ......................................................... 79

Appendix D: References ...................................................................................... 81

Appendix E: Revision Changes........................................................................... 93
1

RECOMMENDATION FOR KEY MANAGEMENT

Part 3: Application-Specific Key Management Guidance

1 Introduction

Application-Specific Key Management Guidance, Part 3 of the Recommendation for Key 

Management is intended primarily to help system administrators and system installers 

adequately secure applications based on product availability and organizational needs and 

to support organizational decisions about future procurements. This document also 

provides information for end users regarding application options left under their control 

in normal use of the application. Recommendations are given for a select set of 

applications, namely:

Section 2 – Public Key Infrastructures (PKI)

Section 3 – Internet Protocol Security (IPsec)

Section 4 – Transport Layer Security (TLS)

Section 5 – Secure/Multipurpose Internet Mail Extensions (S/MIME)

Section 6 – Kerberos

Section 7 – Over-the-Air Rekeying of Digital Radios (OTAR)

Section 8 – Domain Name System Security Extensions (DNSSEC)

Section 9 – Encrypted File Systems (EFS)

Section 10 – Secure Shell (SSH)

The following is provided for each topic:

• A brief description of the system under discussion that is intended to provide 

context for the security guidance,

• Recommended algorithm suites and key sizes and associated security and 

compliance issues,

• Recommendations concerning the use of the mechanism in its current form for the 

protection of Federal Government information,

• Security considerations that may affect the security effectiveness of key 

management processes,

• General recommendations for purchase decision makers, system installers, system 

administrators and end users.

Following Section 10 are five appendices with a glossary, an explanation of acronyms, 

basic information for novice and end users on obtaining and using keys, references for 

documents cited herein, and changes incorporated into this revision.

This document does not reflect a comprehensive view of current products and technical 

specifications. Future versions of this document will include updates to the topics 

covered, and may include additional subjects as new techniques are widely implemented.
2

1.1 Purpose

Part 3 of the Recommendation for Key Management, Application-Specific Key 

Management Guidance, is intended to address the key management issues associated 

with currently available cryptographic mechanisms. General Guidance, Part 1 of the 

Recommendation for Key Management, contains basic key management guidance for 

users, developers and system managers regarding the “best practices” associated with the 

generation and use of the various classes of cryptographic keying material [SP 800-57 

Part 1]. General Organization and Management Requirements, Part 2 of the 

Recommendation, provides a framework and general guidance to support establishing 

cryptographic key management within an organization, and a basis for satisfying the key 

management aspects of statutory and policy-based security planning requirements for 

Federal Government organizations.

This document, Part 3 of the Recommendation, is designed for system installers, system 

administrators and end users of existing key management infrastructures, protocols, and 

other applications, as well as the people making purchasing decisions1for new systems 

using currently available technology. Note that end users who act as their own system 

installers, administrators and purchasing agents may find the guidance intended for 

administrators, installers and purchasers to be beneficial. In centrally managed 

organizations, the organization’s management must establish a security policy that acts as 

a foundation for all end-user guidance.

Recommendations are made for mechanisms designed to protect stored data and data in 

transit. This document will not provide a complete restatement of existing standards or 

implementation directives. Standards and guidelines with this level of detail are 

referenced where appropriate. 

For each of the key management infrastructures, protocols, and applications addressed in 

Part 3, the following is provided:

 A brief description of the system under discussion that is intended to provide 

context for the security guidance provided,

 Recommended algorithm suites and key sizes, and associated security and 

compliance issues,

 Recommendations concerning the use of the mechanism in its current form for the 

protection of Federal Government information, 

 Security considerations that may affect the security effectiveness of key 

management processes, and

 General recommendations for people making purchasing decisions, system 

installers, system administrators and end users.

The logistics of how one should obtain, store or transfer keys or key pairs within a given 

application or system are application and implementation-specific and beyond the scope 

 

1 This is not necessarily a procurement officer, but likely a person making the decision on the IT product to 

be used.
3

of this document. In large federal systems, these functions are frequently handled by 

system administrators or completed with direct guidance from system administrators. For 

end users faced with these tasks on their own, an informative appendix has been included 

with general information intended to point the end user in the right direction.

Since some of the infrastructures, protocols and applications addressed in this 

Recommendation will be refined or replaced over time, the guidance provided herein will 

become obsolete. Similarly, it is anticipated that new infrastructures, protocols, and 

applications will be developed. Although this document will be updated as mechanisms 

and techniques evolve, it may not always reflect a comprehensive view of current 

products and technical specifications. Hence, references to version numbers or other 

implementation status information are provided to enable an evaluation of the 

applicability of particular elements of guidance to the specific version of an 

infrastructure, protocol, or application into which a mechanism is integrated.

Note that many of the applications described in Part 3 are currently in use by U.S. 

government agencies. Some of these applications were developed and implemented prior 

to the release of Part 1 of this Recommendation, and therefore, may not follow all of the 

principles identified in Part 1 [SP 800-57 Part 1]. The use of current implementations of 

these applications may be necessary until more carefully designed applications are 

available. It is very important that each implementation that does not comply with NIST 

standards and guidelines be evaluated for associated risks and that steps be taken to 

mitigate those risks as discussed in this Recommendation. 

1.2 Requirement Terms

This Recommendation often uses “requirement” terms; these terms have the following 

meaning in this document:

1. shall: This term is used to indicate a requirement of a Federal Information 

Processing Standard (FIPS) or a requirement that must be fulfilled to claim 

conformance to this Recommendation. Note that shall may be coupled with not to 

become shall not.

2. should: This term is used to indicate an important recommendation. Ignoring the 

recommendation could result in undesirable results. Ignoring recommendations to 

accommodate the acceptance of messages protected with commonly used, 

unapproved cryptography may create interoperability issues. Ignoring 

recommendations to select new products with approved, seldom-used 

cryptographic mechanisms may leave an organization ill-prepared to migrate 

away from mechanisms that will soon be inappropriate for the protection of 

federal systems. Note that should may be coupled with not to become should 

not.

1.3 General Protocol Considerations

There are a number of general issues associated with the protocols discussed in Part 3.

Four of these issues are briefly discussed in order to familiarize the reader with concepts 
4

that will be repeated throughout the document and to help frame the upcoming 

discussions: 

 Mandatory-to-implement vs. optional-to-implement,

 Cryptographic negotiation,

 Single or multi-use keys, and

 Algorithm and key size transitions.

1.3.1 Mandatory-to-Implement versus Optional-to-Implement

Many of the cryptographic security services described in this document are based on 

public standards (e.g., Internet Engineering Task Force (IETF) Requests for Comment 

(RFCs), American National Standards, etc.). In these standards, algorithms are frequently 

described as mandatory-to-implement or optional-to-implement. Neither of these terms 

provides information about the security of the algorithm.

Mandatory-to-implement algorithms will be in any product that meets the public 

standards, allowing interoperability between products. 

Optional-to-implement algorithms tend to be next-generation algorithms that may 

become mandatory-to-implement algorithms in a future version of the standard. There 

could be considerable delay in the widespread use of these new algorithms for a variety 

of reasons, ranging from a need for supporting hardware or software upgrades, to issues 

of interoperability. For example, an algorithm that is optional-to-implement within an 

S/MIME protocol may not currently be supported by the system’s cryptographic module. 

However, these algorithms often offer improved security that could significantly increase 

the longevity of the system. Therefore, one may want to consider buying products that 

support the optional-to-implement algorithms, even if those algorithms will not be 

available to all end users immediately. 

As previously defined, the terms shall and should are used to provide information about 

whether algorithms have adequate security for use on federal computer networks. As 

such, there may be mandatory-to-implement algorithms that do not provide adequate 

security (e.g., Data Encryption Standard (DES) or RC2), and this document will say they 

shall not be used. Similarly, there may be optional-to-implement algorithms that have 

greater security (e.g., Advanced Encryption Standard (AES)), and this document may say 

that these algorithms should or shall be used in a given situation.

The distinction between mandatory-to-implement and optional-to-implement is important 

when two users on different systems desire to communicate or when different levels of 

security may be required for different applications running on the same system. This is 

further discussed in the next section on cryptographic negotiation.

1.3.2 Cryptographic Negotiation

Parts 1 and 2 of this Recommendation establish a sound basis for selecting appropriate 

cryptographic algorithms and managing the corresponding cryptographic keys. However, 

enforcing these guidelines can be problematic for a number of reasons, including the 
5

unavailability of certain algorithms or key sizes, the preferences of the communicating 

parties or other system limitations. When servers dictate the algorithms used, the server 

may select the algorithms that optimize overall system performance, rather than the ones

that provide the highest level of security. 

In some multi-party protocols where multiple algorithms are supported for the same 

purpose, a client can enforce the rules in Parts 1 and 2 through negotiation within the 

protocol. Some protocols (e.g., S/MIME) allow the initiating client to select the 

cryptographic algorithms without negotiating with the receiving client. In this case, as in 

the case where applications do not permit negotiation, a receiving client may be presented 

with information that has been inadequately protected. For example, a receiving client 

may receive a signed and encrypted S/MIME e-mail message that was encrypted using 

DES and signed with a 512-bit RSA key2. Rejecting such messages does not necessarily 

enhance security (in this case, the message has already been sent over the Internet), but 

the receiving user should be aware that the security services purportedly provided by the 

digital signature and content encryption are suspect and cannot be depended upon. It may 

be appropriate to reject the message or terminate the protocol. A risk assessment and 

subsequent organizational policy may be required to determine the appropriate course of 

action.

In other protocols (e.g., Transport Layer Security (TLS)), the client proposes a set of 

options, and the server chooses from the proposed list during a negotiation phase of the 

protocol. Where negotiation is supported, protocols may be designed to negotiate cipher 

suites or to negotiate each algorithm independently. In either case, a client or server may 

be faced with a situation where the preferred algorithms of the client or server and the 

proposed algorithms of the other party are not of the same security strength, or where 

approved algorithms are not available.

Another issue may arise when a protocol is designed to negotiate algorithms, but not key 

sizes. In such a case, the clients may find themselves communicating with approved 

algorithms, but inadequate key sizes. For example, after negotiating for RSA signatures, 

the client might get a message signed with a 512-bit RSA key3.

Enforcing the recommendations from Parts 1 and 2 may also be complicated by system or 

application design decisions. Systems may have application-specific controls for 

cryptographic algorithms, or they may have system-wide controls. For example, a user 

may wish to restrict one application to using AES, and another to using TDEA, while the 

system design may only allow the use of TDEA. Often the only limitation on public key 

sizes is an indirect limitation through the choice of root Certificate Authority (CA) keys

(see Section 2.1).

When there are a variety of algorithms or key sizes available for a given communication 

protocol, the following questions need to be addressed:

 

2 The DES algorithm and the 512-bit RSA key size do not provide adequate security (see [SP 800-57 Part 

1]).

3 A 512-bit RSA key does not provide an acceptable security strength (see [SP 800-57 Part 1]).
6

 Is negotiation mandatory, optional or unsupported? 

 When negotiation is supported, who proposes the cryptographic mechanisms to be 

used, who selects the mechanisms, and what are the selection criteria?

 Is negotiation based on predefined cipher suites, or is each algorithm proposed 

independently?

 What is the granularity for the negotiation: just algorithms, both algorithms and 

key sizes, combinations of algorithms and/or key sizes, or protocol versions?

 What cannot be specified?

A good start at ensuring communication security in a multi-algorithm setting would be to:

 Limit the list of algorithms available to the application to those best suited for 

users of the system and those needed for interoperability,

 Adopt a policy that disallows sending messages using an inadequate level of 

protection,

 Adopt a policy explaining how to respond to messages received without adequate 

protection, and

 Adopt a policy explaining what to do when faced with a need for secure 

communications with a party using un-approved algorithms or inadequate key 

sizes.

1.3.3 Single or Multi-Use Keys

A major thrust from Part 1 of this Recommendation is that, in general, keys shall not be 

used for multiple cryptographic purposes. For example, the same key shall not be used to 

generate a digital signature and to establish other keying material (see [SP 800-57 Part 1, 

Sec. 8.1.5.1.1.2 ] for the rare exceptions to this guidance). It is less clear as to whether a 

digital signature key, for example, can be used only for a specific application (e.g., 

signing e-mail) or for multiple applications (e.g., for both signing e-mail and signing 

documents). In some cases, it may be acceptable for an application to share keys with 

other applications. In other cases, sharing keys may not be desirable. For example, best

practices indicate that a server’s TLS keys should not be used to support other 

applications. Even where keys are used to perform the same cryptographic operation 

(e.g., digital signatures), sharing keys may be inappropriate because one application 

could be providing one service (e.g., authentication), while a second application could be 

providing a different service (e.g., non-repudiation). It is important to remember that it 

may be a bad idea to use keys for multiple applications.

An agency should perform a risk assessment when considering the use of the same key 

for multiple applications.

1.3.4 Algorithm and Key Size Transition

Part 1 of this Recommendation provides timeframes for transitioning from algorithms 

and key sizes currently in use by many applications and protocols in order to increase the 
7

strength of the security mechanisms in the future [SP 800-57 Part 1]. In many cases, the 

algorithms and key sizes required to provide adequate security are not available within 

the current implementations or are unavailable uniformly across the community of users 

that need to interoperate. Transitions to new algorithms or key sizes will not necessarily 

occur instantaneously, but will require gradual upgrades across a system. For example, a 

system owner may have the need to upgrade his system’s e-mail package before 

upgrading the cryptographic module. Hence, for a period of time, the system may be 

running with an e-mail package capable of TDEA and AES 128 encryption, but a 

cryptographic module that can only handle TDEA. There will be a need to upgrade 

components of a system with new capabilities, while continuing to support the old 

capabilities, until all components have been upgraded.

During this transition period, interaction between components can proceed in one of the 

following ways:

1. Some means is provided to determine when the new security mechanism is 

available to all parties in a given transaction so that it can be used instead of the 

old security mechanism (e.g., using a protocol that negotiates the security 

mechanisms to be used). When the new security mechanism is not available to all 

parties involved in a transaction, the old security mechanism can be used. This 

approach has the advantage that when a set of parties have the newer mechanism, 

their transactions are protected at a higher security level. The disadvantage is that 

those transactions using the old security mechanism are not as well protected; this 

also raises the possibility that the same information could be sent in different 

transactions between two or more sets of parties using security mechanisms of 

different strengths – in effect, nullifying the higher security strength provided by 

the new security mechanism4. 

2. All components use the old security mechanism until all components have been 

updated; at that time, the system immediately transitions to the new capability. 

This approach has the potential problem that all components would not be 

updated by the deadline, thus providing inadequate protections for all information 

during the period following the deadline until such time as all components have 

been upgraded. However, this approach has the advantage that the same data will 

not be sent at two different security levels.5

Most of the applications and protocols discussed in Part 3 require an upgrade of the 

available security mechanisms to be compliant with Part 1. The following sections 

provide guidance on how the existing mechanisms may best be used until appropriate 

upgrades can be made. Organizations and system administrators must determine the 

approach for transitioning to stronger security mechanisms within a system.

 

4 This becomes an issue when higher security-level users are unaware that others may be using a lower 

security-level mechanism to protect the same information.

5 Assuming that data sent before the transition is not also sent after the transition.
8

2 Public Key Infrastructure (PKI)

2.1 Description

A PKI is the most common key management approach for the distribution of public keys. 

As described in SP 800-57 Part 1, Recommendation for Key Management, Part 1: 

General [SP 800-57 Part 1], public keys are used to establish security services after 

obtaining a variety of assurances: assurance of integrity, assurance of domain parameter 

validity (where appropriate), assurance of public key validity, and assurance of private 

key possession. In most cases, applications must also establish the identity of the user 

associated with this key pair. In a PKI, the infrastructure establishes the user’s identity 

and the required assurances to provide a strong foundation for security services in PKIenabled applications and protocols, including IPsec (Section 3), Transport Layer Security 

(Section 4), S/MIME secure e-mail (Section 5) and some versions of Kerberos (Section 

6). This section presents basic guidance for PKI-based key management. For broader and 

more detailed information on PKI, see [SP 800-32].

Public key certificates bind two names to a public key, the user’s name and the issuer’s 

name, using a digital signature generated by the issuer. The user is the party authorized to 

use the private key associated with the public key in the certificate. The issuer is a trusted 

third party that generates and signs the certificate after verifying: the identity of the user; 

the validity of the public key, associated algorithms and any relevant parameters; and the 

user’s possession of the corresponding private key. The issuer is known as a Certificate 

Authority (CA). In many cases, the CA will delegate responsibility for the verification of 

the subject’s identity to a Registration Authority (RA). The certificate is used to distribute 

the user’s public key to other interested parties, known as relying parties, since they rely 

on the assurances provided by the PKI and the certificate creation process.

CAs generally issue a self-signed certificate called a root certificate (sometimes also 

called a trust anchor); this is used by applications and protocols to validate the 

certificates issued by a CA. CA certificates play a key role in many protocols and 

applications, and are generally kept in what is often called a root certificate store. Much 

of the business of properly configuring applications and protocols consists of ensuring 

that only appropriate root certificates are loaded into the root certificate store. In 

Microsoft Windows operating systems, there are root certificate stores that are 

maintained by the operating system for various purposes that are shared by various 

Microsoft protocols and applications, and by other applications that may choose to use 

them. There is a similar “Keychain” facility in the Apple operating systems. Some 

applications, intended to be portable between operating systems, can maintain their own 

root certificate stores and also have a feature that allows them to share a root certificate 

store with other applications.

6

 

6 The various Mozilla browsers and e-mail clients, and the Apache web servers are examples. Microsoft 

Internet Explorer, Outlook and Internet Information Server all use the Windows root certificate store; 

Apple Safari and Mail use the Keychain; and Mozilla Firefox, Thunderbird and SeaMonkey all have their 

own root certificate stores, and they also can share a root certificate store from Mozilla’s Network Security 

Services (NSS) utility.
9

Certificates are generally issued in accordance with a certificate policy. Generally that 

policy can be found on the issuing CA’s website. If an organization’s policy, for 

example, is to accept only certificates that use at least 2048-bit RSA, 2048-bit DSA or 

224-bit elliptic curve cryptography, and either, SHA-224 or SHA-256, then the only 

practical way to ensure that public key sizes meet the requirements is usually to ensure 

that the root certificate store contains only root certificates with a certificate policy that 

requires these algorithms and key sizes in its subordinate certificates. Current 

applications that use PKI will check to ensure that a certificate has been issued under the 

root certificate in the application’s root store, and that it has not been subsequently 

revoked, but will not otherwise check the suitability of the public key or hash algorithms 

used in the certificate – the application will simply use the specified keys to compute the 

mathematically correct results. So, correctly configuring root certificate stores is a critical 

step in key management.

The specifics of where the root certificate store is located and how it is managed for each 

application and protocol are beyond the scope of this Recommendation. Typically, 

however, there are menus for viewing and managing certificate stores in the browser 

applications, but this is subject to change with each product update. There may also be 

utilities and features in the operating system or application for centralized management 

by the system administrator. When a browser or other application encounters an 

unrecognized CA certificate, end users may be prompted to add that certificate to their 

permanent trusted certificate store, temporarily trust the certificate, or reject the 

certificate and close the application.

The most common certificate format is the X.509 version 3 (X.509v3) certificate [RFC 

5280]. In addition to the user and issuer names and the public key, all X.509 certificates 

also include a digital signature, validity (starting and expiring times), and identifiers that 

specify the cryptographic algorithm(s) to be used with the public key and signature.

X.509v3 certificates include an extensibility feature; CAs usually include standard 

extensions in their certificates to indicate which cryptographic operations the public key 

was intended to support, the policy that governed certificate issuance, and where to find 

out if the certificate has been revoked (i.e., an authoritative source for certificate status 

information). CAs may also include “private” extensions in their certificates that contain 

information particular to an application or domain of users.

A relying party is an individual or organization that relies on the certificate and the CA 

that issued the certificate to provide valid information (see Appendix A). Before a relying 

party uses the public key in a certificate, he must determine whether the key used by the 

issuer to sign this certificate can be trusted. In the simplest case, the relying party knows 

about the issuer, and has already decided to trust certificates issued by that CA. CAs that 

a relying party trusts directly are called trust anchors. When multiple trust anchors are 

recognized, the set of trust anchors is referred to as the trust list.

In some cases, a relying party will wish to process user certificates that were signed by 

issuers other than a CA in its trust list. To support this goal, CAs issue cross certificates 

that bind another issuer’s name to that issuer’s public key. Cross certificates are an 

assertion that a public key may be used to verify signatures on other certificates. A 

relying party may be able to develop a certification path – a sequence of certificates –
10

demonstrating that a user’s public key certificate can be trusted, even though it was 

issued by a CA that is not in the relying party’s trust list. All certification paths begin 

with a trust anchor, include zero or more intermediate certificates, and end with the 

certificate that contains the user’s public key.

The entire path must be examined to ensure that the certificates have not been revoked, 

were issued under appropriate policies, and that each public key is suitable for the use to 

which it has been put. This process is known as path validation. 

As noted above, the certificate itself will usually include a pointer to an authoritative 

source for certificate status information. Certificate status information may be provided 

using one of two standard mechanisms:

 An Online Certificate Status Protocol (OCSP) responder [RFC 6960]. An OCSP 

responder is a trusted system, and provides signed status information for a given 

CA, on a per certificate basis, in response to a request from a relying party.

Relying parties can authenticate the response by verifying the OCSP responder’s 

digital signature. As the OCSP responder is providing authoritative status 

information, there shall be a formal relationship between the CA and OCSP 

responder (e.g., a contract).

 An alternative method is to use a certificate revocation list, or CRL. An X.509 

CRL contains a list of certificates issued by a CA that have been revoked, 

indicates when they were revoked, and may include the reason for revocation 

[RFC 5280]. If the serial number of an unexpired certificate does not appear on 

the CRL, then it is still valid. CRLs are digitally signed, like a certificate, so they 

can be distributed through untrusted systems. Most commonly, CRLs are 

distributed via LDAP7directories or web servers. The distribution of CRLs by 

web servers has become more common recently.

In many cases, PKIs will also provide key recovery services (using Recovery Servers) to 

support business continuity. Key recovery services store private keys that support key 

establishment to ensure that the plaintext of encrypted data may be recovered in the 

future. These services can provide the private key to the user in the event of loss or 

failure of their cryptographic module, or to the user’s management when policy or legal 

requirements exist. When supported, this service removes a key management burden 

from PKI-enabled applications. A PKI should include a key recovery services system.

This section provides guidance for general-purpose PKIs when users from different 

organizations need to support a variety of applications. For large, general-purpose PKIs, 

interoperability is an important consideration. Less commonly, PKIs may be deployed to 

support a small, closed community of users or for a single application, where wider 

interoperability is less important. The requirements within this section are focused on 

 

7 Lightweight Directory Access Protocol (LDAP) is a software protocol for enabling anyone to locate 

organizations, individuals, and other resources, such as files and devices in a network, whether on the 

Internet or on a corporate intranet. See [RFC 4511] Lightweight Directory Access Protocol (LDAP): The 

Protocol and [RFC 4512] Lightweight Directory Access Protocol (LDAP): Directory Information Models.
11

large, general-purpose PKIs, such as the Federal PKI. For PKIs requiring less 

interoperability, these requirements should be evaluated for appropriateness within their 

systems. In general, cryptographic algorithm and key size standards should be met by all 

PKIs.

2.2 Security and Compliance Issues

2.2.1 Recommended Key Sizes and Algorithms

Table 2-1 below summarizes the recommended key sizes for key pairs used by PKI users 

and infrastructure components. The PKI uses the term digital signature key to refer to a 

private signature key or public signature verification key (as defined in Part 1) that 

provides a non-repudiation service. The term authentication key is used by the PKI to 

refer to a private authentication key or public authentication key as defined in Part 1. 

Note that both a digital signature key and an authentication key are used with a digital 

signature algorithm.

The dates in this table are consistent with those that appear in Part 1, where:

 A digital signature key is as defined above.

 A key establishment key is an asymmetric key pair used to provide key agreement 

or key transport, and

 A CA and OCSP responder signing key is an asymmetric key pair used to sign 

and verify certificates.

Approved algorithms and key sizes specified in Table 2-1 and certificate expiration dates

are two different things. These algorithms and key sizes are approved for use beyond the 

year 2013. However, a digital signature or key establishment certificate may expire at any 

time, depending on the organization’s security policy.
12

Table 2-1: Recommended Algorithms and Key Sizes

Key Type Algorithms and Key Sizes

Digital Signature keys used 

for authentication

(for Users or Devices)

RSA (2048 bits)

ECDSA (Curve P-256)

Digital Signature keys used 

for non-repudiation

(for Users or Devices)

RSA (2048 bits)

ECDSA (Curves P-256 or P-384) 

CA and OCSP Responder 

Signing Keys

RSA (2048 or 3072bits)

ECDSA (Curves P-256 or P-384)

Key Establishment keys

(for Users or Devices)

RSA (2048 bits)

Diffie-Hellman (2048 bits)

ECDH (Curves P-256 or P-384)

Note that some approved algorithms and key sizes, such as DSA 2048, are omitted to 

enhance interoperability. RSA and ECDSA, which are included in Table 2-1 above, have 

been widely deployed in PKIs. Therefore, they are recommended for use to enhance 

interoperability. However, DSA (2048 and 3072) as specified in [FIPS 186-4] are 

allowed as long as the required security strength is satisfied. For ECDSA, only the two

elliptic curves listed in Table 2-1 above of the elliptic curves are recommended for use in 

PKIs for digital signatures [FIPS 186-4]. Similarly, Elliptic Curve Diffie-Hellman

(ECDH) is recommended to support key establishment, rather than Elliptic Curve MQV. 

While Table 2-1 is focused on the strength of the public key contained in a certificate, the 

strength of the digital signature on a certificate itself is equally important. The signature 

security strength also reflects the security strength of the hash algorithm, and possibly the 

padding scheme8, in addition to the strength of the private key used to generate the 

signature. Table 2-2 below summarizes the recommended algorithms, key sizes, hash 

functions, and padding schemes for signing certificates and CRLs by CAs, and OCSP 

status messages by OCSP responders.

 

8 RSA has two padding schemes used in the PKI: PKCS #1 v1.5, and PSS. The security strength of a digital 

signature generated using ECDSA is not affected by a padding scheme.
13

Table 2-2: Digital Signature Recommendations for CAs and OCSP Responders

Public Key Algorithms 

and Key Sizes Hash Algorithms Padding Scheme

RSA (2048 or 3072 bits) SHA-256 PKCS #1 v1.5, 

PSS

ECDSA (Curve P-256) SHA-256 N/A

ECDSA (Curve P-384) SHA-384 N/A

User certificates containing RSA or Diffie-Hellman public keys should be signed using 

the RSA signature algorithm. User certificates containing elliptic curve public keys 

should be signed using ECDSA.

Not all combinations of algorithms and key sizes are appropriate for the protection of 

Federal Government information. To enhance interoperability, users should obtain 

authentication, signature, and key establishment certificates with complementary 

algorithms for all public keys.9 For most users, signature and key establishment keys 

should provide the same cryptographic strength. Table 2-3 below shows preferred 

combinations for user keys.

While symmetric key cryptography is not strictly required, block ciphers are used in 

practically all PKI implementations and PKI-enabled applications. All components using 

block ciphers shall support the AES-128 algorithm. To support legacy implementations, 

components that process RSA keys should support three-key Triple-DEA [SP 800-67].

Components that support P-384 elliptic curve keys and the SHA-384 algorithm shall 

support AES-256.

Table 2-3: Recommended Combinations for the 

Recommended Algorithms and Key Sizes

Authentication Key Type Signature Key Key Establishment Key

RSA 2048 RSA 2048 RSA 2048

RSA 2048 RSA 2048 Diffie-Hellman 2048

ECDSA P-256 ECDSA P-256 ECDH P-256

ECDSA P-256 ECDSA P-384 ECDH P-384

ECDSA P-384 ECDSA P-384 ECDH P-384

 

9

In general, protocols and applications are designed to use cryptographic algorithms from one 

mathematical family. For example, applications that encounter certificates with ECDSA digital signatures 

would expect to use elliptic curve Diffie-Hellman for key establishment services. Users that obtain an 

ECDSA certificate (i.e., a certificate containing an ECDSA public key to be used for verifying digital 

signatures), and an RSA key establishment certificate (i.e., a certificate containing an RSA public key to be 

used for key establishment), for example, may find they cannot use the keys together in a single 

application. Other combinations of certificates are commonly used (see Table 2-3). It is advisable that 

users obtain authentication, signature, and key establishment certificates that are complementary to ensure 

that the keys can be used together in applications and protocols.
14

2.3 Procurement Guidance

The following provides guidance for those responsible for making decisions about which 

products to purchase in support of a PKI.

2.3.1 CA/RA Software and Hardware:

1. Ensure that CA and RA software supports at least one of these protocols: the 

Certificate Management Protocol (CMP) [RFC 4210], Enrollment over Secure 

Transport (EST) [RFC 7030] and Certificate Management Using Cryptographic 

Message Syntax (CMC); see [RFC 5272].

2. Ensure that all CAs support the generation of certificates and CRLs that conform to 

[RFC 5280]. (Specific requirements with respect to certificate and CRL extensions 

are detailed below.)

3. Ensure that CAs are capable of issuing multiple certificates to users, and for all such

certificates, asserting the key usage extension, including the Extended Key Usage 

extension.

4. Ensure that CAs are capable of including the CRL distribution points extension.

5. Ensure that CAs support the inclusion of HTTP URLs to specify the location of 

CRLs.

6. CAs should support the inclusion of LDAP to specify the location of CRLs.

7. Ensure that CAs are capable of specifying an authoritative OCSP responder in the 

Authority Information Access extension.

8. Each PKI has its own certificate profile, identifying certificate extensions that appear 

in the certificates and CRLs it issues.10 Ensure that CAs are able to generate all 

mandatory extensions in the appropriate profiles. For CAs owned or operated on 

behalf of federal agencies, the following specific guidance applies:

a) Ensure that CAs that implement federal agency-specific policies are able to 

generate certificates and CRLs that meet the agency profile and the Federal PKI 

Certificate Profile [FPKI PROF].

b) Ensure that CAs that implement the Common Policy Framework [COMMON] are

able to generate certificates and CRLs meeting the Shared Services Certificate 

and CRL Profile [COMMON PROF].

9. CAs should support the inclusion of “private” extensions in certificates and CRLs.11

 

10 This profile is often documented explicitly, but may be implicitly specified through the certificate policy.

11 Private extensions are defined by an organization to meet their own unique requirements. Note that 

noncritical private extensions do not impact the interoperability of certificates or CRLs.
15

10. Ensure that CAs support at least one of the following algorithms for digitally signing 

certificates and CRLs: RSA with PKCS#1 v1.5 padding; RSA with PSS Padding 

[RFC 3447], DSA or ECDSA. To maximize flexibility, CAs should support RSA and 

ECDSA.

12

11. Ensure that CAs include backup and archive capabilities to support reconstitution of 

the CA in the event that the root key is corrupted, destroyed or lost, and it is 

necessary to rebuild the CA using a backup root key, rather than simply recovering

the lost state of the CA. CAs should include backup and archive capabilities in order 

to establish when certificates were issued and revoked, and under whose authority.

12. Ensure that CA/RA components are shipped or delivered via controlled methods that 

provide a continuous chain of accountability, from the purchase location to the CA’s 

or RA's physical location.

2.3.2 OCSP Responders:

1. Ensure that OCSP responders conform to RFC 6960, Online Certificate Status 

Protocol [RFC 6960].

2. Ensure that OCSP responders are capable of processing both signed and unsigned 

requests and are capable of processing requests that either include or omit the name of 

the relying party making the request. However, OCSP responders may ignore 

signatures and requester names, if present.

3. Ensure that OCSP responders are capable of processing certificate status requests and 

generating responses for non-error conditions as specified in [RFC 5019].

4. Where supported, the OCSP responder should sign the OCSP response with the 

algorithm and key size used to sign the certificate. Ensure that OCSP responders 

support at least one of the following algorithms for digitally signing response 

messages: RSA with PKCS#1 v1.5 padding; RSA with PSS Padding, DSA or 

ECDSA. The supported algorithms should include the algorithm(s) used by the 

corresponding CA when signing the certificate whose status is in question. To support 

future algorithm transitions by the CA, OCSP responders should support RSA and 

ECDSA.13

 

12 The algorithm used to sign certificates and CRLs in an operational CA is dependent upon both the 

cryptographic module in use and the CA’s software. The selected algorithm must appear in both sets of 

supported algorithms.

13 As with CAs, the algorithm used to sign responses in an operational OCSP responder is dependent upon 

both the cryptographic module in use and the OCSP responder’s software. The selected algorithm must 

appear in both sets of supported algorithms.
16

2.3.3 Cryptographic Modules

1. Ensure that Cryptographic modules for CAs, Key Recovery Servers, and OCSP 

responders are hardware modules validated as meeting FIPS 140-2 Level 3 or higher

[FIPS 140-2].

2. Ensure that cryptographic modules for RAs are hardware cryptographic modules 

validated as meeting FIPS 140-2 Level 2 or higher [FIPS 140-2].

3. Ensure that relying party and user cryptographic modules are validated as meeting 

FIPS 140-2 Level 1 or higher [FIPS 140-2].

2.3.4 Key Recovery Servers

1. If the PKI supports key establishment (i.e., certificates will include key transport or 

key agreement keys), the PKI should include a key recovery mechanism.

2. Implementations should support automated, user-initiated key recovery; key recovery 

by the organization should also be supported.14

2.3.5 Relying Party Software

1. Relying party path validation

a) Ensure that relying party implementations use RFC 5280-conformant path 

validation [RFC 5280].

b) Where interoperability outside a single organization is required (e.g., a single 

federal agency), path validation modules should conform to requirements for an 

Enterprise path validation module (PVM), as specified in NIST Recommendation 

for X.509 Path Validation [X.509 Path].

c) Where interoperability across organizations is required, path validation modules 

should conform to requirements for a Bridge-Enabled PVM, as specified in the 

draft NIST Recommendation for X.509 Path Validation [X.509 Path].

d) Ensure that relying party implementations support CRLs or OCSP for certificate 

status, and should support both.

2. Building certificate paths

a) Ensure that relying party implementations are able to build certification paths.

b) At a minimum, implementations should support http-based certificate retrieval.

c) Relying party implementations should also be able to obtain CA certificates and 

CRLs using LDAP from an organizationally designated local directory, as well as 

locations specified within a user certificate.

 

14 Organizational key recovery should emphasize security and privacy, rather than performance. Dual 

control for recovery of a user’s keys by the organization is strongly recommended.
17

3. Relying parties that work within a single organizational PKI (e.g., a PKI that supports 

a company or agency) should be able to discover paths for user certificates issued by 

CAs that are hierarchically subordinate to the trust anchor CA.

4. Relying parties that accept certificates from other organizations should be able to 

discover paths in non-hierarchical PKIs.

2.3.6 Client Software

1. Ensure that client implementations support multiple private keys and certificates for 

each end user to support different cryptographic services. For example, the client 

implementation should support and differentiate between private keys associated 

with public keys in certificates supporting digital signatures, and private keys 

associated with public keys in certificates supporting key establishment.

2. Ensure that client cryptographic modules are validated at FIPS 140-2 Level 1 or 

higher [FIPS 140-2].

3. Client implementations should support the certificate management protocol 

supported by the organization’s CA.15

2.4 Recommendations for System Installers/Administrators

The system installer and administrator is the person (or people) who are responsible for 

establishing the PKI and who are responsible for the tasks associated with its day-to-day 

operation. The system administrator shall ensure that end users are trained and that the 

organization’s security policy is enforced.

2.4.1 Certificate Issuance

1. CAs shall be configured to ensure that certificates specify public keys with approved 

key sizes, valid domain parameters (if appropriate), and approved algorithms.

2. For maximum interoperability, CAs and users should use RSA key pairs for digital 

signatures and key transport.

3. For maximum security and performance, CAs and users should use elliptic curve key 

pairs for digital signatures and key agreement.

4. When signing certificates or CRLs, CAs shall generate digital signatures using a 

signing algorithm, hash function and a padding scheme (if the signing algorithm is 

RSA) combination specified in Table 2-2.

 

15 Where keys and certificates are stored on smart cards, and all updates are performed at the RA, user 

implementations need not support the certificate management protocol.
18

5. For digital signature certificates, CAs shall sign the certificate using a digital 

signature process (i.e., signature algorithm, hash function and key) whose security 

strength is equal to or greater than the security strength of the subject public key in 

the certificate. For key establishment certificates, CAs may sign the certificate using a 

digital signature process whose security strength is less than the security strength of 

the subject public key in the certificate16.

6. Generating key pairs:

a) Users should generate their own digital signature key pairs.

b) Key establishment key pairs may be generated by the user or by the PKI on the 

user’s behalf; where required, the PKI that generated a user’s key pair may retain 

copies of the key-establishment private key to permit key recovery.

c) CAs should perform proof of possession for all key pairs before issuing 

certificates.

7. CAs shall obtain assurance of public-key validity before issuing certificates.

8. Key usage extension.

a) All certificates issued shall include the key-usage extension.

b) The key-usage extension shall restrict acceptance of the private key to a single 

cryptographic function: either user/entity authentication and verification of 

committed data, or key establishment.

9. All certificates shall include the CRL distribution-points extension to support the 

retrieval of status information.

10. If an OCSP responder is supported, a certificate shall include an appropriate URL in 

the Authority Information Access extension.

11. Certificates should be renewed before they expire and replaced if there is a change in 

the certificate’s contents, such as the domain name or the embedded e-mail address.

2.4.2 Certificate Revocation Requests

1. CAs should be configured to automate revocation processing where practical:

 

16 A public key certificate used for key establishment involves two keys: the subject (key establishment) 

public key, which is used to establish a symmetric key that will protect data, and the signing key of the 

Certification Authority (CA), which is used to sign the certificate. The CA’s signing key needs to be 

secure only until the key-establishment certificate expires, but the subject (key establishment) public key 

needs to be secure as long as the data must be secure, which may be long after the key establishment 

certificate expiration date. As long as the CA’s signing key is secure during the certificate’s lifetime, and 

the certificate has been securely archived, any break of the CA signing key after the expiration of the 

certificate does not affect the validity of the subject (key-establishment) public key or the security that it 

(the subject public key) can provide. For example, if the security strength of the subject (key establishment) 

public key is greater than that of the CA’s signing key, any break of the signing key after the subject public 

key is signed does not affect the security of that public key. Therefore, it is acceptable for a key transport or 

key agreement subject public key to be stronger than the CA key used to sign a certificate containing the 

key agreement or key transport public key.
19

a) CAs should be configured to authenticate and process revocation requests 

electronically.

b) Where the CA can authenticate a digitally signed request submitted by the user of 

the associated key pair or an RA, the request should be handled without manual 

intervention.

2. RAs should be configured to submit digitally signed revocation requests on behalf of 

users or the organization.

2.4.3 Certificate Revocation List Generation

1. To maximize interoperability, all CAs should be configured to generate full CRLs. A 

full CRL is a single CRL that lists all revoked and unexpired certificates issued by a 

particular CA.

2. CAs that serve a large community should generate CRL distribution points in 

addition to full CRLs. Each CRL distribution point lists a subset of the revoked 

certificates for a given CA. The number of certificates covered by a CRL distribution 

point should be limited to a maximum of 250 000 to ensure that the distribution point 

CRLs do not grow to an unmanageable size.

2.4.4 PKI Repositories for the Distribution of Certificates and CRLs

1. PKIs should be configured to provide certificates and CRLs to requesters without 

authentication of the requester.

2. PKI repositories shall be configured to require authenticated access to modify the set 

of certificates and CRLs distributed by the repository.

3. At a minimum, repositories shall support either HTTP version 1.1 or LDAP version 3 

interface.

4. For maximum interoperability, both HTTP and LDAP should be supported.

5. Replication of repositories (e.g., through directory shadowing or web server 

replication) to maximize availability should be considered.

6. PKI repositories should contain all CA certificates issued by or to the corresponding 

PKI.

7. PKI repositories shall contain all current CRLs.
20

2.4.5 OCSP Responders

For federal agencies, detailed configuration guidance for OCSP responders is specified in 

Draft Guidance for OCSP Responders in the U.S. Federal PKI.

17

1. If maximum interoperability is required then:

a) OCSP responders shall not require that requests be signed and shall not limit the 

set of relying parties to which certificate status information is provided.

b) The responders shall generate OCSP basic responses, and the responses shall not

include critical extensions.

2. Where interoperability requirements are limited to a closed community:

a) OCSP responders may require signed requests, and may reject requests from 

entities outside that community.

b) OCSP response messages may include private extensions known within the target 

community.

2.4.6 Backup and Archive

1. To maintain the availability of status information, CAs shall ensure that sufficient 

information is stored in a secure location to reconstitute the CA after a disaster.

2. CAs should archive sufficient information to establish when certificates were 

issued, and under whose authority.

3. As a general rule, audit logs should be maintained, along with any certificates and 

CRLs issued by the CA.

4. User public signature verification keys should be archived, along with their 

corresponding certificates as long as required.

2.4.7 Relying Party Integration and Configuration

1. Path discovery components shall be configured to enable path discovery and 

require the retrieval of status information.

2. Status information should be accepted in both CRL and OCSP formats.

3. Relying party implementations shall be configured to recognize the smallest set 

of acceptable trust anchors possible.

4. For business-to-government and government-to-government applications, federal 

agencies should use either the Common Policy Root CA or an agency CA that is 

cross-certified with the Common Policy Root CA or the Federal Bridge as the 

trust anchor.

 

17 Available at http://cio.nist.gov/esd/emaildir/lists/pkits/doc00000.doc.
21

5. For citizen-to-government applications with limited security requirements (e.g. 

Level 2 e-Authentication requirements as specified in [OMB 04-04]) and high 

interoperability requirements, agency applications may use the pre-installed trust 

anchors provided in COTS products.

6. Path validation modules:

a) For end-user applications and applications with minimal security 

requirements, path validation modules should be configured to accept any 

valid path.

b) For systems with more significant security requirements (e.g., systems using 

PKI to satisfy Level 3 or Level 4 e-Authentication), path validation modules 

should be configured to only accept paths that are valid under appropriate 

policies.

2.5 User Guidance (Subscribers)

In a PKI, the subject is the identity of the user associated with a public key. The subject 

may be a person or a device. For the purposes of this section, the term “user” is means 

either the person associated with a public key, or the administrator of a device associated 

with a public key.

1. Users should generate their own key pairs for digital signatures and 

authentication.

2. Users may generate their own key pairs for key establishment, or the key 

establishment key pairs may be imported from a trusted source.

3. Users shall protect the authenticators (e.g., the PIN or password) that control 

access to their private keys.

4. Users shall request the revocation of their certificates if they believe the 

authenticator or cryptographic module has been stolen, copied or compromised.

5. Users shall control the disposition of “old” key pairs after certificates expire 

unless otherwise controlled in accordance with federal agency policy and 

procedures.

a) Private signature keys should be destroyed after the corresponding 

certificate(s) expire.

b) Private key establishment keys need not be destroyed after the corresponding 

certificate(s) expire. The user should not destroy the private key 

establishment key until all symmetric keys established using this key have 

been recovered or otherwise protected (e.g., by encrypting under a different 

key). Premature destruction of private key establishment keys may prevent 

recovery of the subscriber’s plaintext data.
22

6. Users shall verify all CRLs before rejecting certificates in the CRLs.
23

3 Internet Protocol Security (IPsec)

3.1 Description

IPsec is a suite of protocols for securing Internet communications at the network layer 

and operates within the Internet Protocol (IP). It is frequently used to establish Virtual 

Private Networks (VPNs)18, requiring both parties to share keying material, and enabling 

telecommuters or travelers to gain secure access to their business networks. IPsec 

provides the cryptographic security functions for both versions 4 and 6 of the Internet 

Protocol.

IPsec operates by inserting one of two special IPsec headers after the IP header in each 

message. The Authentication Header (AH) provides integrity protection. The 

Encapsulating Security Protocol (ESP) Header provides confidentiality and/or integrity 

protection. Hereafter, the terms AH and ESP will be used as shorthand for messages 

using AH and ESP headers, respectively. Both ESP and AH provide data origin 

authentication, and optionally provide replay protection. AH protects the IP header and 

the data following the IP header. ESP, when applied directly to a packet (i.e., in transport 

mode), protects the data, but not the IP header. However, ESP in tunnel mode (with a 

new IP header inserted) does protect the original IP header. Furthermore, using ESP with 

automated keying protects the source and destination addresses in the IP header in either 

transport or tunnel mode. Since AH processing introduces unnecessary complexity, and 

since ESP can provide equivalent functionality, the use of AH is not recommended.

There have been three versions of IPsec.19 All new systems should implement IPsec-v320, 

as it has many enhancements not found in the previous versions. However, IPsec-v2 is 

still implemented in numerous current systems, despite the fact that it is obsolete.21

Two classes of key management methods are specified for IPsec: manual keying and 

automated keying. Manual keying involves an agreement (in an unspecified manner) by 

the parties in a communication on the IPsec protections to be applied and the symmetric 

keys to be used. This has a major downside in that it severely limits the scalability of the 

security solution and requires re-keying to be done in an unspecified manner. A Security 

Association (SA, i.e., a relationship between two or more entities that describes how each 

entity will use the security services to communicate securely) and its secret keys cannot 

be easily renewed in the cases where the SA expires, has been used for the maximum 

allowable volume of traffic, or if its keys are compromised.

To use automated keying, an automated negotiation between peers prior to exchanging 

IPsec-protected traffic determines the IPsec protections to be applied and the symmetric 

keys to be used. The same method can be used to maintain, delete, or renegotiate the SA 

 

18 See SP 800-77, Guide to IPsec VPNs [SP 800-77].

19 There are no generally accepted names for IPsec-v3 and IPsec-v2; these terms are used in this document to make the 

requirements more understandable

20 IPsec-v3 is specified in [RFC 4301], [RFC 4302], [RFC 4303] and [RFC 4835].

21 IPsec-v2 is specified in [RFC 2401], [RFC 2402] and [RFC 2406].
24

(e.g., to rekey). This approach permits a decoupling of the key management mechanism 

from the other security mechanisms, thus facilitating the use of alternative key 

management methods without having to modify other security mechanisms.

The preferred automated keying method is IKE, the Internet Key Exchange protocol that 

was designed specifically for use with IPsec. IKE generates the necessary keying material 

for IPsec via an authenticated secure channel between the two IKE peers. There are two 

versions of IKE in use: IKEv1 ([RFC 2407], [RFC 2408] and [RFC 2409]) and IKEv2 

[RFC 5996]; both versions perform mutual authentication, and establish and maintain 

security associations. SAs will be valid for a specified period of time or volume of traffic. 

IKEv1 is still implemented in numerous current systems, despite the fact that it is 

obsolete. These two versions of IKE are not interoperable. IKEv2 was designed to be 

more reliable and efficient than IKEv1; therefore, IKEv2 should be used.

Table 3-1 provides the IETF reference materials for versions 2 and 3 of IPsec.

Table 3-1: Summary of References for IPsec

Version Security 

Architecture Privacy Authentication Automated Key Management

IPsec-v2 RFC 2401 RFC 2406 RFC 2402, 

RFC 2406

RFC 2407, RFC 2408, 

RFC 2409

IPsec-v3 RFC 4301 RFC 4303 RFC 4302, 

RFC 4303 RFC 5996

The IPsec security mechanisms are not tied to any specific cryptographic algorithms; in 

fact, many algorithms and modes have IETF Requests For Comment (RFCs) describing 

their use with IPsec. This, however, can result in a situation where there are so many 

choices for typical system administrators to make that it is difficult to achieve 

interoperability. To improve interoperability in IPsec-v3, two cipher suites: VPN-A and 

VPN-B were specified [RFC 4308]. However, these two cipher suites are not NISTapproved cipher suites. Four additional cipher suites have been defined in [RFC 6379]: 

Suite-B-GCM-128, Suite-B-GCM-256, Suite-B-GMAC-128 and Suite-B-GMAC-256

and they are NIST-approved.

Implementers may allow the individual selection of security algorithms (i.e., rather than 

selecting one of the pre-specified suites of algorithms) specified in [RFC 6379], but users 

must be aware that picking non-standard groupings of algorithms may result in limited 

interoperability. However, when IPsec is used in the context of a VPN, security policy 

can be centrally managed, thus ensuring interoperability without the use of pre-defined 

cipher suites. Current IETF algorithm guidance is in [RFC 7321].
25

3.2 Security and Compliance Issues

3.2.1 Cryptographic Algorithms

Table 3-2 below gives cryptographic algorithm recommendations for use within IPsec.

The algorithms that are specified for IKE are used to protect IKE’s own traffic. The 

algorithms used in ESP and AH are used to provide IPsec protection to data traffic; for 

these algorithms to be used within ESP or AH, IKE must be capable of negotiating their 

use.

In Table 3-2 below, column four lists the IETF conformance requirements as specified in 

the RFCs by using the three IETF requirement levels: MUST, SHOULD and MAY to 

indicate whether the algorithm needs to be implemented. See [RFC 2119] for definitions 

of these requirement levels and further information on IETF Conformance language.

Column five, however, states federal conformance requirements using two levels: 

Mandatory and Optional. Mandatory means that the feature is required to be available in 

an implementation, and Optional means that implementation of the technique is 

permitted.

Table 3-2: Cryptographic Algorithm Recommendations

Protocol Cryptographic 

Service Algorithm/Mode

IETF

Requirement 

RFC 7321

Federal 

Requirement

ESP Encryption

TDEA in CBC 

mode MAY

Optional; if used,

TDEA shall use 

three distinct 

keys

ESP Encryption

AES with 128-bit 

keys in CBC mode MUST Mandatory

ESP Encryption

AES-128 in 

Counter mode MAY

Optional; if used, 

shall be used 

with integrity 

protection 

ESP or AH Integrity 

Protection

HMAC SHA1-96

(key strength shall

be equal to or 

greater than 112 

bits)

MUST Mandatory
26

Protocol Cryptographic 

Service Algorithm/Mode IETF Requirement

Federal 

Requirement

ESP or AH Integrity 

Protection

HMAC SHA-256-

128

(key strength shall

be equal to or 

greater than 112 

bits)

[RFC 4868]

SHOULD Optional

ESP

Encryption and 

Integrity 

Protection

AES-128 in 

Galois/Counter 

Mode

[RFC 4106], 

[RFC 4835] Optional

ESP

Encryption and 

Integrity 

Protection

AES-128 in 

Counter mode with 

CBC-MAC

[RFC 4309], 

[RFC 4835]

MAY

Optional

ESP or AH Integrity 

Protection

AES-128 in GMAC 

Mode [RFC 4543]Optional

IKEv1 or

IKEv2 Encryption TDEA in CBC mode MAY

Optional; if used,

TDEA shall use 

three distinct 

keys

IKEv1 or

IKEv2 Encryption AES-128 in CBC mode MUST Mandatory

IKEv1 or

IKEv2

Pseudo-random 

function HMAC-SHA1 

[RFC 4109], 

[RFC 4307]

MUST

Mandatory

IKEv1 or

IKEv2

Pseudo-random

Function

HMAC-SHA-256 [RFC 4868]

SHOULD

Optional

IKEv1 or

IKEv2

Diffie-Hellman

Group 24

2048-bit MODP [RFC 5114] Mandatory
27

Protocol Cryptographic 

Service Algorithm/Mode IETF Requirement

Federal 

Requirement

IKEv1 or

IKEv2

Diffie-Hellman

Group 14

2048-bit MODP [RFC 4109], 

[RFC 4307]

SHOULD

Optional

IKEv1 or

IKEv2

Elliptic Curve 

Diffie-Hellman P-256 or P-384

[FIPS 186-4], 

[RFC 5903]

MAY

Should

IKEv1 or

IKEv2 Integrity

HMAC-SHA1-96

(key strength shall

be equal to or 

greater than 112 

bits)

[RFC 4109], 

[RFC 4307]

MUST

Mandatory

IKEv1 or

IKEv2 Integrity

HMAC-SHA256-

128

key strength shall

be equal to or 

greater than 112 

bits)

[RFC 4868]

SHOULD Optional

IKEv1 Peer 

Authentication

2048-bit RSA with 

SHA256

[RFC 4109]

SHOULD Mandatory

IKEv1 Peer 

Authentication DSA with SHA256 [RFC 4109]MAY Mandatory

IKEv2 Peer 

Authentication

2048-bit RSA with 

SHA256

[RFC 5996]

MUST Mandatory

IKEv2 Peer 

Authentication DSA with SHA256 [RFC 5996]MAY Optional

IKEv1 or

IKEv2

Peer

Authentication

ECDSA-256 or 

ECDSA-384

[RFC 4754]

MAY Should

In [RFC 4835] and [RFC 2410], ESP provides options for NULL integrity protection or 

NULL encryption, which means that either no integrity protection would be applied or no 

encryption would be used, respectively; however, [RFC 4835] specifies that at least one 

of them MUST be applied. In the RFCs, NULL integrity protection (often referred to as 

NULL authentication) is intended for use in situations where confidentiality is required 

without the need for integrity protection. NULL integrity protection shall not, and in fact 

cannot, be used with NULL encryption. ESP, for example, could send unencrypted 

packets, (encryption set to NULL), but would be required to integrity-protect them, for 

example, by using HMAC-SHA1. On the other hand, ESP could send packets encrypted 

with AES-128 in CBC mode, but omit the integrity check (integrity protection set to 

NULL).
28

However, to be compliant with this Recommendation, IPsec ESP-protected traffic shall

always be integrity-protected, either through the use of an integrity-protection algorithm,

such as HMAC-SHA1-96, or through the use of a combined-mode algorithm, such as 

AES-128 in Galois/Counter Mode. Therefore, encrypted ESP shall not be used with 

NULL integrity-protection.

When IPsec-protected traffic is integrity-protected, an Integrity Check Value (ICV) is 

stored in the Integrity Check Value field of the ESP payload [RFC 4303] or in the 

Integrity Check Value field of the Authentication Header (AH) [RFC 4302]; this field is 

referred to as the “Authentication Data” field in IPsec-v2 ([RFC 2406], [RFC 2402]).

When HMAC is used for integrity protection, the length of the ICV value is at most the 

size of the output value of the hash function. For example, the ICV value for HMACSHA1-96 and HMAC-SHA256-128 is 96 and 128 bits, respectively [RFC 4868].

Although the IETF still recommends supporting AES-XCBC-MAC ([RFC 3566], [RFC 

4434], [RFC 7321]), it is not approved for use by the Federal Government. As such, 

AES-XCBC-MAC shall not be used for integrity protection.

A new class of algorithms, called combined-mode algorithms, appears in the above table.

They can be negotiated by IKE and used within IPsec-v3 cipher suites. These algorithms 

provide both encryption and integrity protection. Two combined-mode algorithms have 

been approved for Federal Government use: AES in Galois/Counter Mode (GCM) [RFC 

4106] and AES in counter mode with CBC-MAC (AES-CCM) [RFC 4309].

There is also a variant of AES-GCM, referred to as AES-GMAC [RFC 4543], that

provides integrity protection, but does not provide encryption. This mode may be used 

within either the ESP or AH header.

The maximum size of the ICV for AES-CCM, AES-GCM and AES-GMAC is 16 bytes.

Implementations shall support an ICV size of 16 bytes for these three algorithms ([RFC 

4309], [RFC 4106], [RFC 4543]).

AES-GCM, AES-CCM, AES-CTR, and AES-GMAC shall not be used with manually 

distributed keys. If the counter value, in AES-CTR or AES-CCM, or the IV value, in 

AES-GCM or AES-GMAC, is used for more than one packet with the same key, the 

security of the algorithm’s confidentiality mechanism is compromised. Since manual 

keying presents a major challenge to this limit, manually distributed keys shall not be 

used with these algorithms. Automated keying using IKE establishes secret keys for the 

two peers within each Security Association, with an extremely small probability of 

duplicate keys.

In previous IETF guidance, single DES using the CBC mode [RFC 2405] was 

mandatory-to-implement; however, this algorithm shall not be used to protect 

information.
29

IPsec allows the individual selection of security algorithms. As an example, an 

implementer using Table 3-2 and following the guidance of Part 1 [SP 800-57 Part 1], 

could select the following algorithms to form an IPsec suite with an overall security 

strength of 112 bits:

 ESP Encryption: AES in CBC mode

 ESP Integrity Protection: HMAC-SHA1

 IKEv2 Encryption: AES in CBC mode

 IKEv2 Pseudo-random function: HMAC-SHA1

 IKEv2 Diffie-Hellman group: 2048-bit MODP

 IKEv2 Integrity: HMAC-SHA1

 IKEv2 Peer Authentication: 2048-bit RSA with SHA-256.

The Suite-B-GCM-128 and Suite-B-GCM-256 suites are both defined in [RFC 6379]. At 

present, these cipher suites are not widely available or deployed. [SP 500-267] states that 

support for these cipher suites is optional. However, wherever practical, implementations 

should be procured that support these cipher suites, and they should be selected for use 

wherever very high performance and security strength are required. As discussed above, 

AES-GCM is a combined-mode algorithm that provides both encryption and integrity 

protection; therefore these suites provide integrity, despite the fact that the integrity 

mechanism is listed in [RFC 6379] as NULL for both suites.

3.2.2 Additional Recommendations

1. The Authentication Header (AH) should not be used in IPsec version 3.

2. IKE should be used for automated key management to ensure a re-keying capability 

and scalability.

3. Once an ESP Security Association has expired or is no longer in use, its ESP 

encryption keys shall continue to be protected by the system and kept secret as long 

as the data they were used to protect needs to be kept secret.

3.3 Procurement Guidance

These recommendations are written to assist individuals responsible for selecting security 

products that include IPsec for the security of the IP layer.

1. Any IPsec system for use within the Federal Government should include an IKE 

implementation for automated key management.

2. Ensure that IPsec implementations include approved algorithms for each IPsec 

security component.
30

3. IPsec implementations should include the algorithms used in the Suite B cipher 

suites.

3.4 Recommendations for System Installers

Systems installers are those individuals that install products that include IPsec for 

security.

1. IKE should be used for automated key management within any IPsec system.

2. NULL encryption shall only be employed when integrity protection is required, but 

confidentiality is not needed.

3. Installers shall select approved algorithms for each security component, as specified 

in Section 3.2.

3.5 Recommendations for System Administrators

System administrators are those individuals responsible for the day-to-day functioning of 

the security product containing IPsec. System administrators shall:

1. Ensure that end users are properly trained and that the organization’s security policy 

is enforced.

2. Ensure that a key used by the product is protected throughout its lifespan.

3.6 Recommendations for End Users

An end user is the individual using a product that relies on IPsec for security. End users 

shall:

1. Be aware of and trained to follow the organization’s security policy for using the 

product.

2. Operate their system as instructed by their organization and system administrator.

4 Transport Layer Security (TLS)

This section was moved to SP 800-52 Revision 1, Guidelines for the Selection, 

Configuration, and Use of Transport Layer Security (TLS) Implementations [SP 800-52].
31

5 Secure/Multipart Internet Mail Extensions (S/MIME)

5.1 Description

Secure/Multipurpose Internet Mail Extensions (S/MIME) provides a consistent way to 

send and receive secure Internet mail. S/MIME is a set of specifications that are defined 

by a series of IETF RFCs, namely [RFC 5751], [RFC 5652], [RFC 2045], [RFC 2046],

[RFC 2047], [RFC 4288], [RFC 4289] and [RFC 2049]. S/MIME provides the following 

cryptographic security services for electronic messaging applications:

 Authentication of a sending party using digital signatures,

 Message integrity and non-repudiation of origin using digital signatures, and

 Confidentiality using encryption.

S/MIME, therefore, requires a suite of algorithms for creating digital signatures, 

generating hash values, establishing keys and encrypting the content of the e-mail, as 

well as some means of establishing and sharing digital identities. Federal 

implementations rely on a public key infrastructure, specifically X.509 PKI, to establish 

S/MIME user identities, to bind those identities to the user’s public key through public 

key certificates, to provide digital signatures, to provide keys to be used for content 

encryption, or to establish symmetric keys for use on a per message basis. For detailed 

information on PKIs, see Section 2 of this Recommendation.

Stored electronic mail encompasses key management issues associated with encrypted 

file integrity and with transmission over a network. It is therefore necessary 1) to 

establish pair-wise and/or multicast (sent to more than one recipient) key management 

relationships between the sender and receiver(s) and 2) to securely store the key(s) 

associated with encrypted e-mail until it is no longer necessary for a recipient to be able 

to decrypt or verify the integrity of the e-mail.

S/MIME is not restricted to e-mail; it can be used with any transport mechanism that 

employs MIME protocols, such as the Hypertext Transfer Protocol (HTTP).

5.2 Security and Compliance Issues

S/MIME products can be implemented with different combinations of security features 

and a variety of cryptographic algorithms. Senders and receivers may have different 

capabilities and may be sending messages protected with algorithms of different 

strengths. This can lead to numerous interoperability issues. Federal clients using secure 

e-mail shall be able to perform the following:

 Send and receive signed messages,

 Send and receive encrypted messages,
32

 Send and receive signed and encrypted messages,

 Request, send and process signed receipts, and

 Process messages from secure e-mail list clients (includes suppressing receipts, as 

required, and nondisclosure of list recipients, as required).

Furthermore, federal systems shall:

 Utilize cryptographic modules that are FIPS 140-1 or FIPS 140-2 validated [FIPS 

140-2],

 Support cryptographic Cipher Suite 1 (see Table 5-1 below), and

 Support X.509 certificates that conform to Federal PKI X.509 Certificates and the 

CRL Extensions Profile.

Federal clients should be capable of sending and processing e-mail with security labels 

and securely binding senders’ certificates to their signatures through the signing 

certificate attribute as described in [RFC 5035].22

The most widely accepted, standard S/MIME profile is [RFC 5751]. Not all 

cryptographic algorithms available for use in support of the features in the profile are 

appropriate for the protection of Federal Government information. The S/MIME 

specifications allow the selection of individual algorithms. However, a number of cipher 

suites have been specified to define a specific combination of algorithms. Federal 

organizations shall use approved algorithms within S/MIME implementations for key 

establishment and transmitting messages. Tables 5-1 through 5-3 specify a variety of 

cipher suites that may be used to protect federal information and information systems

(based on [SP 800-49] and [RFC 6318]). Any of the algorithms listed in the following

tables may be used, in accordance with security strength time frame restrictions given in 

Part 1 and [SP 800-131A] to protect federal information in combinations other than those 

displayed.

Table 5-1: Cipher Suite 1

Mechanism Guidance

Digital Signatures DSA with key sizes ≥ 2048 bits

[FIPS 186-4]

Hash SHA-256

[FIPS 180-4]

Key Agreement Diffie-Hellman with key size ≥ 2048bits

[SP 800-56A]

Encryption AES-128 in CBC mode

[FIPS 197] and [SP 800-38A]

 

22 Both of these services are defined in S/MIME V3 standards [RFC 2634].
33

Table 5-2: Cipher Suite B, Level 1*

Mechanism Guidance

Digital Signatures ECDSA with P-256

[X9.62]

Hash SHA-256

[FIPS 180-4]

Key Agreement ECDH with P-256

[SEC1]

Key Derivation Based on SHA-256

[SEC1]

Key Wrap AES-128

[RFC 3394]

Encryption AES-128 in CBC mode

[FIPS 197] and [SP 800-38A]

*see [RFC 6318]

Table 5-3: Cipher Suite B, Level 2*

Mechanism Guidance

Digital Signatures ECDSA with P-384

[X9.62]

Hash SHA-384

[FIPS 180-4]

Key Agreement ECDH with P-384

[SEC1]

Key Derivation Based on SHA-384

[SEC1]

Key Wrap AES-256

[RFC 3394]

Encryption AES-256 in CBC mode

[FIPS 197] and [SP 800-38A]

* see [RFC 6318]

Federal clients shall be supported by a Public Key Infrastructure with valid Federal PKI 

X.509 certificates for senders and receivers.

Cryptographic modules used in federal systems shall comply with FIPS 140-2 [FIPS 140-

2].

Federal S/MIME implementations may, in accordance with organizational policies, be 

capable of receiving messages protected with algorithm suites that are not approved for 
34

federal use in sending protected messages. In those instances, users should be presented 

with a warning banner explaining that the cryptographic mechanisms used are weak and, 

therefore, that integrity and authentication cannot be assured.

5.3 Procurement Guidance

The following recommendations are for any individual that makes a purchasing decision 

for acquiring an S/MIME-enabled component.

1. In support of security and compatibility across the Federal Government, ensure 

that all federal information systems support Cipher Suite 1.

2. Procurements should support Cipher Suite B, either Level 1, Level 2, or both.

3. Federal clients may support RC2 for use only in the event that users receive 

correspondence encrypted with this weaker and unapproved algorithm.

4. Ensure that federal agencies do not use SHA-1 for digital signature generation; 

however, it can be used for the verification of digital signatures signed using 

SHA-1. Cryptographic algorithm implementations should be modular so as to 

allow for new algorithms.

If an S/MIME client needs to generate a key pair, ensure that the S/MIME client or some 

related administrative utility or function is capable of generating public/ private key pairs 

on behalf of the user.

5.4 Recommendations for System Installers

The system installer is the individual that installs the S/MIME application and performs 

the initial configuration of the system.

1. Federal clients shall be configured to support Cipher Suite 1 for interoperability 

as described above in Section 5.2.

2. Systems shall be configured so that they only permit the use of approved 

cryptographic algorithms and approved key sizes to encrypt or sign new 

messages.

3. Installers should install and configure S/MIME clients so that they default to the 

use of an approved cipher algorithm suite. Furthermore, installers should

configure clients so that there is a straightforward means for end users to change 

default settings and select algorithms as needed for interoperability and in 

accordance with organizational needs and policies.

4. System installers should configure clients so that end users can use unique 

certificates for each security function (e.g., encryption, digital signatures) at their 

disposal.
35

5.5 Recommendations for System Administrators

The System Administrator is the individual who runs the S/MIME application on a dayto-day basis and, on client implementations, interacts with the end user.

1. The system administrator shall ensure that end users are properly trained and that 

the organization’s security policy is enforced.

2. Systems shall be maintained so that they only permit the use of approved 

cryptographic algorithms and approved key sizes to encrypt or sign new 

messages.

3. Administrators should maintain S/MIME clients so that they default to the use of 

an approved cipher algorithm suite. Furthermore, administrators should maintain 

a straightforward means for end users to change default settings and select 

algorithms as needed for interoperability and in accordance with organizational 

needs and policies.

4. System administrators should provide training for users on the relative security 

provided by various cryptographic algorithms and on organizational policies for 

their use.

5. System administrators should provide end users with guidance on how 

certificates and keys are stored and managed, and identify the end user’s related 

responsibilities.

5.6 Recommendations for End Users

An end user is the individual using a client to access the system. Even within a centrally 

managed environment, end users may find that they have a significant amount of control 

over some of the security features within an SMIME implementation.

1. Users shall operate their system as instructed by their organization and system 

administrators.

2. Users should use unique certificates for each security function at their disposal.23

3. Users shall protect their private keys from unauthorized disclosure.

4. Users should not send the same message in both encrypted and plain text.

 

23 If they have not been supplied with certificates by their home organizations, users can obtain certificates 

from a number of organizations via the web.
36

6 Kerberos

6.1 Description

The Kerberos authentication mechanism was developed at the Massachusetts Institute of 

Technology (MIT) to enable the secure authentication of users to Target Servers (TSs) 

over an unprotected network, where client software acts on behalf of a user24. The 

original design and implementation of Kerberos and its first three revisions (i.e., versions 

1 through 4) was primarily the work of Steve Miller, Clifford Neuman, Jerome Saltzer 

and Jeffrey Schiller25. Kerberos is used for local logins, remote (over the network) 

authentication, and for client-to-TS requests. It can also be extended to provide for the 

establishment of cryptographic keys between a client and a TS. Kerberos has been 

designed so that a user and a TS rely on a trusted third party to provide assurance of each 

party’s identity. This assurance is granted by means of tickets and authentication 

information, each encrypted with symmetric keys.

The trusted third party is a Key Distribution Center (KDC), which consists of an 

Authentication Server (AS) and a Ticket Granting Service (TGS). The AS and TGS may 

or may not reside on the same machine. The KDC has a database of user, TS, and TGS 

symmetric keys. All KDC symmetric keys are accessible by the TGS. The user’s key is 

normally created by hashing a user’s password with other information.

An overview of the Kerberos version 5 protocol is shown in Figure 6-1. The following is 

a simplification of the process (e.g., the generation of most keys and the use of most 

cryptographic operations are not specified). For example, tickets and authentication 

information are protected with checksums and encryption when transmitted. 

1. A user logs onto a client by entering a password, from which a user symmetric 

key is generated.

2. The client, acting on behalf of the user, requests a Ticket Granting Ticket from the 

AS.

3. The AS generates a Ticket Granting Ticket, for a specified validity period, and 

sends it to the client.

4. The client provides the Ticket Granting Ticket to the TGS, along with his own 

authentication information, which includes the client identifier and a time stamp.

5. The TGS checks the authentication information and the validity period of the 

Ticket Granting Ticket. The TGS then generates a Target Server Ticket and sends 

it to the client.

 

24 Note that a single client implementation may be used by multiple users, and a single user may use 

multiple client implementations (e.g., a user could access different workstations, each with its own client 

implementation).

25 The design was based in part upon a protocol proposed by Needham and Schroeder [NEED] with 

modifications provided by Denning and Sacco [DENN]. For more detail on the goals, motivations, and 

rationale of Kerberos see [NEUM].
37

6. The client sends authentication information and the Target Server Ticket to the 

TS.

7. The TS checks the authentication information and the validity period of the Target 

Server Ticket; if the information is reasonable, the user is authenticated to the TS.

The protocol may be extended to authenticate the TS to the user, and a ticket may be reused within its validity period.

1

2 3 4 5

TS

AS

Client

TGS

1. User-Client Interface

2. Request for Ticket Granting Ticket

3. Ticket Granting Ticket

4. Request for Target Server Ticket

5. Target Server Ticket

6. Request for Service

6

User

KDC

Figure 6-1: The Kerberos Protocol

Each TGS has its own “realm” of clients and TSs. However, different realms may be 

linked by the sharing of inter-realm keys between TGS's (see Figure 6-2). A client in 

Realm 1 wishing a service on a TS in Realm 2 may obtain a ticket from TGS1 that 

introduces the client to TGS2. This ticket is encrypted with the inter-realm key shared 

between TGS1 and TGS2. The client can then request a ticket from TGS2 for the desired 

service on the TS in Realm 2. Thus, realms may be networked to provide clients with 

inter-realm services.
38

1

4

2

3

5

TS

Realm2

AS

Realm1

Client

Realm1

TGS1

Realm1

1. User-Client Interface

2. Request for Ticket Granting Ticket

3. Ticket Granting Ticket for TGS1

4. Request for Ticket Granting Ticket

5. Ticket Granting Ticket for TGS2

6. Request for Target Server Ticket 

7. Target Server Ticket for TS

8. Request for Service

6

User

TGS2

Realm2

7

8

Figure 6-2: Cross-Realm Authentication

In an alternative Kerberos protocol between the client and the AS (as specified in [RFC 

4556]), either both the user and the AS have key-establishment public key pairs with 

corresponding certificates, or the user has a key-establishment key pair and associated

certificate, and the AS has a digital signature key pair and digital signature certificate. 

The user symmetric key can then be established between the client and the KDC in one of 

two ways:

1. Using key agreement (e.g., Diffie-Hellman) between the AS and the client26, or

2. Using key transport (e.g., RSA), where the AS generates the user symmetric key, 

and sends the key to the client27.

Once the user symmetric key is established, the remainder of the protocol proceeds as 

previously described. In this case, the need for user symmetric keys generated from 

passwords can thus be avoided.

Kerberos implementations may provide an additional authentication capability that is 

called pre-authentication, as described in [RFC 6113]. TLS may also be implemented to 

protect all communication between the clients and KDCs as described in [RFC 6251].

 

26 In this case, both the user and the AS have key-establishment key pairs.

27 In this case, the user has a key-establishment key pair, and the AS has a digital-signature key pair.
39

6.2 Security and Compliance Issues

1. Kerberos version 5 was initially specified in [RFC 1510]. More recently, the 

security was updated in version 5 ([RFC 4120] and [RFC 6649]); however, many 

existing implementations still correspond to the initial RFC.

2. Many current Kerberos implementations based on [RFC 1510] rely on DES for 

symmetric encryption functions. DES is no longer approved for use in protecting 

Federal Government information and has been deprecated as well in Kerberos 

[RFC 6649].

3. If a keyless checksum computation is used for the data integrity of Kerberos 

messages, the integrity of the message may be inadequate.

4. Some Kerberos implementations rely solely on the entropy (i.e., randomness) 

provided by the user password to generate the symmetric client key (the client key 

is a hash of the user’s password). Passwords, in general, do not provide enough 

randomness for generating a key. In such cases, a dictionary attack28 is feasible. If 

passwords are used to generate cryptographic keys, they should be selected to 

maximize the difficulty of a password guessing attack, thus increasing the 

difficulty of an off-line dictionary attack [SP 800-118].29

5. Compromising the client, KDC, or TS could compromise the symmetric keys that 

they contain and thereby compromise parts of the system. In particular, the KDC 

stores keying information for all the KDC users, the TGS, and any TS that 

communicates directly with the KDC TGS. These symmetric keys require 

protection that is commensurate with the protection required for the data that they 

protect (e.g., tickets, other keys, authentication information, and shared data).

6. The TGS has read-only access to the KDC database. If the TGS and database do 

not reside on the same machine, a secure channel is required for the TGS to obtain 

the required TS keys.

7. A failure of the AS or the TGS would prevent all AS users from obtaining new 

tickets and corresponding new services.

8. Clocks must be synchronized in order to accurately assess the validity of clock 

authentication information and tickets. If the TS’s clock is running behind the 

clock of the KDC (or AS), then previous authentication information and tickets 

could be played back to the TS after they have expired.

9. If a Kerberos implementation has a TLS capability, then it should be used when 

the DH key agreement or RSA key transport method discussed above is not used.

 

28 A dictionary attack is a technique for guessing a password by selecting candidate passwords from a list 

of words commonly found in a dictionary, or derived from words commonly found in a dictionary. Each 

selected candidate is tested as though it were the actual password until the result of the test indicates that 

the correct password has been selected.

29 Draft SP 800-118, Guide to Enterprise Password Management, is currently under development [SP 800-

118].
40

6.3 Procurement Guidance

The following recommendations are for any individual that makes a purchasing decision 

for acquiring a Kerberos capability.

1. Ensure that new procurements conform to version 5 ([RFC 4120], [RFC 6649]).

2. Ensure that government procurements specify the inclusion of approved 

symmetric key encryption algorithms (e.g., the Advanced Encryption Standard 

(AES)) [RFC 3962].

3. Ensure that an approved MAC computation (e.g., HMAC-SHA1 or HMACSHA256-128) is available for data integrity with encryption (see [RFC 3962] and 

the IETF Informational Draft, AES Encryption with HMAC-SHA2 for Kerberos 5, 

at: http://tools.ietf.org/html/draft-ietf-kitten-aes-cts-hmac-sha2-04).

4. Kerberos version 5 permits the use of smart cards or tokens (e.g., FIPS 201 

Personal Identity Verification cards [FIPS 201]) to store a user's password. Ensure 

that passwords stored on tokens are randomly generated; therefore, when tokens 

are used, ensure that a means of generating random passwords and securely 

writing them on the token is available and the password generation function is a 

NIST-approved random number generation function [SP 800-90A]. When tokens 

are used, ensure that the manual entry of passwords is not permitted except to 

authenticate the user to the token.

5. If passwords are used to form user symmetric keys, ensure that the password 

mechanism supports the use of strong passwords [SP 800-118], and an approved 

hash algorithm (e.g., SHA-1 or stronger) is used as the hash algorithm.

6. If passwords are generated by users, ensure that the system software enforces a 

strong password policy in accordance with [SP 800-118].

7. Kerberos with public key authentication and subsequent key establishment can 

provide stronger security than the use of password-based keys and should be 

available where PKI mechanisms are available. See [RFC 4556] and [RFC 5349]

for further information. Ensure that the key-establishment methods (i.e., keyagreement or key-transport methods) used have at least 112 bits of security; see 

[SP 800-57 Part 1] and [SP 800-131A] for further information.

8. TLS should be used when the system supports it.

9. Procurement officials should consider whether inter-realm networking is 

necessary and include the capability in the software if it’s needed.

10. Ensure that cryptographic modules used by CAs, TSs and clients are validated at 

FIPS 140-2 Level 1 or higher [FIPS 140-2].

6.4 Recommendations for System Installers

The system installer is any individual(s) that installs a Kerberos capability and performs 

the initial configuration of the system.
41

1. Government systems shall be configured so that approved algorithms (e.g., AES) 

shall be used [RFC 3962], and DES shall not be used [RFC 6649].

2. An approved MAC checksum (e.g., HMAC-SHA1 or HMAC-SHA256-128) shall

be installed and used in all implementations for data integrity purposes (see [RFC 

3962] and the IETF Informational Draft, AES Encryption with HMAC-SHA2 for 

Kerberos 5, at: http://tools.ietf.org/html/draft-ietf-kitten-aes-cts-hmac-sha2-04).

3. The AS, TGS, TSs, and clients shall use strong access control mechanisms30

(physical and logical) for protecting and updating keys.

4. Kerberos version 5 permits the use of smart cards or tokens (e.g., FIPS 201 

Personal Identity Verification cards [FIPS 201]) to store a user's password.

Passwords stored on tokens shall be randomly generated; therefore, when

tokens are used, a means of generating random passwords by a NIST-approved 

random number generation function ([SP 800-90A], [SP 800-90B], [SP 800-

90C]), and securely writing them on the token shall be used. When tokens are 

used, manual entry of passwords shall not be permitted except to authenticate the 

user to the token.

5. Kerberos with public key-based user authentication and key establishment can 

provide stronger security than password-based keys and should be installed where 

PKI mechanisms are available and the software has the capability. See [RFC 

4556] for further information.

6. TLS should be used when the system supports it, see Section 4 for guidelines on 

using TLS.

7. If user passwords are generated by the system, the system shall generate strong 

passwords [SP 800-118].If passwords are used to form user symmetric keys, then 

an approved hash algorithm (e.g., SHA-1 or stronger) shall be used as the 

password-hashing algorithm.

8. If user passwords are used to generate cryptographic keys, the password 

mechanism shall be configured to use and require strong passwords [SP 800-

118].

29

9. A backup AS and TGS should be provided so as to minimize the impact in case 

of operational failure or denial-of-service attacks.

10. Clocks should be synchronized periodically and whenever a new system is 

brought on-line31.

6.5 Recommendations for System Administrators

The System Administrator is any individual(s) who runs a system with a Kerberos 

capability on a day-to-day basis and interacts with the end user.

 

30 Strong access control mechanisms either prevent or detect unauthorized attempts to access or replace 

sensitive data. These controls may be physical (e.g., locks, guards, or alarms) or logical (e.g., encryption, 

data integrity, or entity authentication).

31 See http://www.nist.time.gov.
42

1. System administrators shall ensure that users are properly trained and that the 

organization’s security policy is enforced.

2. The AS, TGS, TS, and client shall be physically secured.

3. Tickets shall be encrypted or physically protected at the client, TS, and TGS sites.

4. If the passwords are generated by the user, then the system administrator shall

develop a policy for selecting strong passwords that is enforced by the software 

[SP 800-118].

29

5. System clocks shall be periodically verified to ensure synchronization.

6.6 Recommendations for End Users

An end user is the individual using the Kerberos capability.

1. If user-selected passwords are allowed, they shall be generated in accordance 

with the organization’s password policy.

2. Users shall protect their password from unauthorized disclosure. If a token 

containing a password or key is provided, users shall protect the token from 

unauthorized use. Users shall report the loss of physical tokens or the 

compromise of passwords.
43

7 Over-The-Air Rekeying (OTAR) Key Management Messages 

(KMMs)

7.1 Description

A key management protocol has been specified for over-the-air rekeying of digital radios

(OTAR) [OTAR]. This protocol has been designed to handle several types of 

cryptographic security, one of which, Type 3, has been designed for unclassified, 

sensitive communications and is discussed herein. The only differences between the 

security types are the cryptographic algorithms used and the security requirements. The 

Type 3 algorithms and security requirements are addressed in both OTAR and OTAR1

[OTAR1]

32

.

For key management, a secure mobile system consists of Key Management Facilities 

(KMFs) and mobile radios that are subordinate to each KMF. Key Management 

Messages (KMMs) are exchanged between each KMF and its subordinate mobile radios 

(see Figure 7-1). Cryptographic keys are transferred from a KMF to a mobile radio and 

protected using a key-wrapping algorithm and key wrapping key; many of the KMMs are 

protected by encrypting the data in the messages; the integrity of the messages is 

protected using a Message Authentication Code (MAC).

Key Management

Facility

O O O O

Mobile Radio Mobile Radio

KMMs

O O Portable Radio

: Normal radio communications (i.e., no KMMs)

Figure 7-1: Radio Communications with OTAR

 

32 Reference [OTAR] provides an overview of the key management techniques and the protocol. Reference 

[OTAR1] specifies the general security requirements for transmitting Type 3 key management messages 

(KMMs), the requirements for wrapping the keys, the techniques used for KMM integrity and the 

mechanism used to protect against replay of the KMMs.
44

Three general types of keys are used in OTAR: a Key-Wrapping Key (KWK)33, a 

Traffic-Encryption Key (TEK) and a key to be used for the computation of a Message 

Authentication Code (MAC).

7.2 Security and Compliance Issues

7.2.1 Cryptographic Algorithms

Although the protocol has been designed to allow the use of any block cipher algorithm 

to apply the cryptographic protection, only three block cipher algorithms have been 

included in the specification: DES, TDEA and AES.

Approval for DES has been withdrawn because DES no longer provides the security that 

is needed to protect Federal Government information.

TDEA, as specified in [SP 800-67], uses three DES encryption/decryption operations 

with a “key bundle” consisting of three separate DES keys. Two versions of TDEA have 

been included in the OTAR specification: a one-key version, whereby all three keys are 

the same for compatibility with DES, and a three-key version (3-TDEA), whereby the 

three keys are different. Since DES is no longer considered secure, the one-key version of 

the TDEA is also no longer considered secure and shall not be used.

7.2.2 Message Authentication and Cryptoperiods

A Message Authentication Code (MAC) is used to authenticate and protect the integrity 

of many of the KMMs as specified in OTAR1, using the CBC-MAC mode of operation. 

The security of a MAC depends, in part, on the block size of the MAC algorithm. AES 

has a larger block size than 3-TDEA, and so the security of AES CBC-MAC is better 

than 3-TDEA CBC-MAC. The OTAR documentation provides no guidance on the length 

of cryptoperiods (i.e., the number of messages or the length of time that a key may be 

used before it must be changed).

For AES, the number of messages that can be authenticated using a given key is, in 

practice, not an issue. However, AES keys shall be periodically updated because of other 

threats to the system, e.g., lost radios or an undetected compromise of a key.

When using 3-TDEA, no more than 1 000 000 messages shall be sent using a given key 

because of threats to the security of the algorithm. However, like AES, it may be prudent 

to update the 3-TDEA keys more frequently because of other threats to the system.

7.2.3 Key Usage

Part 1 of this Recommendation states that keys shall be used for only one purpose.34

However, OTAR1 states that the key used to generate a MAC must be either a key 

 

33 A Key Wrapping Key may also be referred to as a Key Encryption Key (KEK).

34 There is an allowed exception to this rule, but it does not apply to OTAR [SP 800-57 Part 1, Sec. 5.2]).
45

reserved for authentication and integrity protection purposes, or a key derived from a 

Traffic-Encryption Key (TEK) using a key wrapping algorithm. In this latter case, note 

that the TEK might be used for both encryption and for key derivation. In order to 

comply with the recommendation to use a key for only one purpose, the MAC key shall

be a key reserved for a single purpose.

7.2.4 Backup

The KMF should backup all keying material shared with and among the mobile radios so 

that it can be recovered if necessary. When a key is no longer required, it should be 

deleted from both normal operational storage and backup storage.

7.2.5 Rekeying

Procedures shall be in place to rekey all radios in the network in the event of a key 

compromise. If a radio is lost, procedures shall enable rekeying other radios in the 

network so that the lost radio no longer has the capability of communicating securely 

with other radios in the network.

7.2.6 Random bit generators

Keys shall be generated at the KMF using an approved random bit generator that 

provides sufficient randomness for the desired security strength of the cryptographic 

processes. NIST-approved random bit generation methods are specified in [SP 800-90A], 

[SP 800-90B] and [SP 800-90C] (under development).

7.3 Procurement Guidance

The following recommendations are for any individual(s) that makes a purchasing 

decision for acquiring OTAR equipment.

1. Ensure that the AES or TDEA algorithm is included.

2. If TDEA is provided in an implementation, ensure that the three-key version is

included, and the implementation is capable of limiting the number of uses of a single 

TDEA key bundle to 1 000 000.35

3. Ensure that KMFs and radios conform to OTAR and OTAR1.

4. When keys are generated within KMFs, ensure that they are generated using 

approved random bit generators.

5. Ensure that cryptographic modules used by the KMF and the mobile radios are 

validated at FIPS 140-2 Level 1 or higher [FIPS 140-2].

 

35 See Section 7.2.2.
46

6. Ensure that KMFs include backup and archive capabilities to support reconstitution of 

the KMF in the event of a disaster (e.g., fire, earthquake).

7.4 Recommendations for System Installers

The system installer is the individual(s) that installs an OTAR capability and performs 

the initial configuration of the system components.

1. The KMF shall use and have strong physical and logical access control mechanisms 

to protect the cryptographic keys (e.g., physical locks, alarms or password token).

2. Backup KMFs shall be provided.

3. A TEK should not be used for multiple purposes. Reserved MAC keys should be 

used for message authentication and integrity protection.

4. Maximum cryptoperiods for each key type shall be determined at the KMF in 

accordance with the organization’s security policy and Section 7.2.2.

5. Radios shall be accounted for; in the case of a lost or stolen radio, an assessment of 

the effect of a loss of the keys contained in that radio shall be made. The use of any 

key contained in that radio shall be discontinued. Procedures shall be in place for 

replacing these keys if used by the KMF or by other radios.

6. Implementations shall be configured to use the AES or TDEA algorithms, and to 

disallow the use of DES.

7. If TDEA is provided and is to be used, the three-key version shall be used, and the

one-key version shall not be used.

8. For implementations using TDEA in which the cryptoperiod of a key bundle is 

configurable, the cryptoperiod shall be set to a value less than 1 000 000 messages.

7.5 Recommendations for System Administrators

The System Administrator is the individual who manages the OTAR system or its 

components on a day-to-day basis and interacts with the end users.

1. System administrators shall ensure that the organization’s security policy is 

enforced.

2. System administrators shall protect the keying material from disclosure and 

modification.

3. Procedures shall be in place for replacing keys at the end of their cryptoperiod.
47

4. To maintain the availability of the KMF, system administrators shall ensure that 

sufficient information is stored in a secure location to reconstitute the KMF after a 

disaster.

5. Backup KMFs along with a strategy and procedures to transition from a primary 

KMF to a backup KMF, and from a backup KMF to the primary KMF shall be 

established.

6. System administrators shall train end users in the use of their radios and the 

procedures to be followed in the case of lost radios or suspected key 

compromises.

7. Audit logs should be maintained at the KMF with sufficient information to 

indicate which keys are shared by which radios.

7.6 Recommendations for End Users

An end user is the individual using a radio that has an OTAR capability.

1. End Users shall operate radios as instructed by their organization and system 

administrators.

2. End users shall protect their radios from loss and unauthorized access.

3. In the event that a radio is lost or a key is suspected of being compromised, end users 

shall immediately notify the system administrator in accordance with the 

organization’s security policy.
48

8 Domain Name System Security Extensions (DNSSEC)

8.1 Description

The Domain Name System (DNS), as defined in [RFC 1034] and [RFC 1035], is the 

global hierarchical distributed database system for mapping Internet addresses, Simple 

Mail Transfer Protocol (SMTP) servers, and other information to a human-readable 

name. Its main purpose is handling mappings between host domain names and Internet 

addresses, but it can handle other forms of data as well, such as host system information, 

the geographic location of servers, even encoded digital certificates. DNS data is stored 

as individual Resource Records (RRs) each associates a piece of data (e.g., IP address, 

mail server name) with a domain name and an identifying Resource Record type code 

(RR type). All the RRs for a particular organization are stored in an administrative unit 

called a zone. Multiple zones form a domain. A domain is hierarchical, in that one zone 

may act as a delegating parent to one or more child-delegated zones. For example, most 

federal agencies are child delegations under the “.gov” parent zone.

Zone information is maintained on authoritative servers, which are distributed all over 

the Internet to answer queries according to the DNS network protocols. The DNS 

infrastructure is comprised of a small group (or single server) known as a primary master 

authoritative server that has a local zone database, and multiple secondary servers that 

obtain their copies of the zone database from the primary authoritative master server.

Another set of components are caching recursive servers36, which query the authoritative 

servers and cache any replies. On the end user’s client system, software components 

known as resolvers make DNS queries to recursive caches and/or authoritative servers.

Figure 8-1 depicts the relationship between the DNS components.

Figure 8-1: DNS Components

 

36 Caching Recursive Server is sometimes shortened to “caching server” or “recursive server.” However, 

the role remains the same.

Primary 

Authoritative 

Server

Secondary 

Authoritative

Server

Caching 

Server

End User 

Resolver

Zone Transfer 

(TSIG)

Query/Response 

(DNSSEC)

Query/Response 

(DNSSEC) Query/Response 

(DNSSEC)
49

The basic DNS does not have many security features [SP 800-81]. A suite of RFCs has 

been developed to provide security enhancements contained in three IETF documents, 

collectively called the DNS security extensions (DNSSEC) ([RFC 4033], [RFC 4034], 

[RFC 4035]). DNSSEC provides a layer of authentication and integrity protection for any 

kind of data stored in the DNS, including data used by other protocols. For example, 

there are RR types allocated for storing Secure Shell (SSH) keys in the DNS, which then 

rely on DNSSEC to protect the integrity of that information.

8.1.1 DNS Data Authentication

Cryptographically generated public key-based digital signatures provide authentication 

for DNS data. Commonly, there will be two or more digital signature public-key pairs 

(which make up the key set) used to implement DNSSEC in a zone. One key pair is used 

to sign the zone data (referred to as the Zone Signing Key or ZSK), and a separate key 

pair is used to sign the zone key set (known as the Key Signing Key or KSK). While 

some zones may only use one key pair for both ZSK and KSK, it is not recommended for 

federal agency zones. This KSK is also known as the Secure Entry Point (SEP) key for 

the zone – using it, a client can authenticate the ZSK (by validating the signature over the 

ZSK using the KSK public key), and then use the ZSK to authenticate the zone data. The 

KSK is also used to link the security chain37 from the zone to its delegating parent. Since 

the KSK is used to link security from the zone (e.g. “example.gov”) to the delegating 

parent zone (e.g. “.gov”) [RFC 4035], it is often longer lived, and used infrequently (used 

only to sign the zone key set). Multiple digital signature algorithms can be supported, so 

there may be multiple keys (one for each algorithm), as there is no algorithm negotiation 

in DNSSEC, and clients may only understand certain digital signature algorithms. There 

is one mandatory-to-implement algorithm as defined by the IETF, so there is at least one 

agreed-upon digital-signature algorithm that all servers and clients will understand.

Currently, both RSA using SHA-1 and SHA-256 have been specified and can be used 

with DNSSEC zones. Most modern implementations either support both or will do so 

after an upgrade. Zones deploying DNSSEC for the first time can start with RSA using 

SHA-256. Zones that initially deployed with RSA using SHA-1 should migrate to RSA

(2048-bit RSA key) using SHA-256: see [SP 800-131A] for information about guidelines 

for using RSA with different key sizes and hash algorithms. However, both hash 

algorithms (i.e., SHA-1 and SHA-256) should be used to generate digital signatures for 

DNS data for a period of time to ensure that client systems that cannot validate RSA with 

SHA-256 can still authenticate DNS data. The length of this transition period depends on 

the widespread availability and deployment of client-system software that understands 

RSA with SHA-256.

8.1.2 DNS Transaction Authentication

Additional authentication mechanisms are used for server-server communication and 

administrative control. Transaction authentication is performed by computing an HMAC 

 

37 The security chain (also referred to as “chain of authentication”) is the collection of digital signatures and 

public keys that can be used to trace a logical path from the data to be validated back to a trusted, installed 

public key on the client [SP 800-81]. This chain of public keys and signatures is similar to a PKI certificate 

chain (see Section 2.1), but entirely contained within the DNS.
50

over the entire DNS message and a secret random string that is known by both 

authoritative DNS servers in the transaction, and transmitting the result in a Transaction 

Signature (TSIG) RR appended to the original message. Transaction authentication is 

usually used for special transactions, such as zone transfers or dynamic updates. A zone 

transfer is a special query type that is used to keep secondary authoritative servers up-todate with the most recent version of the zone data. Dynamic update is a feature that 

allows an authorized administrator to add or delete DNS data by sending a specially 

formatted message. This is frequently used in local area networks where the Dynamic 

Host Configuration Protocol (DHCP) is used to assign IP addresses dynamically. The 

DHCP server may update the DNS server by sending a dynamic update message to 

reflect network changes.

The currently defined algorithms used in TSIG authentication are HMAC using SHA-1

and the SHA-2 family of hash algorithms (SHA-224, SHA-256, SHA-384 and SHA-512).

The use of SHA-1 is acceptable for current security practices when using HMAC with a 

suitably random secret string. All DNS server administrators taking part in the transaction 

must agree on which algorithm and secret string size will be used for transaction 

authentication and must ensure that all parties have the same secret random string (which 

may include out-of-band transactions to distribute keys).

8.1.3 DNS Cryptographic Algorithms/Schemes, Modes and Combinations

DNS does not support algorithms in isolation, but specifies suites of algorithms and 

schemes. Algorithm/scheme combinations for zone data signing and for message 

authentication are provided in Table 8-1 ([RFC 6944], [RFC 6605]) and Table 8-2 ([RFC 

3645], [RFC 4635]):

Table 8-1: Recommended Algorithm and Scheme Combinations for 

Zone Data Signing

Suite Authentication Digest IETF Status Approved for 

Federal Use38

RSA_SHA-256 RSA SHA-256 Recommended 

to Implement YES

RSA_SHA-512 RSA SHA-512 Recommended 

to Implement YES

ECDSAP256SHA256 ECDSA SHA-256 Recommended 

to Implement YES

ECDSAP384SHA384 ECDSA SHA-384 Recommended 

to Implement YES

 

38 Refer to Part 1 of this guide for approved key lengths and for algorithm lifetimes [SP 800-57 Part 1].
51

Table 8-2: Recommended Message Authentication Algorithms

Suite IETF status Approved for 

Federal Use

HMAC_SHA1 Mandatory YES

HMAC_SHA-224 Optional YES

HMAC_SHA-256 Mandatory YES

HMAC_SHA-384 Optional YES

HMAC_SHA-512 Optional YES

GSS_TSIG39 Optional YES

It should be noted that HMAC-MD5.SIG-ALG.REG.INT is a suite that is widely 

implemented and often set as the default choice. However, it shall not be used for federal 

implementations. Since TSIG message authentication is used between servers where 

there is an existing trust relationship, the administrators must agree on the method used 

and the secret (random) string used with the TSIG method.

Due to message size constraints (See Section 8.1.4 below), large RSA keys may result in 

DNS transaction failures that are often interpreted by clients as DNS failures. It is 

recommended that, instead, a digital signature algorithm that has the same security

strength, but with smaller sized keys be used, such as ECDSA [RFC 6605]. It is 

recommended that DNS administrators plan to migrate to ECDSA for zone signing by 

October 1, 2015, or plan to migrate earlier as soon as it becomes available in DNS 

software components.

8.1.4 Special Considerations for Key Sizes

There are some special considerations needed when choosing the size of the RSA 

DNSSEC signing keys. Early deployments have shown that large RSA keys can result in 

protocol issues, such as response messages that are too large to fit in a standard UDP 

packet. DNSSEC requires the use of larger DNS packet sizes up to 4 KB, but practical 

limits are around 1500 bytes or less.

It is recommended that DNS administrators maintain 1024-bit RSA/SHA-1 and/or 

RSA/SHA-256 ZSK’s until October 1, 2015, or until it is proven that the majority of 

routers, caches and other network middle boxes can handle packet sizes over 1500 bytes 

(if before 2015). However, 1024-bit RSA keys are allowed until the 2015 date to 

accommodate older versions of DNS clients that may be older network components that

cannot handle UDP packets with sizes over 1500 bytes. This is an exception to the 

security guidance provided in [SP 800-131A]. However, this exception on RSA key sizes 

 

39 Generic Security Service Algorithm for Secret Key Transaction Authentication (GSS-TSIG [RFC3645]) 

may be found in some server implementations.
52

does not apply to Key Signing Keys. KSK’s shall follow the guidance set in Part 1 of this 

Recommendation [SP 800-57 Part 1].

It is recommended that zone administrators migrate DNSSEC zone signing algorithms to 

ECDSA by 2015 or when support for ECDSA appears in DNSSEC components, 

whichever is sooner.

To minimize the risk when using 1024-bit RSA ZSK’s with DNSSEC, ZSK’s should be 

changed more frequently: every one to three months, with a signature validity period of 

five to seven days. The ZSK-rollover sequence discussed in [SP 800-81] is recommended 

to maintain a valid chain of authentication in DNS data.

8.1.5 Special Considerations for NSEC3

There is a special variant to DNSSEC that minimizes the risk of information leakage and 

is known as the Hashed Next Secure (NSEC3) RR [RFC 5155]. In DNSSEC, a client can 

map the contents of the zone by sending a series of queries for the Next Secure (NSEC) 

RR type found in error messages. These NSEC RRs provide signed proof that the queried 

name did not exist, but also provides two names that do exist in the zone as part of that 

proof. NSEC3 attempts to minimize this information leakage of zone names by using the 

hash values of the two existing names (currently using SHA-1 only). However, this 

requires the server and client to be able to perform multiple SHA-1 hash calculations 

during runtime; note that this method could be used to mount a Denial of Service attack 

against the server if multiple requests are made.

NSEC3 was designed to solve a specific class of information leakage that could lead to a 

complete mapping of network resources in a DNS zone. NSEC3 deployment risks are 

often greater than the usefulness provided by using NSEC3, unless there is an overriding 

need to deploy NSEC3 beyond zone content protection (examples include protecting 

personally identifying information that may be contained in the DNS). However, it is a 

good idea to use NSEC3-aware client software, because a client may access a zone that 

uses NSEC3 RRs with DNSSEC.

System installers and administrators should develop a transition plan to migrate from 

SHA-1 to SHA-256, and to do so when SHA-256 becomes available in major software 

distributions. This would involve deploying both SHA-1 and SHA-256-based NSEC3 

RRs until it is observed that SHA-256-aware implementations have become widely used 

in the Internet community.

8.2 Security/Compliance Issues

1. Even though 1024-bit RSA keys are allowed until October 1, 2015 to accommodate 

older versions of DNS clients, 2048-bit RSA keys are strongly recommended for use.

2. Although not strictly necessary to the specification, a Key Signing Key should be 

used to maintain security chains from the parent zone (e.g., .gov) to the zone (e.g.,

nist.gov). This KSK should be securely transmitted to the delegating parent 

according to the policy and procedure established by the parent zone.
53

TSIG-shared secret strings should be random for use in providing integrity protection for 

DNS message transactions, and generated at appropriate security strengths. The system 

installers using the TSIG secret string shall agree on which TSIG algorithm to use.

8.3 Procurement Guidance

The following recommendations are for any individual that makes a purchasing decision 

for acquiring DNSSEC-capable components for their network infrastructure.

1. Ensure that DNSSEC utilities use FIPS140-2 validated cryptographic modules [FIPS 

140-2].

2. DNS server software should generate and serve NSEC3 RRs, if required by zone 

policy.

3. Ensure that DNS server software use an approved random bit generator to generate 

random strings for use with TSIG message authentication using HMAC that is 

consistent with the hash-algorithm security-strength recommendations in Part 1.

4. Ensure that DNSSEC-enabled versions of network applications are purchased as 

required by security policy, if available.

5. Ensure that DNSSEC software implementing SHA-256 is included in procurements 

when available.

8.4 Recommendations for System Installers

The system installer is the individual(s) that installs a DNS component and performs the 

initial configuration of the component.

8.4.1 Recommendations for System Installers (Authoritative Servers)

1. Authoritative server installers shall configure a DNS authoritiative server to serve 

DNSSEC-signed zone data.

2. Authoritiative server installers shall use an approved random bit generator (as 

discussed in [SP 800-90A]) to create and configure an initial, random secret string for 

use with TSIG in transactions.

3. Authoritative servers shall be configured to generate and sign zone data with a key 

pair that is consistent with the key size recommendations for digital signatures as 

specified in Part 1.

4. Authoritative servers shall be configured to generate and sign the key set with a key 

pair that is consistent with the key sizes recommended for digital signatures, as 

specified in Part 1.

5. Authoritative servers shall be configured to generate and use a random secret string 

for zone transfer-message authentication (via TSIG) between primary and secondary 

servers. The security strength of the random bit generator process shall support the 

security strength required by the servers.

6. Authoritative servers shall be configured to generate and use a separate shared secret 
54

string for dynamic update-message authentication (via TSIG). The security strength 

of the random bit generator process shall support the security strength required by the 

servers.

8.4.2 Recommendations for System Installers (Caching Recursive Servers)

1. Recursive caching server installers shall configure DNS servers to be DNSSECaware.

2. Recursive caching server installers shall install at least one public key used for 

DNSSEC validation. The key(s) shall be kept up-to-date to insure successful 

validations.

8.4.3 Recommendations for System Installers (Client Systems)

1. Client systems shall be configured to send DNS queries to a DNSSEC-enabled 

caching recursive server.

2. Client systems should be configured to use DNSSEC-enabled applications, if they are 

available.

8.5 Recommendations for System Administrators

The System Administrator is the individual who runs the DNS application on a day-today basis and interacts with the end user.

8.5.1 Recommendations for System Administrators (Authoritative Server)

1. The organization security policy regarding the Authoritative servers shall be 

enforced.

2. Cryptographic keys shall be protected as specified in Part 1.

3. System administrators shall replace zone data-signature Resource Records before the 

end of their validity period.

4. Zone data-signature Resource Records shall be replaced if the private key associated 

with the public key is compromised, when the administrator of the DNS zone leaves 

the organization, and for other reasons listed in the organization’s security policy.

5. Administrators shall utilize methods for handling and protecting the private key (e.g., 

using a smart card that requires appropriate user authentication).

6. Administrators shall follow the key lifecycle procedures found in NIST Special 

Publication 800-81, Secure Domain Name System (DNS) Deployment Guide [SP 800-

81].

8.5.2 Recommendations for System Administrators (Caching Recursive Servers)

1. Server administrators shall ensure that there is an organization security policy for 

using the Caching Recursive Server.

2. Cryptographic keys shall be adequately protected (see Part 1).

3. The trust anchors for DNS validating caches should be kept up to date.
55

8.5.3 Recommendations for System Administrators (Client Systems)

1. Server administrators shall ensure that there is an organization security policy for 

using the client systems.

2. Server administrators shall ensure that users are properly trained.

8.6 Recommendations for End Users

An end user is the individual using a client to access the system.

1. End users shall operate their client systems as instructed by their organization and 

system administrator.
56

9 Encrypted File Systems (EFS)

9.1 Description

The encryption of data files and complete disk volumes presents a somewhat different set 

of key management issues than those for encryption of network-communicated data.

While network communications security focuses on the privacy and integrity of 

information in transit, storage (e.g., file) security focuses on the privacy and integrity of 

persistent data and the secure sharing of this data. The key management guidance 

surrounding file and volume encryption are sufficiently similar to consolidate into a 

single section.

Unlike the previous sections, where the protocols and/or standards have been thoroughly 

studied by the network security community, the commercial solutions for file encryption 

utilize a wide variety of security schemes and methods for storing keys. Due to this range 

of solutions, this section will not be comprehensive, but will cover a variety of methods 

used for file encryption.

The most important questions that designers of a file encryption system must answer are:

 How are keys used in the system, and what protection are they afforded?

 Where are the keys stored on the system?

 Does the method scale upward for numerous user communities (without requiring 

an impractical number of keys to be stored)?

9.1.1 Number of Keys Required

For an Encrypted File System, keys are used to encrypt a file or group of files. The 

system can either encrypt each file with a distinct symmetric key or encrypt a set of files 

using the same symmetric key. In the first case, it is very easy to provide access to a 

sharing user, for example, by simply giving the key to that user. In this way, only that 

single file can be accessed by the sharing user without providing access to any of the 

other files in the system. The drawback with this first method is that if many files are 

encrypted, the model can quickly become unwieldy, since a key is required for each 

encrypted file and must be provided to the sharing user.

In the second case, many fewer keys are required, which eases the key distribution 

process. However, when a sharing user requires access to a single file, giving access to 

that user is more problematic. By simply sending a key to the sharing user, access would 

be provided to all of the owner’s40 files that were encrypted using that key, rather than to 

the single file.

However, there are several cryptographic management actions that could be used to grant 

access to an individual file in the second case above. One option to limit access in this 

second system is for the owner or system to decrypt the file and re-encrypt it using a new 

key for transmission to the sharing user (e.g., using network security mechanisms and 

session keys). In the increasingly rare case of both users sharing a common file server, 

 

40 An owner could be an individual or a group of individuals or processes that share the key.
57

the decrypted and re-encrypted file would be placed in the sharing user’s file space. This 

would require significant processing overhead and a key management protocol for 

exchanges between the owner and the sharing user, or between the file system 

management process and the sharing user. This process requires proper protection for 

these new keys at the same security strength as the original key protecting that file.

Another option is for the sharing user to be provided with the encrypted file and the 

owner’s decryption key that could be used to decrypt all of the owner’s files, including 

the one provided to the sharing user. System overhead is reduced, and it may be possible 

to protect the key from third-party administrators, but it is unlikely that the owner would 

agree that the requester should be granted access to all files protected under the common 

key.

Having provided more extreme examples in the previous two cases, the following is a 

more common approach that is used. In this case, each user on the system has an 

asymmetric key pair. Each owner’s file is encrypted under a different randomly generated 

symmetric File Encrypting Key (FEK). The FEK is then encrypted using the public key 

of the owner and stored with the encrypted file. When the owner of a file wants to share it 

with another user, the owner decrypts the encrypted FEK (using her private key) and then 

encrypts the FEK using the sharing user’s public key. The encrypted file and re-encrypted 

FEK can then be provided to the requester. This system has several advantages. First, the 

owner needs to manage only a single asymmetric key pair. Second, it permits easier file 

sharing between users. Third, it is very efficient because files do not have to be reencrypted in order to be shared. Finally, the system need not manage any keys separately, 

since the asymmetric keys are managed by the owners, and the FEKs are stored in 

encrypted form with the files.

The owner can, of course, use different keys to encrypt different files or sets of files. The 

fewer files that the owner chooses to encrypt with a given key results in more keys and 

associations (e.g., associations of keys with file identities, file groups, individual 

identities, or access groups) that would need to be maintained.

Another important concept within an encrypted file system that must be considered is 

how data recovery is implemented. If a user loses his keys, without a data recovery 

capability within the system, the user’s data is permanently lost. As such, it is vital that 

some form of data recovery, such as master administrator passwords, be included.41 This 

requires a file encryption system that allows multiple passwords to decrypt the file, one 

of which is provided to the user, and the second is provided to the administrator, in the 

form of a master administrator password. Another possibility is that the administrator has 

a method of storing the user’s passwords for use only when the user has lost or forgotten 

their password.

When the scope of the system expands from a pair of individuals to a large network or 

internet-work, the factors associated with the management of keys can become unwieldy.

Key management challenges associated with large systems include the following:

 

41 The specifics of how data recovery is accomplished are beyond the scope of this document.
58

 Maintaining context in the face of global data placement (many owners and large 

quantities of data).

 Very large numbers of keys to manage and distribute.

 If numerous keys are stored in a single location within an EFS, that site provides 

an attractive target for an adversary  a single point of trust for a large domain.

 Difficulty in accounting for revoked users (individuals who have left an 

organization, whose subscriptions have expired, or otherwise should not be 

authorized to access the file any longer).

 Reassignment of ownership of protected data to another individual or 

organization.

 Recovery of data in the event of lost keys (e.g., the case of archived encrypted

data that is encrypted and stored by an individual who has left the organization 

and cannot be found).

 A sharing user who has been provided the keys to a number of the owner’s files, 

then provides the keys, and the owner’s data, to additional users.

9.1.2 Access to Symmetric Keys used in File Encryption

After the decision has been made about the number of files to be protected with a single 

symmetric key, key management questions can be considered. How does the File 

Encryption System generate the encryption keys? How will the keys be stored and 

protected? This section identifies common ways of answering these questions, as well as 

discussing their strengths and weaknesses. As technology advances, additional techniques 

will be developed, and as such, the list below is not complete nor should be considered 

mandatory.

Consider common answers to the important questions above. First, how do file 

encryption systems generate symmetric keys? A simple method is to derive the key from 

a password as described in [SP 800-132]. In this case, the security of the system depends 

on the randomness of the password; normally, passwords do not contain enough 

randomness to be used for generating keys (i.e., they can be guessed relatively easily). A 

standard dictionary attack can often recover weak passwords, so a strong password is 

vital for the security of this type of system. It is preferable to utilize a good random bit 

generator within the system to generate keys. Approved random bit generators can be 

found in [SP 800-90A], [SP 800-90B] and [SP 800-90C].

Next, consider the question of how to protect the keys. There is a great deal of effort 

underway by the Trusted Computing Group (TCG) to develop secure storage of keys on 

computers. As this effort continues to mature, the Trusted Platform Module chip, through 

its key cache management, offers another format for protecting keys used in EFS.

Then, consider where these keys will be stored. If keys need to be stored, they could 

simply be stored on the computer itself or on a hardware token. Alternatively, the key 

could be split into two (or more) key components with, for example, one component 

stored on a hardware token and the other key component stored on the computer itself. If 

a split key is employed, the method used to combine the key components is important; 

performing an XOR operation on equal length key components is better than simply 
59

concatenating the components. Common hardware tokens include smartcards. The 

advantage of using a hardware token is that if the user stores the hardware token away 

from the computer, and the key is split between the token and the computer, an adversary 

needs to recover both pieces of hardware to recover the key. Additional security may be 

provided by encrypting the key splits, perhaps by using a password.

There are many permutations of answers to the questions above. Four examples of how 

these questions can be answered will be considered, along with the pros and cons of each 

system. It is important to consider the specific environment in which the File Encryption 

System will be used, as that will usually point to a specific type of system that is 

preferable for that case.

The first example that will be considered is a file encryption system that uses a single 

symmetric key to encrypt every file on the system. This single key is generated using a 

method in [SP 800-132] from a user’s password.

The second example is a system that utilizes per-file encryption keys, which are stored on 

the hard disk, encrypted by a key encryption key. The key encryption key (which is also 

used to decrypt each file encryption key) is securely stored on the hard drive (e.g., using 

the Trusted Platform Module (TPM) [TPM]).

The third example is a system that utilizes per-file encryption keys that are split into two 

key components that will be XORed to recreate the key, with one key component stored 

on a hardware token and the other component derived from a password (e.g., using a 

method in [SP 800-132] to derive the key).

The fourth example uses per-file encryption keys, which are encrypted under the file 

owner’s asymmetric private key as previously described. This system is common in 

current file-encryption packages, while the previous three are extreme to show the pros 

and cons of the systems more clearly.
60

Table 9-1: Summary of File Encryption Examples

Method Pros Cons

Example 1:

SP 800-132

- Least expensive solution.

- Utilizing a strong password 

can result in reasonable 

security.

- Less secure because the 

security is dependent only on 

the strength of the password.

Example 2:

Key Encryption 

Key

- Secure storage directly in the 

computer.- Secure from 

external software attack 

and physical threat.

- Relatively new technology.

- Keys are stored on the same 

computer as the file.

Example 3:

Hardware Token

- Splits key.- Requires two 

hardware pieces to decrypt.

- Highly secure if implemented 

correctly.

- If the files or token are lost, 

the files will stay secure.

- More expensive.

- If the token is lost, the files 

cannot be decrypted.

Example 4:

Asymmetric user 

owned Key 

Encryption Key

- No plaintext keys stored in 

the computer.

- Efficient file sharing.

- Highly secure if a token is 

used.

- Compromise of a user’s 

private key compromises 

only the user’s files.

- Requires the user to manage 

his/her own key pair.

- Requires either a user 

password or a user token.

9.2 Security and Compliance Issues

1. Any encrypted file system shall employ approved cryptography if it is to be used for 

the protection of Federal Government information.

2. Keys derived from passwords shall use strong passwords to maximize the difficulty 

of an off-line exhaustion attack [SP 800-118].

9.3 Recommendations for Procurement Officials

The following recommendations are for any individual(s) that makes a purchasing 

decision for acquiring an EFS component.

1. Ensure that an encrypted file system includes a data-recovery capability (e.g., master 

administrator password) so that the data is not lost in the event that a user forgets his

password or the user is unavailable. Data recovery is vital in this type of system.
61

2. Ensure that any EFS system that derives keys from passwords has the capability of 

enforcing the use of strong passwords.

3. To increase the security of encrypted file systems, the system should use a hardware 

token or a TPM for the storage of the keys.

9.4 Recommendations for System Installers

The system installer is the individual(s) that installs EFS components and performs the 

initial configuration of the components.

1. When an EFS system that utilizes passwords for security is installed, the installer 

shall require that strong passwords be enforced by the EFS. This maximizes the 

difficulty of an off-line exhaustion attack.

2. The system installer should ensure that the database of keys is protected by 

encryption to ensure the security of the system. In addition, if the key is split by the 

EFS system, the key component stored on a hardware token should be protected.

9.5 Recommendations for System Administrators

The system administrator is the individual that manages the EFS on a day-to-day basis 

and interacts with the end user.

1. The system administrator shall ensure that the organization’s security policy is 

enforced.

2. Key recovery procedures shall be in place to ensure that users can recover their data 

if they lose their authentication information (password, token data, etc). A method for 

data recovery personnel to authenticate these users should be in place prior to the 

recovery of the user’s keys or files.

3. If the EFS includes a master administrator password for use in data recovery by the 

system administrator, the system administrator shall utilize a strong password.

4. System administrators shall provide training and security guidance to the end users of 

the system that, at a minimum, focuses on passwords, data-recovery procedures, and 

user configuration of their system to utilize the authentication features within the 

system.

9.6 Recommendations for End Users

An end user is the individual that uses the EFS to secure and share their information.

1. If user-selected passwords are used within the product, end users shall utilize strong 

passwords to maximize the difficulty of an off-line exhaustion attack.

2. End users shall follow guidance provided by the system administrator regarding the 

use of an Encrypted File System product.

3. End users shall inform a system administrator if they have lost their hardware token 

or forgotten their password.
62

10 The Secure Shell (SSH)

10.1 Description

SSH is a protocol between clients and servers for secure remote login and other secure 

network services over an insecure network or the Internet. The Internet Engineering Task 

Force (IETF) governs the SSH protocol [RFC 4251]

42

. The SSH protocol consists of 

three major components: the Transport Layer Protocol [RFC 4253], the User 

Authentication Protocol [RFC 4252] and the Connection Protocol [RFC 4254].

10.1.1 Transport Layer Protocol (SSH-TLP)

The Transport Layer Protocol (TLP) provides server authentication, confidentiality, and 

integrity with perfect forward secrecy43. The establishment of an SSH connection is 

initiated using the TLP, which negotiates the algorithms to be used and authenticates the 

server. TLP does not provide client authentication.

The algorithm-negotiation step of the protocol is used to determine the algorithms to be 

used by the client and server for key agreement (called key exchange in the protocol), 

server authentication, encryption and data integrity. The encryption and data-integrity 

algorithms that protect communications from the client to the server and from the server 

to the client are independently selected, i.e., the encryption algorithm protecting 

communication from the client to the server may be different than the encryption 

algorithm protecting communication from the server to the client.

After the algorithm negotiation is completed, the TLP authenticates the server to the 

client in a key-exchange process [RFC 4253, Sec. 8]) that provides keys and IVs for the 

selected cryptographic algorithms. The key-exchange process provides assurance that the 

server is the owner of the public key, but additional steps may be required to verify the 

server’s identity; see Section 10.2.1.2 for a discussion on the verification of the server’s 

public host key.

The protocol provides data- integrity protection by choosing a MAC algorithm for each 

communication direction between the server and the client. However, the protocol also 

allows this service (i.e., data-integrity protection) to be disabled.

10.1.2 The User Authentication Protocol (UAP)

The User Authentication Protocol authenticates the client to the server. After the TLP is 

completed, the server and the client communicate using an encrypted SSH tunnel44 that 

uses the selected encryption algorithm(s) from the negotiation process and the keys from 

 

42 This section discusses SSH version 2, sometimes called SSH2 or SSH-2. SSH1 never became a standard 

protocol and was later replaced by the SSH2 protocol, which is the SSH protocol in [RFC 4251].

43 Perfect forward secrecy is a cryptographic property of a key-establishment method in which the 

compromise of a currently established session key or long-term private key does not cause the compromise 

of any earlier established session keys.

44 An encrypted tunnel is an end-to-end communication connection where all of the data traffic going 

through the connection is encrypted.
63

the key exchange (see the SSH TLP protocol discussion in Section 10.1.1). Using the 

encrypted SSH tunnel, the server can securely perform any client authentication. The 

UAP provides a single authenticated tunnel for the SSH connection protocol.

10.1.3 Connection Protocol (CP)

The Connection Protocol [RFC 4254] multiplexes the encrypted tunnel used by SSH into 

several logical channels. The CP defines how interactive login sessions, remote execution 

of commands, forwarded TCP/IP connections, and forwarded X11 connections [X11] can 

be run simultaneously over an established SSH Transport Layer and User Authentication 

connection.

10.2 Security and Compliance Issues

10.2.1 TLP Issues

10.2.1.1 Algorithm Negotiation

In this step of the TLP, the algorithms to be used for the key exchange (i.e., key 

agreement), public key authentication, data encryption and message authentication are 

selected.

1. The SSH server and client should choose the same NIST-approved cryptographic 

algorithms for both communication directions (data streams) for a particular 

cryptographic service. For example, the same encryption algorithm and MACgeneration algorithm should be used for both communication directions to 

provide confidentiality and integrity protection, respectively.

Note: The cryptographic algorithms selected for use depend on the algorithms that 

are supported by both the server and the client, and by the client’s preference 

levels for these algorithms in each of its algorithm lists offered in the negotiation; 

the client’s preference level is indicated by the order in which the algorithms are 

listed. See [RFC 4253, Sec. 7.1] for the defined procedure for selecting the 

cryptographic algorithms.

2. Suite B cryptographic algorithms are recommended for use when supported by 

both the client and server systems. Suite B cryptographic algorithms are specified 

in [RFC 6239]. They are:

 Key agreement (key exchange): ecdh-sha2-nistp256 and ecdh-sha2-nistp384 

(see Section 10.2.1.2).

 Public key algorithm (for server and client authentications): x509v3-ecdsasha2-nistp256 and x509v3-ecdsa-sha2-nistp384 (see Section 10.2.1.3).

The public keys are conveyed in X.509 version 3 certificates.

 Encryption and MAC: AEAD_AES_128_GCM and AEAD_AES_256_GCM

(see Sections 10.2.1.4 and 10.2.1.5).

10.2.1.2 Key Agreement /Key Exchange Algorithms

Two families of Diffie-Hellman key-agreement/key-exchange algorithms have been 

specified for use in SSH: those based on finite fields and those based on elliptic curves.
64

 RFC 4253 specifies two finite-field key-exchange methods: “diffie-hellmangroup1-sha1” and “diffie-hellman-group14-sha1” [RFC4253]. Since diffiehellman-group1-sha1 uses 1024-bit keys, which provide less than 112 bits of 

security strength, it shall not be used. However, note that the use of SHA-1, in 

this case, is acceptable.

 RFC 4419 specifies two additional finite-field Diffie-Hellman key-exchange 

methods: “diffie-hellman-group-exchange-sha1” and “diffie-hellman-groupexchange-sha256” [RFC 4419]. Although the modulus length (i.e., key size) that 

can be supported by these two methods is between 1024 and 8192 bits, a modulus 

length of at least 2048 bits shall be used.

 RFC 5656 specifies key-agreement options based on elliptic curves that can be 

used instead of the options described above [RFC 5656]. Two of the options are 

mandatory-to-implement for the Federal Government when supporting Elliptic 

Curve Cryptography for SSH: “ecdh-sha2-nistp256” and “ecdh-sha2-nistp384”. 

These options specify the use of elliptic curve Diffie Hellman with the 

appropriate, NIST-approved hash function and curve: “ecdh-sha2-nistp256”

specifies the use of SHA-256 and the nistp256 curve, while “ecdh-sha2-nistp384”

specifies the use of SHA-384 and the nistp384 curve [RFC 5656]. See [FIPS 186-

4] for information about the curves.

10.2.1.3 Public Key Authentication Algorithms

Public-key authentication algorithms are the digital-signature algorithms that can be used 

in the key exchange specified in [RFC 4253, Sec. 8] to perform server authentication.

RFC 4253 specifies two digital signature algorithms: RSA and DSA (which is called 

“ssh-dss” in the RFC) using SHA-1 as the hash function for server authentication. It is 

important to note that according to [SP 800-131A], SHA-1 is no longer allowed for 

generating digital signatures. However, in this protocol, SHA-1 is allowed for server 

authentication, as long as the public key size of the signing function (either RSA or DSA) 

is at least 2048 bits. The protocol allows a server’s public key to be used without 

validation (i.e., without obtaining assurance of public key validity). For Federal 

Government use, the client shall obtain assurance of public key validity, as required in 

[SP 800-89], before digital signature verification can be performed.

Additional authentication algorithms for SSH are specified in [RFC 5656]. These use the 

Elliptic Curve Digital Signature Algorithm (ECDSA) using the curves specified in [FIPS 

186-4]. Table 10-1 identifies the requirements for the implementation of the ECDSA 

algorithm and the curves to be used by the Federal Government.

Table 10-1: Public Key Authentication Methods Using Elliptic Curves

ECDSA Hash 

Algorithm IETF Federal Government

ecdsa-sha2-nistp256 SHA-256 Mandatory Mandatory

ecdsa-sha2-nistp384 SHA-384 Mandatory Mandatory
65

[RFC 6187] specifies the use of X.509 version 3 certificates to convey public keys for the 

digital-signature algorithms. It also formally defines 2048-bit RSA with SHA-256 as a 

method for use in SSH. This combination provides 112 bits of security and is allowed. It 

is important to note that the hash algorithm used with the digital signature algorithms for 

authentication can be different than the hash function HASH in the key exchange and the 

key-derivation functions of SSH TLP.

10.2.1.4 Encryption Algorithms

Several encryption algorithms are available for SSH; Federal Government applications

shall use NIST-approved algorithms. The currently specified encryption algorithms and 

mode(s) of operation for SSH that are NIST-approved are 3DES-CBC, AES-128-CBC, 

AES-192-CBC and AES-256-CBC [RFC 4253], and AEAD_AES_128_GCM and 

AEAD_AES_256_GCM [RFC 5647]. 3DES-CBC is mandatory for IETF 

implementation.

In the protocol, when either AEAD_AES_128_GCM or AEAD_AES_256_GCM is 

selected as the encryption algorithm for protecting an encrypted tunnel (client-to-server 

or server-to-client), it is also chosen as the MAC algorithm. A different algorithm or set 

of algorithms may be selected for communication in each direction. It should be noted 

that when AEAD_AES_128_GCM or AEAD_AES_256_GCM is chosen (for encryption 

and MAC generation), the algorithm is executed only once (rather than once for 

encryption and another time for integrity protection), because these modes provide both 

confidentiality and integrity protections at the same time.

When AEAD_AES_128_GCM or AEAD_AES_256_GCM is used, the SSH packetlength field is not encrypted, but is processed as additional authenticated (plaintext) data. 

See [RFC 5647] for more details on using these algorithms in SSH.

10.2.1.5 Message Authentication Codes (MAC) Algorithms

Several MAC algorithms are available for SSH; Federal Government applications shall

use NIST-approved algorithms. The currently specified MAC algorithms for SSH that are 

NIST-approved are HMAC-SHA1 (with MAC tags of 160 bits), HMAC-SHA1-96 (with 

MAC tags of 96 bits [RFC 4253]), AEAD_AES_128_GCM and AEAD_AES_256_GCM 

[RFC 5647].

As explained in Section 10.2.1.4, AEAD_AES_128_GCM or AEAD_AES_256_GCM 

can only be chosen as the MAC algorithm when also chosen as the encryption algorithm.

10.2.1.6 Public Host-Key Verification

After the algorithm negotiation is completed, the TLP authenticates the server to the 

client in a key-exchange process [RFC 4253, Sec. 8]. In this key exchange, a DiffieHellman (DH) key exchange is performed, producing two values: a DH value45 K and an 

exchange hash value called H. H is the hash value of K and other data [RFC 4253, Sec. 8] 

for a complete specification of H). The generated H and K are subsequently used as 

inputs to a key-derivation function [RFC 4253, Sec. 7.2] for the specification of this 

function) to generate IVs and keys for the selected cryptographic algorithms.

 

45 In the key-agreement schemes in [SP 800-56A], K would be considered to be the shared secret.
66

To prove that the server is actually the owner of the public key (called the public host key 

in the protocol), the server generates a digital signature using its private key (called the 

private host key in the protocol) on H (i.e., data that is known by both the client and the 

server). If the client can verify the digital signature using the public host key, then the 

client has assurance that the server is the owner of the public key. However, the identity 

of the server has not been verified unless the public host key is verified by the client 

before the signature on H is verified.

Verifying the public host key is performed by 1) comparing the server name and its 

public host key against a database of trusted server names and public host keys, or 2) 

using the public host key certificate (i.e., the server’s public key certificate), or 3) using 

an out-of-band verification method using DNSSEC upon initiating SSH TLP46. A 

verification of the association between the public host key and the server name is 

essential to the security of the SSH connection. If the connection is not verified, then the 

connection is subject to a man-in-the-middle attack; details of this vulnerability are 

addressed in [RFC 4251]. Therefore, the association of a public host key (i.e., the server’s 

key) to a server name shall be verified for every TLP session, and the TLP shall fail if 

the verification fails.

 When the first method of server authentication is used, and it is the only method 

available for the client, the protocol shall continue only when a match is found. 

Note that the protocol does not explicitly require the connection to fail when the 

public host key is not verified, however; this Recommendation requires a 

discontinuation of the protocol in this case.

 For the second method, the TLP specified in [RFC 4253] allows the client to 

either a) accept the presented public key certificate without verification of the 

association between the public key and the server name, or b) verify the certificate 

and continue only if the public host key is verified (see Section 2 for details on 

how to verify a public key in a public key certificate). For Federal Government 

use, option b) shall be used; that is, when this second method is used, the protocol 

shall not continue unless the server’s certificate has been successfully verified.

 When an out-of-band verification method using DNSSEC is used (method three), 

the server’s identity and public key shall not be accepted unless the fingerprint 

(hash value) of the public host key matches a fingerprint of the public key in the 

“SSHFP” resource record(s) of the server. “SSHFP” resource record(s) shall be 

verified before the fingerprint is accepted as a legitimate fingerprint. See [RFC 

4255] for details about this method.

Other server-authentication methods may be defined later for the protocol.

It is important to note that the key exchange specified in [RFC 4253, Sec. 8] contains a 

Diffie-Hellman primitive specified in [SP 800-56A] that generates a shared secret. In [SP 

800-56A], a complete key-agreement scheme contains a specific key-derivation method 

(e.g., a key-derivation function) that uses the shared secret to derive keying material. In 

SSH, the shared secret is provided, instead, to the key-derivation function specified in

 

46 Details of the DNSSEC can be found in [RFC 4255].
67

[RFC 4253, Sec. 7.2]. This key-derivation function has been approved in [SP 800-135].

Therefore, the Diffie-Hellman key exchange in [RFC 4253, Sec. 8], combined with the 

key-derivation function in [RFC 4253, Sec. 7.2] is an approved key-agreement method 

for SSH TLP.

10.2.2 UAP Issues

There are many authentication methods that the server can use to authenticate the client. 

The required method that the server must support is public-key authentication, named 

“publickey” in the UAP [RFC 4252, Sec. 7]. This requires the use of a digital-signature 

algorithm as specified in Section 10.2.1.3 except the digital signatures using SHA-1. 

SHA-1 digital signatures shall not be used for client authentication. See [RFC 4252, Sec. 

7] for specific details on how client authentication using one of the digital-signature 

algorithms is done. For Federal Government applications, the digital-signature algorithms 

used for client authentication shall meet the requirements specified in [SP 800-131A].

In the “publickey” authentication method, proof-of-possession of the valid private-key is

considered to be a proof of the identity of the client. Therefore, sharing a private-key

among different clients (users) is prohibited. However, in current practice, many 

organizations allow multiple clients (users) to share a private key. To address this issue 

and other issues related to identity management, NIST recently published Draft NISTIR 

7966, Security of Automated Access Management Using Secure Shell (SSH) [NISTIR 

7966].

Beside the “publickey” method, two other authentication methods have been defined for 

this protocol. The first method is using passwords. The second method is using the

private key of a host system that is trusted by the server in this SSH connection; the latter

method is called “host-based authentication”.

 For Federal Government use, authentication using passwords shall not be used if 

the TLP does not provide an encrypted tunnel, because the password would be 

sent as cleartext.

 For the “host-based authentication” method [RFC 4252, Sec. 9], the client is a 

user of the host system that is attempting to authenticate to the server. In this 

method, the client knows the private signature key of the host. The client uses this 

private key to generate a digital signature on behalf of this host during the

authentication process with the server in the SSH connection. The server verifies 

the digital signature to authenticate the host system. If the authentication is 

successful, and the client (user) is an authorized user associated with this host, 

then this user (the client) is considered to be authenticated by the SSH server.

This “host-based authentication” method should not be used for client 

authentication because the method does not provide direct, cryptographic 

assurance of the identity of the client to the server - the server must trust the host 

system to obtain the correct identity of the client.

Also, client authentication using Kerberos (see Section 6) is also described in [RFC 

4462]. It is important to note that RFC 4462 specifies an authentication method called 

“Authentication Using GSS-API Key Exchange” [RFC 4462, Sec. 4]. This authentication 
68

method shall not be used as a stand-alone authentication method because the method 

does not prove the identity of the client.

10.3 Procurement Guidance

The following recommendation is for any individual that makes a purchasing decision for 

acquiring SSH client and server implementations.

1. Ensure that client and server implementations support at least one of these NISTapproved encryption algorithms: AES-128-CBC, AES-192-CBC and AES-256-

CBC, in addition to 3DES-CBC, which is already mandatory to implement for the

protocol. Implementations that support all of these encryption algorithms should

be selected.

2. When AEAD_AES_128_GCM or AEAD_AES_256_GCM is used in a 

negotiating encryption-algorithm list, it must also be in the corresponding MACalgorithm list. Ensure that an implementation either enforces this automatically or 

provides it as a configurable option.

3. Implementations should be chosen that support Suite B cryptographic algorithms 

as described in Section 10.2.1.1 above.

4. Ensure that client and server implementations support public-key authentication 

using public-key certificates.

5. Ensure that client and server implementations allow a protocol to be discontinued 

when the verification of the public-key-certificate fails.

6. Ensure that client implementations allow preference setting for implemented 

cryptographic algorithms.

7. Ensure that server implementations allow the configuration of the cryptographic 

algorithms to be used.

8. Ensure that server implementations are able to disallow the passwordauthentication method when encryption is not used in the client-to-server traffic 

direction.

9. Ensure that server implementations are able to disallow the “host-based 

authentication method”.

10.4 Recommendations for System Installers

The system installer is the individual(s) that installs the SSH server and client 

applications and performs the initial configuration of the system. The system installer 

shall:

1. Install and/or configure the server and client to use only approved cryptographic 

algorithms.

2. Set the connection to fail when a public-key certificate is not verified at both 

sides: server and client.

3. Configure preferences in negotiating cryptographic-algorithm lists according to 

the organization’s security guidelines for the client. For example, if the 
69

organization prefers the use of AES-256, then AES-256 must be configured to 

have the highest preference among all of the available encryption choices.

4. Configure the use of only approved cryptographic algorithms by the server.

5. Consider disabling the “host-based authentication” option at the server if the 

server is not willing to communicate with all possible users/clients at the host 

machine.

6. When AEAD_AES_128_GCM or AEAD_AES_256_GCM is in a negotiating 

encryption-algorithm list, configure it to also be its companion MAC-algorithm if 

this is not done automatically.

7. Suite B cryptographic algorithms should be set with the highest preferences in the 

client-side application. This will result in the selection of Suite B cryptographic 

algorithms to be used when the server supports them.

The installer shall make sure that all of the cryptographic components and required plugins are installed so that cryptographic operations in SSH can function properly when 

protecting the data traffic.

10.5 Recommendations for System Administrators

System administrators are those individuals responsible for the day-to-day functioning of 

the SSH server and client applications. System administrators shall:

1. Configure the client to verify the server’s public-key certificate in the keyexchange protocol in TLP and discontinue the protocol if the verification fails.

2. Configure the server to authenticate the client by verifying the client’s certificate, 

if provided, when a public-key algorithm for authentication is used in UAP.

3. Ensure that end users are properly trained to follow the organization’s security 

policy for the selection, use and protection of passwords.

4. Ensure that the organization’s security policy is enforced.

5. Ensure the protection of private key(s) associated with the server’s certificate and 

the client’s certificate from disposal, leaks or unauthorized access is/are properly 

configured.

10.6 Recommendations for End Users

An end user is the individual using the SSH client application to securely connect to the 

SSH server. End users shall:

1. Be aware of and trained to follow the organization’s security policy for using the 

product.

2. Operate their system as instructed by their organization and system administrator.
70

Appendix A: Glossary

The terms provided below are defined as they are used in this document. The same terms 

may be defined differently in other documents.

Term Definition

3-TDEA Three key TDEA as specified in [SP 800-67].

Access control Restricts access to resources only to privileged entities.

Access-control 

mechanism

A method for restricting access to some resource.

Approved FIPS-approved and/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 or 3) specified in a list of NIST approved 

security functions.

Archive See Key-management archive.

Asymmetric-key 

algorithm

See Public-key cryptographic algorithm.

Authentication A process that establishes the origin of information, or 

determines an entity’s identity.

Authentication code See Message Authentication Code.

Authorization Access privileges granted to an entity; conveys an “official” 

sanction to perform a given security function or activity.

Availability Timely, reliable access to information by authorized entities.

Backup A copy of information to facilitate recovery, if necessary.

CBC-MAC A mode of operation for block cipher algorithms.

Certificate See Public-key certificate.

Certification 

authority

The entity in a Public Key Infrastructure (PKI) that is 

responsible for issuing certificates, and exacting compliance to 

a PKI policy.

Checksum A method used to protect the integrity of data by detecting 

errors in that data.

Ciphertext Data in its encrypted form.

Compromise The unauthorized disclosure, modification, substitution or use 

of sensitive data (e.g., keying material and other security related 

information).
71

Confidentiality The property that sensitive information is not disclosed to 

unauthorized entities.

Cryptographic key 

(key)

A parameter used in conjunction with a cryptographic algorithm 

that determines its operation in such a way that an entity with 

knowledge of the key can reproduce or reverse the operation, 

while an entity without knowledge of the key cannot. Examples 

include:

1. the transformation of plaintext data into ciphertext data,

2. the transformation of ciphertext data into plaintext data,

3. the computation of a digital signature from data,

4. the verification of a digital signature,

5. the computation of an authentication code from data,

6. the verification of an authentication code from data and a 

received authentication code,

7. the computation of a shared secret that is used to derive 

keying material. 

Cryptographic 

module

The set of hardware, software, and/or firmware in which 

approved security functions are implemented

Cryptoperiod The time span during which a specific key is authorized for use 

or in which the keys for a given system or application may 

remain in effect.

Data integrity A property whereby data has not been altered in an 

unauthorized manner since it was created, transmitted or stored. 

In this Recommendation, the statement that a cryptographic 

algorithm "provides data integrity" means that the algorithm is 

used to detect unauthorized alterations.

Decryption The process of changing ciphertext into plaintext using a 

cryptographic algorithm and key.

DES The Data Encryption Standard that was specified in FIPS 46 

(now withdrawn).

Digital signature The result of a cryptographic transformation of data that, when 

properly implemented, provides the services of:

1. origin authentication,

2. data integrity, and

3. signer non-repudiation.

Distribution See Key distribution.

Encryption The process of changing plaintext into ciphertext using a 

cryptographic algorithm and key.

Entity An individual (person), organization, device or process.

Hash algorithm See Hash function.
72

Hash-based message 

authentication code 

(HMAC)

A message authentication code that uses an approved keyedhash function.

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 free) It is computationally infeasible to find any 

two distinct inputs that map to the same output.

Hash value The result of applying a hash function to information.

Identifier A bit string that is associated with a person, device or 

organization. It may be an identifying name, or may be 

something more abstract (for example, a string consisting of an 

IP address and timestamp), depending on the application.

Integrity The property that sensitive data has not been modified or 

deleted in an unauthorized and undetected manner. In this 

Recommendation, the statement that a cryptographic algorithm 

"provides integrity" means that the algorithm is used to detect 

unauthorized modifications or deletions.

Key See Cryptographic key.

Key agreement A key-establishment scheme whose resultant keying material is 

a function of information contributed by two or more 

participants, so that no party can predetermine the value of the 

keying material.

Key Bundle A set of keys used during one operation, typically a TDEA 

operation.

Key component A parameter used in conjunction with other key components in 

an approved security function to form a plaintext cryptographic 

key or perform a cryptographic function. 

Key distribution The transport of a key and other keying material from an entity 

that either owns or generates the key to another entity that is 

intended to use the key.

Key-encryption key A cryptographic key that is used for the encryption or 

decryption of other keys.

Key establishment A procedure that results in keying material that is shared among 

different parties.

Key management The activities involving the handling of cryptographic keys and 

other related security parameters (e.g., IVs and passwords) 

during the entire life cycle of the keys, including their 

generation, storage, establishment, entry and output, and 
73

destruction.

Key-management 

archive

A function in the lifecycle of keying material; a repository 

containing keying material of historical interest.

Key pair A public key and its corresponding private key; a key pair is 

used with a public-key algorithm.

Key recovery Mechanisms and processes that allow authorized entities to 

retrieve keying material from key backup or an archive.

Key transport A key-establishment procedure whereby one entity (the sender) 

selects a value for secret keying material and then securely 

distributes that value to another party (the receiver).

Key wrapping A method of encrypting keys (along with associated integrity 

information) that provides both confidentiality and integrity 

protection using a symmetric key.

Keying material The data (e.g., keys and IVs) necessary to establish and 

maintain cryptographic keying relationships.

Message 

Authentication Code 

(MAC)

A cryptographic checksum on data that uses a symmetric key to

detect both accidental and intentional modifications of data.

Non-repudiation A service that is used to provide assurance of the integrity and 

origin of data in such a way that the integrity and origin can be 

verified by a third party as having originated from a specific 

entity in possession of the private key of the claimed signatory.

Nonce A time-varying value that has, at most, a negligible chance of 

repeating. For example, a nonce could be a random value that is 

generated anew for each instance of a nonce, a timestamp, a 

sequence number, or some combination of these.

Owner For an asymmetric key pair, the entity that owns the private 

key, whether that entity generated the key pair or a trusted party 

generated the key pair for the entity.

In Encrypted File Systems, the file owner has control over the 

file and grants access of the file to others. A file may have one 

or more owners. An owner could be an individual or a group of 

individuals or processes

Password A string of characters (letters, numbers and other symbols) that 

are used to authenticate an identity or to verify access 

authorization.

Payload A part of the data stream representing the user information and 

user overhead in a communication.

Plaintext Intelligible data that has meaning and can be understood 
74

without the application of decryption.

Private key A cryptographic key, used with a public-key cryptographic 

algorithm that is uniquely associated with an entity and is not 

made public. In an asymmetric (public) cryptosystem, the 

private key is associated with a public key. Depending on the 

algorithm, the private key may be used to:

1. Compute the corresponding public key,

2. Compute a digital signature that may be verified by the 

corresponding public key,

3. Decrypt data that was encrypted by the corresponding 

public key, or

4. Compute a piece of common shared data, together with 

other information.

Protocol A special set of rules used by two or more entities that describe 

the message order and data structures for information 

exchanged between the entities.

Public key A cryptographic key used with a public-key cryptographic 

algorithm that is uniquely associated with an entity and that 

may be made public. In an asymmetric (public) cryptosystem, 

the public key is associated with a private key. The public key 

may be known by anyone and, depending on the algorithm, may 

be used to:

1. Verify a digital signature that is signed by the 

corresponding private key,

2. Encrypt data that can be decrypted by the corresponding 

private key, or

3. Compute a piece of shared data.

Public-key 

(asymmetric) 

cryptographic 

algorithm

A cryptographic algorithm that uses two related keys, a public 

key and a private key. The two keys have the property that 

determining the private key from the public key is 

computationally infeasible.

Public-key certificate A set of data that uniquely identifies an entity, contains the 

entity's public key and possibly other information, and is 

digitally signed by a trusted party, thereby binding the public 

key to the entity. 

Public Key 

Infrastructure (PKI)

A framework that is established to issue, maintain and revoke 

public-key certificates.

Reconstitute Rebuilding a service system (possibly with a different

infrastructure) using previously saved security information 

and/or keying material, rather than simply restarting a system 

from a backup.

Rekey A new key replaces another key; the “value” of the new key is 

entirely independent of the “value” of the old key.
75

Relying party An individual or organization that relies on the certificate and 

the CA that issued the certificate to verify the identity of the 

user; the validity of the public key, associated algorithms and 

any relevant parameters; and the user’s possession of the 

corresponding private key.

Secret key A cryptographic key that is used with a secret-key (symmetric) 

cryptographic algorithm that is uniquely associated with one or 

more entities and is not made public. The use of the term 

“secret” in this context does not imply a classification level, but 

rather implies the need to protect the key from disclosure.

Security association A relationship between two network entities in which security 

information is shared and used to support secure 

communication between the two entities.

Security services Mechanisms used to provide confidentiality, data integrity, 

authentication or non-repudiation of information.

Self-signed 

certificate

A public-key certificate whose digital signature may be verified 

by the public key contained within the certificate. The signature 

on a self-signed certificate protects the integrity of the data, but 

does not guarantee authenticity of the information. The trust of 

self-signed certificates is based on the secure procedures used to 

distribute them.

Shall This term is used to indicate a requirement of a Federal 

Information Processing Standard (FIPS) or a requirement that 

must be fulfilled to claim conformance to this 

Recommendation. Note that shall may be coupled with not to 

become shall not.

Shared secret A value that is generated during a key-agreement scheme; the 

shared secret is used to derive keying material for a symmetrickey algorithm.

Should This term is used to indicate a very important requirement. 

While the “requirement” is not stated in a FIPS, ignoring the 

requirement could result in undesirable results. Note that 

should may be coupled with not to become should not.

Signature verification Uses a digital-signature algorithm and a public key to verify a 

digital signature.

Symmetric key A single cryptographic key that is used with a secret 

(symmetric) key algorithm.

Symmetric-key 

algorithm

A cryptographic algorithm that uses one shared key, a secret 

key.

Threat Any circumstance or event with the potential to adversely 

impact a system through unauthorized access, destruction, 
76

disclosure, modification of data or denial of service.

Triple DES/Triple 

DEA (TDEA)

Triple Data Encryption Algorithm, specified in [SP 800-67].

Unauthorized 

disclosure

An event involving the exposure of information to entities not 

authorized access to the information.

User’s name (in a 

certificate)

The name of the party authorized to use the private key 

associated with the public key in the certificate; the subject of 

the certificate.

X.509 public-key 

certificate

The public key for a user (or device) and a name for the user 

(or device), together with some other information, rendered unforgeable by the digital signature of the certification authority 

that issued the certificate, encoded in the format defined in the 

ISO/ITU-T X.509 standard.
77

Appendix B: Acronyms

AES Advanced Encryption Standard

AH Authentication Header

AS Authentication Server

CA Certificate Authority

CBC Cipher Block Chaining

CBC-MAC Cipher Block Chaining Message Authentication Code

CMVP Cryptographic Module Validation Program

COTS Commercial Off-the-Shelf

CRL Certificate Revocation List

CTR Counter Mode

DES Data Encryption Standard

DHCP Dynamic Host Configuration Protocol

DNS Domain Name System

DSA Digital Signature Algorithm

EC Elliptic Curve

ECDH Elliptic Curve Diffie-Hellman

ECDSA Elliptic Curve Digital Signature Algorithm

EFS Encrypted File System

ESP Encapsulating Security Protocol

FEK File Encryption Key

GCM Galois Counter Mode

GMAC Galois Message Authentication Code

HMAC Hash-based Message Authentication Code

HTTP Hypertext Transfer Protocol

ICV Integrity Check Value

IETF Internet Engineering Task Force

IKE Internet Key Exchange

IP Internet Protocol

IV Initialization Vector

KDC Key Distribution Center

KMF Key Management Facility

KMM Key Management Message

KSK Key Signing Key

KWK Key Wrapping Key

LDAP Lightweight Directory Access Protocol

MAC Message Authentication Code

MD5 Message-Digest Algorithm 5

NIST National Institute of Standards and Technology
78

NSEC Next Secure

OCSP Online Certificate Status Protocol

OMB Office of Management and Budget

OTAR Over The Air Rekeying

PIN Personal Identification Number

PKCS Public Key Cryptography Standard 

PKI Public Key Infrastructure

PRF Pseudorandom Function

PVM Path Validation Module

RA Registration Authority

RFC Request for Comment

RR Resource Record

RSA Rivest, Shamir, and Adleman.

S/MIME Secure/Multipart Internet Mail Extensions

SA Security Association

SEP Secure Entry Point

SHA Secure Hash Algorithm

SMTP Simple Mail Transfer Protocol

SSL Secure Socket Layer

TCG Trusted Computing Group

TDEA Triple Data Encryption Algorithm

TEK Traffic Encryption Key

TGS Ticket Granting Service

TLS Transport Layer Security

TPM Trusted Platform Module

TS Target Server

TSIG Transaction Signature

URL Uniform Resource Locator

VPN Virtual Private Network

XOR Exclusive-Or operation

ZSK Zone Signing Key
79

Appendix C: A Word to Novice End Users

Cryptographic keys are frequently categorized by the algorithms they are used with, the 

operations they are used to perform and by the number of times they can be used. Keys 

will either be asymmetric and used with an asymmetric algorithm, or symmetric and used 

with a symmetric algorithm. Asymmetric keys are generated as key pairs: a private key

that must be kept secret, and a related public key, that may not be a secret. In general, the 

private key performs one operation, for example, a digital signature, and the public key 

does the complementary operation, in this case, signature verification. In the case of 

symmetric keys, end users need to treat the key as a secret shared value and use the same 

value for complementary cryptographic functions, such as encryption and decryption.

Some asymmetric keys are static and intended for long-term use, while others are 

ephemeral and expire after their use with one message or session. The public key of a 

static asymmetric-key pair is often provided in a public-key certificate, while an 

ephemeral public key is not. Although the concept of static vs. ephemeral also applies to 

symmetric keys, a short-term symmetric key is often called a session key, rather than an 

ephemeral key. No specific terminology is used for a long-term symmetric key. An 

application or protocol may be supported by some combination of such keys.

A PKI is the foundation of many current key-management processes and is used in many 

of the protocols or applications described in this Recommendation, as well as other 

security protocols and applications. Some understanding of the role of a PKI and public 

key certificates in key management is very helpful to setting up security protocols or 

applications properly. A long-term, or “static” public key is generally combined with the 

name of the key’s “owner” in an electronic document called a public-key certificate.

While certificates can be self-issued and signed (that is, the party that created the key pair 

can sign his name and the public key with the corresponding private key), most 

certificates are digitally signed with the private key of a trusted entity called a 

Certification Authority.

An end user may typically have one or more certificates, and may have different 

certificates for different applications, e.g., for e-mail and for authentication to web sites.

As Federal Personal Identity Verification (PIV) cards are issued to federal employees and 

contractors, most federal users will have a personal smart card that contains one or more 

personal certificates issued to them by their agency. Other specific applications may 

require “soft” certificates, usually kept on the user’s computer, and possibly issued by 

commercial CAs. For example, browser products, such as Internet Explorer, Firefox or 

Safari, typically implement a mechanism to generate key pairs, send the public keys to a 

CA and return a public-key certificate for that key to the browser. The certificates are 

then kept in a user-certificate store on the user’s computer, which can be managed in a 

manner similar to a root-certificate store (see Section 2). The process and interfaces for 

generating keys, and for requesting, downloading and installing certificates are specific to 

both the individual user client product and, sometimes, to the CA itself; however, the 

websites of CAs often have pages that guide the user through the key generation and 

certificate issuance process for the common browser products. Users can share 

certificates via e-mail or public-key infrastructures, smart cards or other memory tokens.
80

Similarly, secure web servers have TLS server certificates, with certain specific 

characteristics; note that, although federal users are required to use TLS rather than SSL, 

the certificates are identical. The Subject Name in these certificates follows specific rules 

so that the domain name of the server is included in the Subject Name field of the 

certificate. Commercial CAs sell TLS certificates, and, where it is important to reach a 

general population of non-government users, it may be desirable to get a TLS certificate 

from a CA that has its root certificate widely distributed “out of the box” in the certificate 

stores of Microsoft Windows, Macintosh OS X, and the various Mozilla browsers. This 

will allow most users to verify the server certificate. However, it is important to review 

the certificate policy and choices that may be available from the CAs selling TLS

certificates for use on federal agency web servers in order to ensure that the certificates 

meet the requirements stated herein and in [SP 800-57 Part 1].
81

Appendix D: References

[COMMON] X.509 Certificate Policy for the U.S. Federal PKI Common Policy Framework, 

Version 1.23, Federal Public Key Infrastructure Policy Authority, May 5, 2014, 

105 pp.

http://www.idmanagement.gov/sites/default/files/documents/FCPCA%20CP%20v

1%2023_0.pdf [accessed 12/23/14].

[COMMON PROF] X.509 Certificate and Certificate Revocation List (CRL) Extensions Profile for the 

Shared Secret Providers (SSP) Program, Federal PKI Policy Authority, Shared 

Service Provider Working Group, January 7, 2008, 52 pp.

http://www.idmanagement.gov/sites/default/files/documents/CertCRLprofileForC

P.pdf [accessed 12/23/14].

[DENN] D. Denning and G. M. Sacco, “Timestamps in Key Distribution Protocols,”

Communications of the ACM, vol. 24, no. 8, pp. 533-536, August 1981.

http://dx.doi.org/10.1145/358722.358740.

[FIPS 140-2] U.S. Department of Commerce, Security Requirements for Cryptographic 

Modules, Federal Information Processing Standards (FIPS) Publication 140-2, 

National Institute of Standards and Technology, May 25, 2001 (including change 

notices as of December 3, 2002), 69 pp.

http://csrc.nist.gov/publications/fips/fips140-2/fips1402.pdf [accessed 12/23/14].

[FIPS 180-4] U.S. Department of Commerce, Secure Hash Standard (SHS), Federal Information 

Processing Standards (FIPS) Publication 180-4, National Institute of Standards and 

Technology, March 2012, 37 pp.

http://csrc.nist.gov/publications/fips/fips180-4/fips-180-4.pdf [accessed 12/23/14].

[FIPS 186-4] U.S. Department of Commerce, Digital Signature Standard (DSS), Federal 

Information Processing Standards (FIPS) Publication 186-4, National Institute of 

Standards and Technology, July 2013, 130 pp.

http://dx.doi.org/10.6028/NIST.FIPS.186-4.

[FIPS 197] U.S. Department of Commerce, Advanced Encryption Standard (AES), Federal 

Information Processing Standards (FIPS) Publication 197, National Institute of 

Standards and Technology, November 2001, 51 pp.

http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf [accessed 12/23/14].

[FIPS 201] U.S. Department of Commerce, Personal Identity Verification (PIV) of Federal 

Employees and Contractors, Federal Information Processing Standards (FIPS) 

Publication 201-2, National Institute of Standards and Technology, August 2013, 

87 pp.

http://dx.doi.org/10.6028/NIST.FIPS.201-2.

[FPKI PROF] Federal Public Key Infrastructure (PKI) X.509 Certificate and CRL Extensions 

Profile, Booz-Allen & Hamilton, Inc., and National Institute of Standards and 

Technology, October 12, 2005, 33 pp.

http://www.idmanagement.gov/sites/default/files/documents/fpki_certificate_profil

e_0.pdf [accessed 12/23/14].
82

[NEED] R. M. Needham and M. D. Schroeder, “Using Encryption for Authentication in 

Large Networks of Computers,” Communications of the ACM, vol. 21, no. 12, pp. 

993-999, December 1978.

http://dx.doi.org/10.1145/359657.359659.

[NEUM] B. C. Neuman and T. Y. Ts’o, “Kerberos: An Authentication Service for Computer 

Networks,” IEEE Communications Magazine, vol. 32, no. 9, pp. 33-38, September 

1994.

http://dx.doi.org/10.1109/35.312841.

[NISTIR 7966] T. Ylonen, K. Scarfone and M. Souppaya, Security of Automated Access 

Management Using Secure Shell (SSH), National Institute of Standards and 

Technology, (DRAFT) NISTIR 7966, August 2014, 43 pp.

http://csrc.nist.gov/publications/PubsNISTIRs.html [accessed 12/23/14].

[OMB 04-04] Office of Management and Budget (OMB), E-Authentication Guidance for 

Federal Agencies, OMB Memorandum M-04-04, December 16, 2003, 17 pp.

http://www.whitehouse.gov/sites/default/files/omb/assets/omb/memoranda/fy04/m

04-04.pdf [accessed 12/23/14].

[OTAR] Project 25— Digital Radio Over-the-Air-Rekeying (OTAR) Protocol, TIA/EIA

102.AACA, April 2001.

[OTAR1] Project 25–Digital Radio Over-the-Air-Rekeying (OTAR) Protocol—Addendum 

1—Key Management Security Requirements for Type 3 Block Encryption 

Algorithms, TIA/EIA 102.AACA-1, November 2002.

[RFC 1034] P. Mockapetris, Domain Names – Concepts and Facilities, Internet Engineering 

Task Force (IETF) RFC 1034, November 1987.

http://www.ietf.org/rfc/rfc1034.txt [accessed 12/23/14].

[RFC 1035] P. Mockapetris, Domain Names – Implementation and Specification, Internet 

Engineering Task Force (IETF) RFC 1035, November 1987.

http://www.ietf.org/rfc/rfc1035.txt [accessed 12/23/14].

[RFC 1510] J. Kohl and C. Neuman, The Kerberos Network Authentication Service (V5), 

Internet Engineering Task Force (IETF) RFC 1510, September 1993.

http://www.ietf.org/rfc/rfc1510.txt [accessed 12/23/14].

[RFC 2045] N. Freed and N. Borenstein, Multipurpose Internet Mail Extensions (MIME) Part 

One: Format of Internet Message Bodies, Internet Engineering Task Force (IETF) 

RFC 2045, November 1996.

http://www.ietf.org/rfc/rfc2045.txt [accessed 12/23/14].

[RFC 2046] N. Freed and N. Borenstein, Multipurpose Internet Mail Extensions (MIME) Part 

2: Media Types, Internet Engineering Task Force (IETF) RFC 2046, November 

1996.

http://www.ietf.org/rfc/rfc2046.txt [accessed 12/23/14].
83

[RFC 2047] K. Moore, MIME (Multipurpose Internet Mail Extensions) Part Three: Message 

Header Extensions for Non-ASCII Text, Internet Engineering Task Force (IETF) 

RFC 2047, November 1996.

http://www.ietf.org/rfc/rfc2047.txt [accessed 12/23/14].

[RFC 2049] N. Freed and N. Borenstein, Multipurpose Internet Mail Extensions (MIME) Part 

Five: Conformance Criteria and Examples, Internet Engineering Task Force 

(IETF) RFC 2049, November 1996.

http://www.ietf.org/rfc/rfc2049.txt [accessed 12/23/14].

[RFC 2119] S. Bradner, Key words for use in RFCs to Indicate Requirement Levels, Internet 

Engineering Task Force (IETF) RFC 2119, March 1997.

http://www.ietf.org/rfc/rfc2119.txt [accessed 12/23/14].

[RFC 2246] T. Dierks and C. Allen, The TLS Protocol Version 1.0, Internet Engineering Task 

Force (IETF) RFC 2246, January 1999 (obsoleted by RFC 4346).

http://www.ietf.org/rfc/rfc2246.txt [accessed 12/23/14].

[RFC 2401] S. Kent and R. Atkinson, Security Architecture for the Internet Protocol, Internet 

Engineering Task Force (IETF) RFC 2401, November 1998 (obsoleted by RFC 

4301).

http://www.ietf.org/rfc/rfc2401.txt [accessed 12/23/14].

[RFC 2402] S. Kent and R. Atkinson, IP Authentication Header, Internet Engineering Task 

Force (IETF) RFC 2402, November 1998 (obsoleted by RFC 4302 and RFC 

4835).

http://www.ietf.org/rfc/rfc2402.txt [accessed 12/23/14].

[RFC 2404] C. Madson and R. Glenn, The Use of HMAC-SHA-1-96 within ESP and AH, 

Internet Engineering Task Force (IETF) RFC 2404, November 1998.

http://www.ietf.org/rfc/rfc2404.txt [accessed 12/23/14].

[RFC 2405] C. Madson and N. Doraswamy, The ESP DES-CBC Cipher Algorithm with 

Explicit IV, Internet Engineering Task Force (IETF) RFC 2405, November 1998.

http://www.ietf.org/rfc/rfc2405.txt [accessed 12/23/14].

[RFC 2406] S. Kent and R. Atkinson, IP Encapsulating Security Payload (ESP), Internet 

Engineering Task Force (IETF) RFC 2406, November 1998 (obsoleted by RFC 

4303 and RFC 4835).

http://www.ietf.org/rfc/rfc2406.txt [accessed 12/23/14].

[RFC 2407] D. Piper, The Internet IP Security Domain of Interpretation for ISAKMP, Internet 

Engineering Task Force (IETF) RFC 2407, November 1998 (obsoleted by RFC 

5996).

http://www.ietf.org/rfc/rfc2407.txt [accessed 12/23/14].

[RFC 2408] D. Maughan, M. Schertler, M. Schneider, and J. Turner, Internet Security 

Association and Key Management Protocol (ISAKMP), Internet Engineering Task 

Force (IETF) RFC 2408, November 1998 (obsoleted by RFC 5996).

http://www.ietf.org/rfc/rfc2408.txt [accessed 12/23/14].
84

[RFC 2409] D. Harkins and D. Carrel, The Internet Key Exchange (IKE), Internet Engineering 

Task Force (IETF) RFC 2409, November 1998 (obsoleted by RFC 5996).

http://www.ietf.org/rfc/rfc2409.txt [accessed 12/23/14].

[RFC 2410] R. Glenn and S. Kent, The NULL Encryption Algorithm and its Use with IPsec, 

Internet Engineering Task Force (IETF) RFC 2410, November 1998.

http://www.ietf.org/rfc/rfc2410.txt [accessed 12/23/14].

[RFC 2451] R. Pereira and R. Adams, The ESP CBC-Mode Cipher Algorithms, Internet 

Engineering Task Force (IETF) RFC 2451, November 1998.

http://www.ietf.org/rfc/rfc2451.txt [accessed 12/23/14].

[RFC 2631] E. Rescorla, Diffie-Hellman Key Agreement Method, Internet Engineering Task 

Force (IETF) RFC 2631, June 1999.

http://www.ietf.org/rfc/rfc2631.txt [accessed 12/23/14].

[RFC 2634] P. Hoffman (ed.), Enhanced Security Services for S/MIME, Internet Engineering 

Task Force (IETF) RFC 2634, June 1999.

http://www.ietf.org/rfc/rfc2634.txt [accessed 12/23/14].

[RFC 3394] J. Schaad and R. Housley, Advanced Encryption Standard (AES) Key Wrap 

Algorithm, Internet Engineering Task Force (IETF) RFC 3394, September 2002.

http://www.ietf.org/rfc/rfc3394.txt [accessed 12/23/14].

[RFC 3447] J. Jonsson and B. Kaliski, Public-Key Cryptography Standards (PKCS) #1: RSA 

Cryptography Specifications Version 2.1, Internet Engineering Task Force (IETF) 

RFC 3447, February 2003.

http://www.ietf.org/rfc/rfc3447.txt [accessed 12/23/14].

[RFC 3566] S. Frankel and H. Herbert, The AES-XCBC-MAC-96 Algorithm and its Use with 

IPsec, Internet Engineering Task Force (IETF) RFC 3566, September 2003.

http://www.ietf.org/rfc/rfc3566.txt [accessed 12/23/14].

[RFC 3602] S. Frankel, R. Glenn and S. Kelly, The AES-CBC Cipher Algorithm and its Use 

with IPsec, Internet Engineering Task Force (IETF) RFC 3602, September 2003.

http://www.ietf.org/rfc/rfc3602.txt [accessed 12/23/14].

[RFC 3645] S. Kwan, P. Garg, J. Gilroy, L. Esibov, J. Westhead and R. Hall, Generic Security 

Service Algorithm for Secret Key Transaction Authentication for DNS (GSS-TSIG), 

Internet Engineering Task Force (IETF) RFC 3645, October 2003.

http://www.ietf.org/rfc/rfc3645.txt [accessed 12/23/14].

[RFC 3686] R. Housley, Using Advanced Encryption Standard (AES) Counter Mode with IPsec 

Encapsulating Security Payload (ESP), Internet Engineering Task Force (IETF) 

RFC 3686, January 2004.

http://www.ietf.org/rfc/rfc3686.txt [accessed 12/23/14].

[RFC 3962] K. Raeburn, Advanced Encryption Standard (AES) Encryption for Kerberos 5, 

Internet Engineering Task Force (IETF) RFC 3962, February 2005.

http://www.ietf.org/rfc/rfc3962.txt [accessed 12/23/14].
85

[RFC 4033] R. Arends, R. Austein, M. Larson, D. Massey and S. Rose, DNS Security 

Introduction and Requirements, Internet Engineering Task Force (IETF) RFC 

4033, March 2005.

http://www.ietf.org/rfc/rfc4033.txt [accessed 12/23/14].

[RFC 4034] R. Arends, R. Austein, M. Larson, D. Massey and S. Rose, Resource Records for 

the DNS Security Extensions, Internet Engineering Task Force (IETF) RFC 4034, 

March 2005.

http://www.ietf.org/rfc/rfc4034.txt [accessed 12/23/14].

[RFC 4035] R. Arends, R. Austein, M. Larson, D. Massey and S. Rose, Protocol Modifications 

for the DNS Security Extensions, Internet Engineering Task Force (IETF) RFC 

4035, March 2005.

http://www.ietf.org/rfc/rfc4035.txt [accessed 12/23/14].

[RFC 4106] J. Viega and D. McGrew, The Use of Galois/Counter Mode (GCM) in IPsec 

Encapsulating Security Payload (ESP), Internet Engineering Task Force (IETF) 

RFC 4106, June 2005.

http://www.ietf.org/rfc/rfc4106.txt [accessed 12/23/14].

[RFC 4109] P. Hoffman, Algorithms for Internet Key Exchange version 1 (IKEv1), Internet 

Engineering Task Force (IETF) RFC 4109, May 2005.

http://www.ietf.org/rfc/rfc4109.txt [accessed 12/23/14].

[RFC 4120] C. Neuman, T. Yu, S. Hartman and K. Raeburn, The Kerberos Network 

Authentication Service (V5), Internet Engineering Task Force (IETF) RFC 4120, 

July 2005.

http://www.ietf.org/rfc/rfc4120.txt [accessed 12/23/14].

[RFC 4210] C. Adams, S. Farrell, T. Kause and T. Mononen, Internet X.509 Public Key 

Infrastructure Certificate Management Protocol (CMP), Internet Engineering Task 

Force (IETF) RFC 4210, September 2005.

http://www.ietf.org/rfc/rfc4210.txt [accessed 12/23/14].

[RFC 4251] T. Ylonen and C. Lonvick (ed.), The Secure Shell (SSH) Protocol Architecture, 

Internet Engineering Task Force (IETF) RFC 4251, January 2006.

http://www.ietf.org/rfc/rfc4251.txt [accessed 12/23/14].

[RFC 4252] T. Ylonen and C. Lonvick (ed.), The Secure Shell (SSH) Authentication Protocol, 

Internet Engineering Task Force (IETF) RFC 4252, January 2006.

http://www.ietf.org/rfc/rfc4252.txt [accessed 12/23/14].

[RFC 4253] T. Ylonen and C. Lonvick (ed.), The Secure Shell (SSH) Transport Layer Protocol, 

Internet Engineering Task Force (IETF) RFC 4253, January 2006.

http://www.ietf.org/rfc/rfc4253.txt [accessed 12/23/14].

[RFC 4254] T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, Internet 

Engineering Task Force (IETF) RFC 4254, January 2006.

http://www.ietf.org/rfc/rfc4254.txt [accessed 12/23/14].
86

[RFC 4255] J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell (SSH) Key 

Fingerprints, Internet Engineering Task Force (IETF) RFC 4255, January 2006.

http://www.ietf.org/rfc/rfc4255.txt [accessed 12/23/14].

[RFC 4288] N. Freed and J. Klensin, Media Type Specifications and Registration Procedures, 

Internet Engineering Task Force (IETF) RFC 4288, December 2005.

http://www.ietf.org/rfc/rfc4288.txt [accessed 12/23/14].

[RFC 4289] N. Freed and J. Klensin, Multipurpose Internet Mail Extensions (MIME) Part 

Four: Registration Procedures, Internet Engineering Task Force (IETF) RFC 

4289, December 2005.

http://www.ietf.org/rfc/rfc4289.txt [accessed 12/23/14].

[RFC 4301] S. Kent and K. Seo, Security Architecture for the Internet Protocol, Internet 

Engineering Task Force (IETF) RFC 4301, December 2005.

http://www.ietf.org/rfc/rfc4301.txt [accessed 12/23/14].

[RFC 4302] S. Kent, IP Authentication Header, Internet Engineering Task Force (IETF) RFC 

4302, December 2005.

http://www.ietf.org/rfc/rfc4302.txt [accessed 12/23/14].

[RFC 4303] S. Kent, IP Encapsulating Security Payload (ESP), Internet Engineering Task 

Force (IETF) RFC 4303, December 2005.

http://www.ietf.org/rfc/rfc4303.txt [accessed 12/23/14].

[RFC 4307] J. Schiller, Cryptographic Algorithms for Use in the Internet Key Exchange 

Version 2 (IKEv2), Internet Engineering Task Force (IETF) RFC 4307, December 

2005.

http://www.ietf.org/rfc/rfc4307.txt [accessed 12/23/14].

[RFC 4308] P. Hoffman, Cryptographic Suites for IPsec, Internet Engineering Task Force 

(IETF) RFC 4308, December 2005.

http://www.ietf.org/rfc/rfc4308.txt [accessed 12/23/14].

[RFC 4309] R. Housley, Using Advanced Encryption Standard (AES) CCM Mode with IPsec 

Encapsulating Security Payload (ESP), Internet Engineering Task Force (IETF) 

RFC 4309, December 2005.

http://www.ietf.org/rfc/rfc4309.txt [accessed 12/23/14].

[RFC 4346] T. Dierks and E. Rescorla, The Transport Layer Security (TLS) Protocol Version 

1.1, Internet Engineering Task Force (IETF) RFC 4346, April 2006 (obsoleted by 

RFC 5246).

http://www.ietf.org/rfc/rfc4346.txt [accessed 12/23/14].

[RFC 4419] M. Friedl, N. Provos and W. Simpson, Diffie-Hellman Group Exchange for the 

Secure Shell (SSH) Transport Layer Protocol, Internet Engineering Task Force 

(IETF) RFC 4419, March 2006.

http://www.ietf.org/rfc/rfc4419.txt [accessed 12/23/14].
87

[RFC 4434] P. Hoffman, The AES-XCBC-PRF-128 Algorithm for the Internet Key Exchange 

Protocol (IKE), Internet Engineering Task Force (IETF) RFC 4434, February 

2006.

http://www.ietf.org/rfc/rfc4434.txt [accessed 12/23/14].

[RFC 4462] J. Hutzelman, J. Salowey, J. Galbraith and V. Welch, Generic Security Service 

Application Program Interface (GSS-API) Authentication and Key Exchange for 

the Secure Shell (SSH) Protocol, Internet Engineering Task Force (IETF) RFC 

4462, May 2006.

http://www.ietf.org/rfc/rfc4462.txt [accessed 12/23/14].

[RFC 4511] J. Sermersheim (ed.), Lightweight Directory Access Protocol (LDAP): The 

Protocol, Internet Engineering Task Force (IETF) RFC 4511, June 2006.

http://www.ietf.org/rfc/rfc4511.txt [accessed 12/23/14].

[RFC 4512] K. Zeilenga, Lightweight Directory Access Protocol (LDAP): Directory 

Information Models, Internet Engineering Task Force (IETF) RFC 4512, June 

2006.

http://www.ietf.org/rfc/rfc4512.txt [accessed 12/23/14].

[RFC 4543] D. McGrew and J. Viega, The Use of Galois Message Authentication Code 

(GMAC) in IPsec ESP and AH, Internet Engineering Task Force (IETF) RFC 

4543, May 2006.

http://www.ietf.org/rfc/rfc4543.txt [accessed 12/23/14].

[RFC 4556] L. Zhu and B. Tung, Public Key Cryptography for Initial Authentication in 

Kerberos (PKINIT), Internet Engineering Task Force (IETF) RFC 4556, June 

2006.

http://www.ietf.org/rfc/rfc4556.txt [accessed 12/23/14].

[RFC 4635] D. Eastlake III, HMAC SHA TSIG Algorithm Identifiers, Internet Engineering Task 

Force (IETF) RFC 4635, August 2006.

http://www.ietf.org/rfc/rfc4635.txt [accessed 12/23/14].

[RFC 4754] D. Fu and J. Solinas, IKE and IKEv2 Authentication Using the Elliptic Curve 

Digital Signature Algorithm (ECDSA), Internet Engineering Task Force (IETF) 

RFC 4754, January 2007.

http://www.ietf.org/rfc/rfc4754.txt [accessed 12/23/14].

[RFC 4835] V. Manral, Cryptographic Algorithm Implementation Requirements for 

Encapsulating Security Payload (ESP) and Authentication Header (AH), Internet 

Engineering Task Force (IETF) RFC 4835, April 2007.

http://www.ietf.org/rfc/rfc4835.txt [accessed 12/23/14].

[RFC 4868] S. Kelly and S. Frankel, Using HMAC-SHA-256, HMAC-SHA-384, and HMACSHA-512 with IPsec, Internet Engineering Task Force (IETF) RFC 4868, May 

2007.

http://www.ietf.org/rfc/rfc4868.txt [accessed 12/23/14].
88

[RFC 5019] A. Deacon and R. Hurst, The Lightweight Online Certificate Status Protocol 

(OCSP) Profile for High-Volume Environments, Internet Engineering Task Force 

(IETF) RFC 5019, September 2007.

http://www.ietf.org/rfc/rfc5019.txt [accessed 12/23/14].

[RFC 5035] P. Hoffman, Enhanced Security Services (ESS) Update: Adding CertID Algorithm 

Agility, Internet Engineering Task Force (IETF) RFC 5035, August 2007.

http://www.ietf.org/rfc/rfc5035.txt [accessed 12/23/14].

[RFC 5114] M. Lepinski and S. Kent, Additional Diffie-Hellman Groups for Use with IETF 

Standards, Internet Engineering Task Force (IETF) RFC 5114, January 2008.

http://www.ietf.org/rfc/rfc5114.txt [accessed 12/23/14].

[RFC 5155] B. Laurie, G. Sisson, R. Arends and D. Blacka. DNS Security (DNSSEC) Hashed 

Authenticated Denial of Existence, Internet Engineering Task Force (IETF) RFC 

5155, March 2008.

http://www.ietf.org/rfc/rfc5155.txt [accessed 12/23/14].

[RFC 5246] T. Dierks and E. Rescorla, The Transport Layer Security (TLS) Protocol Version 

1.2, Internet Engineering Task Force (IETF) RFC 5246, August 2008.

http://www.ietf.org/rfc/rfc5246.txt [accessed 12/23/14].

[RFC 5272] J. Schaad and M. Myers, Certificate Management over CMS (CMC), Internet 

Engineering Task Force (IETF) RFC 5272, June 2008.

http://www.ietf.org/rfc/rfc5272.txt [accessed 12/23/14].

[RFC 5280] D. Cooper, S. Santesson, S. Farrell, S. Boeyen, R. Housley, and W. Polk, Internet 

X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) 

Profile, Internet Engineering Task Force (IETF) RFC 5280, May 2008.

http://www.ietf.org/rfc/rfc5280.txt [accessed 12/23/14].

[RFC 5349] L. Zhu, K. Jaganathan and K. Lauter, Elliptic Curve Cryptography (ECC) Support 

for Public Key Cryptography for Initial Authentication in Kerberos (PKINIT), 

Internet Engineering Task Force (IETF) RFC 5349, September 2009.

http://www.ietf.org/rfc/rfc5349.txt [accessed 12/23/14].

[RFC 5430] M. Salter, E. Rescorla and R. Housley, Suite B Profile Transport Layer Security 

(TLS), Internet Engineering Task Force (IETF) RFC 5430, March 2009.

http://www.ietf.org/rfc/rfc5430.txt [accessed 12/23/14].

[RFC 5647] K. Igoe and J. Solinas, AES Galois Counter Mode for the Secure Shell Transport 

Layer Protocol, Internet Engineering Task Force (IETF) RFC 5647, August, 2009.

http://www.ietf.org/rfc/rfc5647.txt [accessed 12/23/14].

[RFC 5652] R. Housley, Cryptographic Message Syntax (CMS), Internet Engineering Task 

Force (IETF) RFC 5652, September 2009.

http://www.ietf.org/rfc/rfc5652.txt [accessed 12/23/14].
89

[RFC 5656] D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the Secure Shell 

Transport Layer, Internet Engineering Task Force (IETF) RFC 5656, December 

2009.

http://www.ietf.org/rfc/rfc5656.txt [accessed 12/23/14].

[RFC 5751] B. Ramsdell and S. Turner, Secure/Multipurpose Internet Mail Extensions 

(S/MIME) Version 3.2 Message Specification”, Internet Engineering Task Force 

(IETF) RFC 5751, January 2010.

http://www.ietf.org/rfc/rfc5751.txt [accessed 12/23/14].

[RFC 5903] D. Fu and J. Solinas, Elliptic Curve Groups modulo a Prime (ECP Groups) for 

IKE and IKEv2, Internet Engineering Task Force (IETF) RFC 5903, June 2010.

http://www.ietf.org/rfc/rfc5903.txt [accessed 12/23/14].

[RFC 5996] C. Kaufman, P. Hoffman, Y. Nir and P. Eronen, Internet Key Exchange Protocol 

Version 2 (IKEv2), Internet Engineering Task Force (IETF) RFC 5996, September 

2010.

http://www.ietf.org/rfc/rfc5996.txt [accessed 12/23/14].

[RFC 6113] S. Hartman and L. Zhu, A Generalized Framework for Kerberos PreAuthentication, Internet Engineering Task Force (IETF) RFC 6113, April 2011.

http://www.ietf.org/rfc/rfc6113.txt [accessed 12/23/14].

[RFC 6187] K. Igoe and D. Stebila, X.509v3 Certificates for Secure Shell Authentication, 

Internet Engineering Task Force (IETF) RFC 6187, March 2011.

http://www.ietf.org/rfc/rfc6187.txt [accessed 12/23/14].

[RFC 6239] K. Igoe, Suite B Cryptographic Suites for Secure Shell (SSH), Internet Engineering 

Task Force (IETF) RFC 6239, May 2011.

http://www.ietf.org/rfc/rfc6239.txt [accessed 12/23/14].

[RFC 6251] S. Josefsson, Using Kerberos Version 5 over the Transport Layer Security (TLS) 

Protocol, Internet Engineering Task Force (IETF) RFC 6251, May 2011.

http://www.ietf.org/rfc/rfc6251.txt [accessed 12/23/14].

[RFC 6318] R. Housley and J. Solinas, Suite B in Secure/Multipurpose Internet Mail 

Extensions (S/MIME), Internet Engineering Task Force (IETF) RFC 6318, June 

2011.

http://www.ietf.org/rfc/rfc6318.txt [accessed 12/23/14].

[RFC 6379] L. Law and J. Solinas, Suite B Cryptographic Suites for IPsec, Internet 

Engineering Task Force (IETF) RFC 6379, October 2011.

http://www.ietf.org/rfc/rfc6379.txt [accessed 12/23/14].

[RFC 6605] P. Hoffman and W.C.A. Wijngaards, Elliptic Curve Digital Signature Algorithm 

(DSA) for DNNSEC, Internet Engineering Task Force (IETF) RFC 6605, April 

2012.

http://www.ietf.org/rfc/rfc6605.txt [accessed 12/23/14].
90

[RFC 6649] L. H. Astrand and T. Yu, Deprecate DES, RC4-HMAC-EXP, and Other Weak 

Cryptographic Algorithms in Kerberos, Internet Engineering Task Force (IETF) 

RFC 6649, July 2012.

http://www.ietf.org/rfc/rfc6649.txt [accessed 12/23/14].

[RFC 6944] S. Rose, Applicability Statement: DNS Security (DNSSEC) DNSKEY Algorithm 

Implementation Status, Internet Engineering Task Force (IETF) RFC 6944, April 

2013.

http://www.ietf.org/rfc/rfc6944.txt [accessed 12/23/14].

[RFC 6960] S. Santesson, M. Myers, R. Ankney, A. Malpani, S. Galperin and C. Adams, X.509 

Internet Public Key Infrastructure Online Certificate Status Protocol – OCSP, 

Internet Engineering Task Force (IETF) RFC 6960, June 2013.

http://www.ietf.org/rfc/rfc6960.txt [accessed 12/23/14].

[RFC 7030] M. Pritikin, P. Yee and D. Harkins (eds.), Enrollment over Secure Transport, 

Internet Engineering Task Force (IETF) RFC 7030, October 2013.

http://www.ietf.org/rfc/rfc7030.txt [accessed 12/23/14].

[RFC 7321] D. McGrew and P. Hoffman, Cryptographic Algorithm Implementation 

Requirements and Usage Guidance for Encapsulating Security Payload (ESP) and 

Authentication Header (AH), Internet Engineering Task Force (IETF) RFC 7321, 

August 2014.

http://www.ietf.org/rfc/rfc7321.txt [accessed 12/23/14].

[SEC1] Certicom Research, SEC1: Elliptic Curve Cryptography, Standards for Efficient 

Cryptography Group, version 2.0, May 21, 2009.

http://www.secg.org/sec1-v2.pdf [accessed 12/23/14].

[SP 500-267] D. Montgomery, S. Nightingale, S. Frankel and M. Carson, A Profile for IPv6 in 

the U.S. Government - Version 1.0, National Institute of Standards and 

Technology, NIST Special Publication 500-267, July 2008, 84 pp.

http://www.nist.gov/manuscript-publication-search.cfm?pub_id=900150 [accessed 

12/23/14].

[SP 800-32] D. R. Kuhn, V. C. Hu, W. T. Polk and S.-j. Chang, Introduction to Public Key 

Technology and the Federal PKI Infrastructure, National Institute of Standards 

and Technology, NIST Special Publication 800-32, February 26, 2001, 54 pp.

http://csrc.nist.gov/publications/nistpubs/800-32/sp800-32.pdf [accessed 

12/23/14].

[SP 800-38A] M. Dworkin, Recommendations for Block Cipher Modes of Operation: Methods 

and Techniques, National Institute of Standards and Technology, NIST Special 

Publication 800-38A 2001 Edition, December 2001, 66 pp.

http://csrc.nist.gov/publications/nistpubs/800-32/sp800-32.pdf [accessed 

12/23/14].

[SP 800-49] C. M. Chernick, Federal S/MIME V3 Client Profile, National Institute of Standards 

and Technology, NIST Special Publication 800-49, November 2002, 27 pp.

http://csrc.nist.gov/publications/nistpubs/800-49/sp800-49.pdf [accessed 

12/23/14].
91

[SP 800-52] T. Polk, K. McKay and S. Chokhani, Guidelines for the Selection, Configuration 

and Use of Transport Layer Security (TLS) Implementations, National Institute of 

Standards and Technology, NIST Special Publication 800-52 Revision 1, April 

2014, 67 pp.

http://dx.doi.org/10.6028/NIST.SP.800-52r1.

[SP 800-56A] E. Barker, L. Chen, A. Roginsky and M. Smid, Recommendation for Pair-Wise 

Key Establishment Schemes Using Discrete Logarithm Cryptography, National 

Institute of Standards and Technology, NIST Special Publication 800-56A

Revision 2, May 2013, 138 pp.

http://dx.doi.org/10.6028/NIST.SP.800-56Ar2.

[SP 800-56B] E. Barker, L. Chen and D. Moody, Recommendation for Pair-Wise Key 

Establishment Schemes Using Integer Factorization Cryptography, National 

Institute of Standards and Technology, NIST Special Publication 800-56B

Revision 1, September 2014, 131 pp.

http://dx.doi.org/10.6028/NIST.SP.800-56Br1.

[SP 800-57 Part 1] E. Barker, W. Barker, W. Burr, W. Polk and M. Smid, Recommendation for Key 

Management – Part 1: General, National Institute of Standards and Technology, 

NIST Special Publication 800-57 Part 1 Revision 3, July 2012, 147 pp.

http://csrc.nist.gov/publications/nistpubs/800-57/sp800-57_part1_rev3_general.pdf

[accessed 12/23/14].

[SP 800-67] W. C. Barker and E. Barker, Recommendation for the Triple Data Encryption 

Algorithm (TDEA) Block Cipher, National Institute of Standards and Technology, 

NIST Special Publication 800-67 Revision 1, January 2012, 34 pp.

http://csrc.nist.gov/publications/nistpubs/800-67-Rev1/SP-800-67-Rev1.pdf

[accessed 12/23/14].

[SP 800-77] S. Frankel, K. Kent, R. Lewkowski, A. D. Orebaugh, R. W. Ritchey and S. R. 

Shama, Guide to IPsec VPNs, National Institute of Standards and Technology, 

NIST Special Publication 800-77, December 2005, 126 pp.

http://csrc.nist.gov/publications/nistpubs/800-77/sp800-77.pdf [accessed 

12/23/14].

[SP 800-81] R. Chandramouli and S. Rose, Secure Domain Name System (DNS) Deployment 

Guide, National Institute of Standards and Technology, NIST Special Publication 

800-81-2, September 2013, 130 pp.

http://dx.doi.org/10.6028/NIST.SP.800-81-2.

[SP 800-89] E. Barker, Recommendation for Obtaining Assurances for Digital Signature 

Applications, National Institute of Standards and Technology, NIST Special 

Publication 800-89, November 2006, 38 pp.

http://csrc.nist.gov/publications/nistpubs/800-89/SP-800-89_November2006.pdf

[accessed 12/23/14].

[SP 800-90A] E. Barker and J. Kelsey, Recommendation for Random Number Generation Using 

Deterministic Random Bit Generators, National Institute of Standards and 

Technology, NIST Special Publication (DRAFT) 800-90A, November 2014, 112 

pp.

http://csrc.nist.gov/publications/PubsSPs.html [accessed 12/23/14].
92

[SP 800-90B] E. Barker and J. Kelsey, Recommendation for the Entropy Sources Used for 

Random Bit Generation, National Institute of Standards and Technology, NIST 

Special Publication (DRAFT) 800-90B, August 2012 [September 2013], 78 pp.

http://csrc.nist.gov/publications/PubsSPs.html [accessed 12/23/14].

[SP 800-90C] E. Barker and J. Kelsey, Recommendation for Random Bit Generator (RBG) 

Constructions, National Institute of Standards and Technology, NIST Special 

Publication (DRAFT) 800-90C, August 2012 [September 2013], 50 pp.

http://csrc.nist.gov/publications/PubsSPs.html [accessed 12/23/14].

[SP 800-118] K. Scarfone and M. Souppaya, Guide to Enterprise Password Management, 

National Institute of Standards and Technology, NIST Special Publication 

(DRAFT) 800-118, April 2009, 38 pp.

http://csrc.nist.gov/publications/PubsSPs.html [accessed 12/23/14].

[SP 800-131A] E. Barker and A. Roginsky, Transitions: Recommendation for Transitioning the 

Use of Cryptographic Algorithms and Key Lengths, National Institute of Standards 

and Technology, NIST Special Publication 800-131A, January 2011, 27 pp.

http://csrc.nist.gov/publications/nistpubs/800-131A/sp800-131A.pdf [accessed 

12/23/14].

[SP 800-132] M. S. Turan, E. Barker, W. Burr and L. Chen, Recommendation for PasswordBased Key Derivation, Part 1: Storage Applications, National Institute of 

Standards and Technology, NIST Special Publication 800-132, December 2010, 18 

pp.

http://csrc.nist.gov/publications/nistpubs/800-132/nist-sp800-132.pdf [accessed 

12/23/14].

[SP 800-135] Q. Dang, Recommendation for Existing Application-Specific Key Derivation 

Functions, National Institute of Standards and Technology, NIST Special 

Publication 800-135 Revision 1, December 2011, 23 pp.

http://csrc.nist.gov/publications/nistpubs/800-135-rev1/sp800-135-rev1.pdf

[accessed 12/23/14].

[TPM] Trusted Computing Group, TPM Main Specification Version 1.2, Revision 103

[Parts 1 (Design Principles), 2 (TPM Structures) and 3 (Commands)], July 9, 

2007.

http://www.trustedcomputinggroup.org/resources/tpm_main_specification

[accessed 12/23/14].

[X9.62] American National Standards Institute, Public Key Cryptography for the Financial 

Services Industry: the Elliptic Curve Digital Signature Algorithm (ECDSA), ANSI 

X9.62:2005, November 2005.

[X11] X.Org Foundation, Documentation for the X Window System Version 11 Release 

7.6 (X11R7.6), December 2010.

http://www.x.org/releases/X11R7.6/doc/index.html [accessed 12/23/14].

[X.509 Path] D. Cooper and W. T. Polk, NIST Recommendation for X.509 Path Validation, 

Version 0.5, Draft Special Publication 800-XXX, May 3, 2004, 28 pp.

http://csrc.nist.gov/groups/ST/crypto_apps_infra/documents/NIST_Recommendati

on_for_X509_PVMs.pdf [accessed 12/17/14].
93

Appendix E: Revision Changes

This revision updates cryptographic requirements for the protocols and applications in the 

document so that the required security strengths as specified in [SP 800-131A] can be 

achieved. This revision also adds the security-related updates from the protocols 

addressed in the document. In addition, this revision adds a new section for Secure Shell 

(SSH). Specific changes in this revision include the following.

Section 2:

1) Additional clarification about the recommended algorithms and key sizes for 

digital signature and key establishment certificates has been added.

2) 1024-bit RSA and DSA are no longer approved for generating digital signatures 

after 2013.

3) For CA/RA Software and Hardware (Section 2.3.1), support for LDAP is now 

recommended (using “should”), not required (using “shall”) as in the original 

version.

4) For Relying Party Software (Section 2.3.5), supporting either OCSP or CRLs for 

certificate status is required. In the first version of this publication, supporting 

CRLs was required.

5) Removed required support for ““anyExtendedKeyUsage” feature.

Section 3:

1) Information about VPN-cipher suites has been updated to include the Suite-B 

VPN cipher suites.

2) Table 3-2 and the cryptographic algorithms recommendation have been revised.

Section 4:

1) A revision of the TLS section will be in SP 800-52 revision 1 [SP 800-52] in the 

future.

Section 5:

1) The discussion of the cipher suites (ALSs) has been updated. New requirements 

in using these cipher suites have been added to meet the security requirements 

specified in [SP 800-131A].

2) In Section 5.3, item 3. The “should” was changed to “may”.

Section 6:

1) A requirement for 112 bits of security after the year 2013 has been stated for 

public key authentication and key establishment.

2) Support for using TLS for authentication and key establishment has been added.
94

Section 7:

 In Section 7.2.3, two occurrences of “should” changed to “shall” in this version.

Section 8:

1) A “should” requirement has been added for migrating to 2048-bit RSA with SHA256 by the end of 2013.

2) Approval for RSA and DSA with SHA-1 for zone-data signing has been limited 

to the end of 2013; see Table 8-1.

3) Message authentication algorithms have been revised in Table 8-2.

Section 9: no major changes in this section.

Section 10: new section.