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NIST Special Publication 800-52 Revision 1
Guidelines for the Selection, Configuration, and Use of Transport Layer
Security (TLS) Implementations
April 2014
August 29, 2019
SP 800-52 Rev. 1 is superseded in its entirety by the publication of SP 800-52
Rev. 2.

Superseding Publication(s) (if applicable)
The attached publication has been superseded by the following publication(s):Series/Number
Title
Author(s)Publication Date(s)URL/DOINIST Special Publication 800-52 Revision 2
Guidelines for the Selection, Configuration, and Use of Transport Layer
Security (TLS) Implementations
Kerry A. McKay; David A. Cooper
August 2019https://doi.org/10.6028/NIST.SP.800-52r2

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https://csrc.nist.gov/publications/detail/sp/800-52/rev-2/final

Date updated: August 29, 2019

NIST Special Publication 800-52
Revision 1

Guidelines for the Selection,
Configuration, and Use of
Transport Layer Security (TLS)
Implementations
Tim Polk
Kerry McKay
Santosh Chokhani

http://dx.doi.org/10.6028/NIST.SP.800-52r1

C O M P U T E R

S E C U R I T Y

NIST Special Publication 800-52
Revision 1

Guidelines for the Selection,
Configuration, and Use of Transport
Layer Security (TLS) Implementations
Tim Polk
Kerry McKay
Computer Security Division
Information Technology Laboratory
Santosh Chokhani
CygnaCom Solutions
McLean, VA
http://dx.doi.org/10.6028/NIST.SP.800-52r1

April 2014

U.S. Department of Commerce
Penny Pritzker, Secretary

National Institute of Standards and Technology
Patrick D. Gallagher, Under Secretary of Commerce for Standards and Technology and Director

Authority
This publication has been developed by NIST to further its statutory responsibilities under theFederal Information Security Management Act (FISMA), Public Law (P.L.) 107-347. NIST is
responsible for developing information security standards and guidelines, including minimum
requirements for Federal information systems, but such standards and guidelines shall not apply
to national security systems without the express approval of appropriate Federal officials
exercising policy authority over such systems. This guideline is consistent with the requirementsof the Office of Management and Budget (OMB) Circular A-130, Section 8b(3), Securing Agency
Information Systems, as analyzed in Circular A-130, Appendix IV: Analysis of Key Sections.
Supplemental information is provided in Circular A-130, Appendix III, Security of Federal
Automated Information Resources.
Nothing in this publication should be taken to contradict the standards and guidelines mademandatory and binding on Federal agencies by the Secretary of Commerce under statutory
authority. Nor should these guidelines be interpreted as altering or superseding the existing
authorities of the Secretary of Commerce, Director of the OMB, or any other Federal official.
This publication may be used by nongovernmental organizations on a voluntary basis and is not
subject to copyright in the United States. Attribution would, however, be appreciated by NIST.
National Institute of Standards and Technology Special Publication 800-52 Revision 1
Natl. Inst. Stand. Technol. Spec. Publ. 800-52 Revision 1, 66 pages (April 2014)
http://dx.doi.org/10.6028/NIST.SP.800-52r1
CODEN: NSPUE2

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

Comments on this publication may be submitted to:
National Institute of Standards and Technology
Attn: Computer Security Division, Information Technology Laboratory
100 Bureau Drive (Mail Stop 8930) Gaithersburg, MD 20899-8930
Email: SP80052-comments@nist.gov

ii

Reports on Computer Systems Technology
The Information Technology Laboratory (ITL) at the National Institute of Standards andTechnology (NIST) promotes the U.S. economy and public welfare by providing
technical leadership for the Nation’s measurement and standards infrastructure. ITL
develops tests, test methods, reference data, proof of concept implementations, andtechnical 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
Transport Layer Security (TLS) provides mechanisms to protect sensitive data during
electronic dissemination across the Internet. This Special Publication provides guidance
to the selection and configuration of TLS protocol implementations while making
effective use of Federal Information Processing Standards (FIPS) and NISTrecommended cryptographic algorithms, and requires that TLS 1.1 configured with FIPSbased cipher suites as the minimum appropriate secure transport protocol andrecommends that agencies develop migration plans to TLS 1.2 by January 1, 2015. This
Special Publication also identifies TLS extensions for which mandatory support must be
provided and other recommended extensions.
Keywords
information security; network security; SSL; TLS; Transport Layer Security

Acknowledgements
The authors, Tim Polk and Kerry McKay of NIST, and Santosh Chokhani of CygnaCom
Solutions would like to thank the many people who assisted with the development of thisdocument. In particular we would like to acknowledge Matthew J. Fanto and C. Michael
Chernick of NIST and Charles Edington III and Rob Rosenthal of Booz Allen and Hamilton who
wrote the initial published version of this document.

iii

Table of Contents
EXECUTIVE SUMMARY ......................................................................................................... VI
1

INTRODUCTION ................................................................................................................ 1
1.1
1.2
1.3
1.4

2

TLS OVERVIEW ................................................................................................................. 4
2.1
2.2
2.3
2.4
2.5
2.6
2.7

3

BACKGROUND ................................................................................................................ 1
HISTORY OF TLS ............................................................................................................ 1
SCOPE ............................................................................................................................. 2
DOCUMENT CONVENTIONS ............................................................................................ 3
HANDSHAKE PROTOCOL ................................................................................................ 4
SHARED SECRET NEGOTIATION ..................................................................................... 5
CONFIDENTIALITY .......................................................................................................... 6
INTEGRITY ...................................................................................................................... 6
AUTHENTICATION .......................................................................................................... 7
ANTI-REPLAY ................................................................................................................. 7
KEY MANAGEMENT ....................................................................................................... 7

MINIMUM REQUIREMENTS FOR TLS SERVERS ..................................................... 9
3.1
PROTOCOL VERSION SUPPORT ....................................................................................... 9
3.2
SERVER KEYS AND CERTIFICATES ................................................................................. 9
3.2.1 Server Certificate Profile......................................................................................... 10
3.2.2 Obtaining Revocation Status Information for the Client Certificate ....................... 13
3.2.3 Server Public Key Certificate Assurance................................................................. 13
3.3
CRYPTOGRAPHIC SUPPORT .......................................................................................... 14
3.3.1 Cipher Suites............................................................................................................ 14
3.3.2 Validated Cryptography .......................................................................................... 19
3.4
TLS EXTENSION SUPPORT ........................................................................................... 20
3.4.1 Mandatory TLS Extensions ...................................................................................... 20
3.4.2 Conditional TLS Extensions .................................................................................... 21
3.4.3 Discouraged TLS Extensions ................................................................................... 22
3.5
CLIENT AUTHENTICATION ........................................................................................... 23
3.5.1 Path Validation ........................................................................................................ 23
3.5.2 Trust Anchor Store................................................................................................... 24
3.5.3 Checking the Client Key Size ................................................................................... 25
3.5.4 Server Hints List ...................................................................................................... 25
3.6
SESSION RESUMPTION .................................................................................................. 26
3.7
COMPRESSION METHODS ............................................................................................. 26
3.8
OPERATIONAL CONSIDERATIONS ................................................................................. 26
3.9
SERVER RECOMMENDATIONS ...................................................................................... 27
3.9.1 Recommendations for Server Selection ................................................................... 27
3.9.2 Recommendations for Server Installation and Configuration ................................. 27
3.9.3 Recommendations for Server System Administrators .............................................. 31

4

MINIMUM REQUIREMENTS FOR TLS CLIENTS .................................................... 33
4.1
PROTOCOL VERSION SUPPORT ..................................................................................... 33
4.2
CLIENT KEYS AND CERTIFICATES ................................................................................ 33
4.2.1 Client Certificate Profile ......................................................................................... 33
4.2.2 Obtaining Revocation Status Information for the Server Certificate ...................... 35
4.2.3 Client Public Key Certificate Assurance ................................................................. 36

iv

4.3
CRYPTOGRAPHIC SUPPORT .......................................................................................... 36
4.3.1 Cipher Suites............................................................................................................ 36
4.3.2 Validated Cryptography .......................................................................................... 36
4.4
TLS EXTENSION SUPPORT ........................................................................................... 37
4.4.1 Mandatory TLS Extensions ...................................................................................... 37
4.4.2 Conditional TLS Extensions .................................................................................... 37
4.4.3 Discouraged TLS Extensions ................................................................................... 38
4.5
SERVER AUTHENTICATION .......................................................................................... 38
4.5.1 Path Validation ........................................................................................................ 39
4.5.2 Trust Anchor Store................................................................................................... 40
4.5.3 Checking the Server Key Size .................................................................................. 40
4.5.4 User Interface .......................................................................................................... 40
4.6
SESSION RESUMPTION .................................................................................................. 41
4.7
COMPRESSION METHODS ............................................................................................. 41
4.8
OPERATIONAL CONSIDERATIONS ................................................................................. 41
4.9
CLIENT RECOMMENDATIONS ....................................................................................... 41
4.9.1 Recommendations for Client Selection .................................................................... 42
4.9.2 Recommendations for Client Installation and Configuration.................................. 42
4.9.3 Recommendations for Client System Administrators............................................... 45
4.9.4 Recommendations for End Users............................................................................. 45
APPENDIX AACRONYMS ................................................................................................. 47

APPENDIX B

INTERPRETING CIPHER SUITE NAMES ............................................. 48

APPENDIX C

PRE-SHARED KEYS ................................................................................... 50

APPENDIX D

FUTURE CAPABILITIES........................................................................... 52

D.1
ADDITIONAL/ALTERNATE WEB SERVER CERTIFICATE VALIDATION MECHANISMS .. 52
D.1.1 Sovereign Keys ........................................................................................................ 52
D.1.2 Certificate Transparency ......................................................................................... 52
D.1.3 Perspectives and Convergence ................................................................................ 53
D.1.4 DANE ....................................................................................................................... 53
D.2
CHECKING SERVER/CLIENT KEY SIZE ......................................................................... 54
D.3
ENCRYPT-THEN-MAC EXTENSION .............................................................................. 54
APPENDIX EREFERENCES .............................................................................................. 55

TablesTable 3-1: TLS Server Certificate Profile ..................................................................................... 11
Table 3-2: Cipher Suites for RSA Server Certificates ................................................................... 16
Table 3-3: Additional TLS 1.2 Cipher Suites for RSA Server Certificates................................... 16
Table 3-4: Cipher Suites for ECDSA Server Certificates ............................................................. 16
Table 3-5: Additional TLS 1.2 Cipher Suites for ECDSA Server Certificates ............................. 17
Table 3-6: Cipher Suites for DSA Server Certificates .................................................................. 17
Table 3-7: Additional TLS 1.2 Cipher Suites for DSA Server Certificates .................................. 17
Table 3-8: Cipher Suites for DH Server Certificates ..................................................................... 17
Table 3-9: Additional TLS 1.2 Cipher Suites for DH Server Certificates..................................... 18
Table 3-10: Cipher Suites for ECDH Server Certificate ............................................................... 18
Table 3-11: Additional TLS 1.2 Cipher Suites for ECDH Server Certificate ............................... 18
Table 4-1: TLS Client Certificate Profile ...................................................................................... 34
Table C-1: Pre-shared Key Cipher Suites ..................................................................................... 50v

Executive Summary
Office of Management and Budget (OMB) Circular A-130, Management of Federal
Information Resources, requires managers of publicly accessible information repositories
or dissemination systems that contain sensitive but unclassified data to ensure thatsensitive data is protected commensurate with the risk and magnitude of the harm thatwould result from the loss, misuse, or unauthorized access to or modification of suchdata. Given the nature of interconnected networks and the use of the Internet to share
information, protection of this sensitive data can become difficult if proper mechanisms
are not employed to protect the data. Transport layer security (TLS) provides such amechanism to protect sensitive data during electronic dissemination across the Internet.
TLS is a protocol created to provide authentication, confidentiality, and data integrity
between two communicating applications. TLS is based on a precursor protocol called
the Secure Sockets Layer Version 3.0 (SSL 3.0) and is considered to be an improvement
to SSL 3.0. SSL 3.0 is specified in [RFC6101]. The Transport Layer Security version 1(TLS 1.0) specification is an Internet Request for Comments [RFC2246]. Each document
specifies a similar protocol that provides security services over the Internet. TLS 1.0 has
been revised to version 1.1, as documented in [RFC4346], and TLS 1.1 has been further
revised to version 1.2, as documented in [RFC5246]. In addition, some extensions havebeen defined to mitigate some of the known security vulnerabilities in implementations
using TLS. These vulnerabilities are not necessarily weaknesses in TLS, but in how
applications use TLS.
This Special Publication provides guidance to the selection and configuration of TLS
protocol implementations while making effective use of Approved cryptographic
schemes and algorithms. In particular, it requires that TLS 1.1 be configured with cipher
suites using Approved schemes and algorithms as the minimum appropriate securetransport protocol 1. It also recommends that agencies develop migration plans to TLS
1.2, configured using Approved schemes and algorithms, by January 1, 2015. Wheninteroperability with non-government systems is required, TLS 1.0 may be supported.
This Special Publication also identifies TLS extensions for which mandatory support
must be provided and other recommended extensions.
Use of the recommendations provided in this Special Publication would promote:•

More consistent use of authentication, confidentiality and integrity mechanisms
for the protection of information transport across the Internet;•

Consistent use of recommended cipher suites that encompass NIST-Approved
algorithms and open standards;

1 While SSL 3.0 is the most secure of the SSL protocol versions, it is not approved for use in the protection of Federal
information because it relies in part on the use of cryptographic algorithms that are not Approved. TLS versions
1.1 and 1.2 are approved for the protection of Federal information, when properly configured. TLS version 1.0 isapproved only when it is required for interoperability with non-government systems and is configured according
to these guidelines.

vi•Protection against known and anticipated attacks on the TLS protocol; and•Informed decisions by system administrators and managers in the integration oftransport layer security implementations.

While these guidelines are primarily designed for Federal users and systemadministrators to adequately protect sensitive but unclassified U.S. Federal Government
data against serious threats on the Internet, they may also be used within closed network
environments to segregate data. (The client-server model and security services discussed
also apply in these situations). This Special Publication supersedes NIST Special
Publication 800-52. This Special Publication should be used in conjunction with existing
policies and procedures.

vii

Guidelines for TLS Implementations

1 Introduction
Many networked applications rely on the Secure Sockets Layer (SSL) and Transport
Layer Security (TLS) protocols to protect sensitive data transmitted over insecurechannels. The Internet’s client-server model and communication protocol designprinciples have been described in many books, such as [Rescorla01], [Comer00], and[Hall00]. TLS requires the existence of a Public Key Infrastructure (PKI) that generates
public key certificates in compliance with [RFC5280]. Books such as [Adams99] and[Housley01], as well as technical journal articles (e.g., [Polk03]) and NIST publications(e.g., [SP800-32]), describe how PKI can be used to protect information in the Internet.
This document assumes that the reader of these guidelines is familiar with public key
infrastructure concepts, including, for example, X.509 certificates; and SSL/TLS
protocols. The references cited above and in Appendix E further explain the backgroundconcepts that are not fully explained in these guidelines.

1.1 Background
The TLS protocol is used to secure communications in a wide variety of onlinetransactions. Such transactions include financial transactions (e.g., banking, trading
stocks, e-commerce), healthcare transactions (e.g., viewing medical records or scheduling
medical appointments), and social transactions (e.g., email or social networking). Any
network service that handles sensitive or valuable data, whether it is personallyidentifiable information (PII), financial data, or login information, needs to adequately
protect that data. TLS provides a protected channel for sending data between the server
and the client. The client is often, but not always, a web browser.
TLS is a layered protocol that runs on top of a reliable transport protocol – typically thetransmission control protocol (TCP). Application protocols, such as the Hypertext
Transfer Protocol (HTTP) and the Internet Message Access Protocol (IMAP), can run
above TLS. TLS is application independent, and used to provide security to any two
communicating applications that transmit data over a network via an application protocol.
It can be used to create a virtual private network (VPN) that connects an external system
to an internal network, allowing that system to access a multitude of internal services andresources as if it were in the network.

1.2 History of TLS
The SSL protocol was designed by the Netscape Corporation 2 to meet security needs ofclient and server applications. Version 1 of SSL was never released. SSL 2.0 was
released in 1995, but had well-known security vulnerabilities, which were addressed by
the 1996 release of SSL 3.0. During this timeframe, Microsoft Corporation released a
protocol known as Private Communications Technology (PCT), and later released ahigher performance protocol known as the Secure Transport Layer Protocol (STLP).
2 Commercial company names are used for historical reference purposes only. No product endorsement is intended or
implied.

1

Guidelines for TLS Implementations

PCT and STLP never commanded the market share that SSL 2.0 and SSL 3.0
commanded. The Internet Engineering Task Force (IETF), a technical working groupresponsible for developing Internet standards to ensure communications compatibility
across different implementations, attempted to resolve, as best it could, security
engineering and protocol incompatibility issues between the protocols. The IETF
standards track Transport Layer Security Protocol Version 1.0 (TLS 1.0) emerged andwas codified by the IETF as [RFC2246]. While TLS 1.0 is based on SSL 3.0, and thedifferences between them are not dramatic, they are significant enough that TLS 1.0 and
SSL 3.0 do not interoperate. TLS 1.0 is also referred to as SSL 3.1.
TLS 1.0 does incorporate a mechanism by which a TLS 1.0 implementation can negotiate
to use SSL 3.0 with requesting entities as if TLS were never proposed. However,
because SSL 3.0 is not approved for use in the protection of Federal information (Section
D.9 of [FIPS140Impl]), TLS must be properly configured to ensure that the negotiation
and use of SSL 3.0 never occurs when Federal information is to be protected.
TLS 1.1 was developed to address discovered weaknesses in TLS 1.0, primarily in theareas of initialization vector selection and padding error processing. Initialization vectors
were made explicit 3 to prevent a certain class of attacks on the Cipher Block Chaining(CBC) mode of operation used by TLS. The handling of padding errors was altered totreat a padding error as a bad message authentication code, rather than a decryption
failure. In addition, the TLS 1.1 RFC acknowledges attacks on CBC mode that rely on
the time to compute the message authentication code (MAC). [RFC4346] states that todefend against such attacks, an implementation must process records in the same manner
regardless of whether padding errors exist. Further implementation considerations for
CBC modes, not included in [RFC4346], are discussed in Section 3.3.1.1.
TLS 1.2 made several cryptographic enhancements, particularly in the area of hashfunctions, with the ability to use or specify SHA-2 family algorithms for hash, MAC, andPseudorandom Function (PRF) computations. TLS 1.2 also adds support forauthenticated encryption with associated data (AEAD) cipher suites.

1.3 Scope
Security is not a single property possessed by a single protocol. Rather, security includes
a complex set of related properties that together provide the required information
assurance characteristics and information protection services. Security requirements areusually derived from a risk assessment to the threats or attacks an adversary is likely tomount against a system. The adversary is likely to take advantage of implementation
vulnerabilities found in many system components, including computer operating systems,
application software systems, and the computer networks that interconnect them. Thus,in order to secure a system against a myriad of threats, security must be judiciously
placed in the various systems and network layers.
These guidelines focus only on security within the network, and they focus directly on
the small portion of the network communications stack that is referred to as the transport
3 The initialization vector (IV) must be sent; it cannot be derived from a state known by both parties, such as theprevious message.

2

Guidelines for TLS Implementations

layer. Several other NIST publications address security requirements in the other parts ofthe systems and network layers. Adherence to these guidelines only protects the data intransit. Other applicable NIST Standards and guidelines should be used to ensure
protection of systems and stored data.
These guidelines focus on the common use where clients and servers must interoperatewith a wide variety of implementations, and authentication is performed using public key
certificates. To promote interoperability, these guidelines (and the RFCs that define the
TLS protocol) establish mandatory features and cipher suites that conforming
implementations must support. There are, however, much more constrained
implementations of TLS servers, where security is needed, but broad interoperability is
not required and the cost of implementing unused features may be prohibitive. For
example, minimal servers are often implemented in embedded controllers and network
infrastructure devices such as routers and then used with browsers to remotely configure
and manage the devices. The use of an appropriate subset of the capabilities specified inthese guidelines may be acceptable in such cases.
The scope is further limited to TLS when used in conjunction with TCP/IP. For example,
Datagram TLS (DTLS) is outside the scope of these guidelines. NIST may issue separate
guidelines for DTLS at a later date.

1.4 Document Conventions
Throughout this document, key words are used to identify requirements. The key words“shall”, “shall not”, “should”, and “should not” are used. These words are a subset of
the IETF Request For Comments (RFC) 2119 key words, and have been chosen based onconvention in other normative documents [RFC2119]. In addition to the key words, thewords “need”, “can”, and “may” are used in this document, but are not intended to benormative. The key word “Approved” is used to indicate that a scheme or algorithm is
described in a Federal Information Processing Standard (FIPS) or is recommended by
NIST.
The recommendations in this document are grouped by server recommendations andclient recommendations. Section 3 provides detailed guidance for the selection andconfiguration of TLS servers. Section 3.9.1 summarizes guidance that applies to theselection of TLS server implementations, Section 3.9.2 summarizes guidance that applies
to the configuration of TLS server implementations, and Section 3.9.3 contains guidance
for system administrators that are responsible for maintaining the server. Section 4
provides detailed guidance for the selection, configuration, and use of TLS clients.
Section 4.9.1 summarizes guidance that applies to the selection of TLS client
implementations, Section 4.9.2 summarizes guidance that applies to the configuration of
TLS client implementations, Section 4.9.3 summarizes guidance for systemadministrators responsible for maintaining TLS clients, and Section 4.9.4 contains
guidance for end users.

3

Guidelines for TLS Implementations

2 TLS Overview
TLS exchanges records over the TLS record protocol. A TLS record contains several
fields, including version information, application protocol data, and the higher-levelprotocol used to process the application data. TLS protects the application data by using a
set of cryptographic algorithms to ensure the confidentiality, integrity, and authenticity of
exchanged application data. TLS defines several protocols for connection managementthat sit on top of the record protocol, where each protocol has its own record type. These
protocols, discussed in Section 2.1, are used to establish and change security parameters,
and communicate error and warning conditions to the server and client. Sections 2.2
through 2.6 describe the security services provided by the TLS protocol and how thosesecurity services are provisioned. Section 2.7 discusses key management.

2.1 Handshake Protocol
There are three subprotocols in the TLS protocol that are used to control the session
connection: the handshake, change cipher spec 4, and alert protocols. The TLS handshake
protocol is used to negotiate the session parameters. The alert protocol is used to notify
the other party of an error condition. The change cipher spec protocol is used to change
the cryptographic parameters of a session. In addition, the client and the server exchange
application data that is protected by the security services provisioned by the negotiatedcipher suite. These security services are negotiated and established with the handshake.
The handshake protocol consists of a series of message exchanges between the client andthe server. The handshake protocol initializes both the client and server to use optional
cryptographic capabilities by negotiating a cipher suite of algorithms and functions,
including key establishment, digital signature, confidentiality and integrity algorithms.
Clients and servers can be configured so that one or more of the following security
services are negotiated during the handshake: confidentiality, message integrity,
authentication, and replay protection. A confidentiality service provides assurance thatdata is kept secret, preventing eavesdropping. A message integrity service provides
confirmation that unauthorized data modification is detected, thus preventing undetected
deletion, addition, or modification of data. An authentication service provides assuranceof the sender or receiver’s identity, thereby detecting forgery. Replay protection ensuresthat an unauthorized user does not capture and successfully replay previous data. In
order to comply with these guidelines, both the client and the server shall be configured
for data confidentiality and integrity services. Note that the anti-replay service is implicit
when data contains monotonically increasing sequence number and data integrity is
assured.
The handshake protocol is used to optionally exchange X.509 public key certificates 5 toauthenticate the server and the client to each other. In order to comply with these4 In these guidelines, “change cipher spec” refers to a protocol, and “ChangeCipherSpec” refers to the message used inthat protocol
5 The use of X.509 public key certificates is fundamental to TLS. For a comprehensive explanation of X.509 publickey certificates see [Adams99] or [Housley01]. In these guidelines, the terms “certificate” and “public key
certificate” are used interchangeably.

4

Guidelines for TLS Implementations

guidelines, the server always presents an X.509 public key certificate that complies withthe requirements stated elsewhere in these guidelines. For client-authenticatedconnections, the client also presents an X.509 public key certificate that complies withthe requirements stated elsewhere in these guidelines.
The handshake protocol is responsible for establishing the session parameters. The clientand server negotiate algorithms for authentication, confidentiality and integrity, as well asderive symmetric keys and establish other session parameters, such as data compression.
The negotiated set of authentication, confidentiality, and integrity algorithms is called thecipher suite.
When all the security parameters are in place, the ChangeCipherSpec message is used to
inform the other side to begin using the negotiated security services agreed to during thehandshake. All messages sent after the ChangeCipherSpec message are protected (i.e.,encrypted and/or integrity protected) using the negotiated cipher suite and derived
symmetric keys.
Finished messages, sent immediately following the ChangeCipherSpec messages, provide
integrity checks for the handshake messages. Each Finished message is protected using
the negotiated cipher suite and the derived session keys. Each side keeps a hash of all of
the handshake messages exchanged up to but not including their Finished message (e.g.,
the Finished message sent by the server includes the Finished message sent by the clientin the hash). The hash value is sent through a pseudorandom function (PRF) keyed by
the master secret key to form the Finished message. The receiving side decrypts theprotected Finished message and compares it to its output of the PRF on the hashedmessages. If the PRF values differ, the handshake has been modified or an error has
occurred in the key management, and the connection is aborted. If the PRF values are thesame, there is high assurance that the entire handshake has cryptographic integrity –nothing was modified, added or deleted and all key derivation was done correctly.
Alerts are used to convey information about the session, such as errors or warnings. For
example, an alert can be used to signal a decryption error (decrypt_error) or that accesshas been denied (access_denied). Some alerts are used for warnings, and others are
considered fatal and lead to immediate termination of the session. A close_notify alert
message is used to signal normal termination of a session. Like all other messages after
the handshake protocol is completed, alert messages are encrypted and optionallycompressed.
Details of the handshake, change cipher spec and alert protocols are outside the scope ofthese guidelines; they are described in [RFC5246].

2.2 Shared Secret Negotiation
The client and server establish keying material during the TLS handshake protocol. The
derivation of the premaster secret depends on the key exchange method that is agreed
upon. For example, when the Rivest Shamir Adleman (RSA) algorithm is used for the
key exchange, the premaster secret is generated by the client and sent to the server in a
ClientKeyExchange message, encrypted with the server’s public key. When DiffieHellman is used as the key exchange algorithm, the client and server send each other theirparameters, and the resulting key is used as the premaster secret. The premaster secret,5

Guidelines for TLS Implementations

along with random values exchanged by the client and server in the hello messages, isused to compute the master secret. The master secret is used to derive session keys,
described in Sections 2.3 and 2.4, which are used by the negotiated security services toprotect the data exchanged between the client and the server, thus providing a securechannel for the client and the server to communicate. Anti-replay protection is implicitly
provided, since each packet has a monotonically increasing sequence number.
The establishment of these secrets is secure against eavesdroppers. When the TLS
protocol is used in accordance with these guidelines, the application data, as well as thesecrets, are not vulnerable to attackers who place themselves in the middle of theconnection. The attacker cannot modify the handshake messages without being detectedby the client and the server because the Finished message, exchanged after security
parameter establishment, provides integrity protection to the entire exchange. In otherwords, an attacker cannot modify or downgrade the security of the connection by placing
itself in the middle of the negotiation.
A premaster secret is securely established by the client using the RSA key transfer,
Diffie-Hellman (DH or DHE) key agreement, or Elliptic Curve DH (ECDH or ECDHE).

2.3 Confidentiality
Confidentiality is provided for a communication session by the negotiated encryption
algorithm for the cipher suite and the encryption keys derived from the master secret andrandom values, one for encryption by the client (the client write key), and another forencryption by the server (the server write key). The sender of a message (client orserver) encrypts the message using a derived encryption key; the receiver uses the same
key to decrypt the message. Both the client and server know these keys, and decrypt themessages using the same key that was used for encryption. The encryption keys arederived from the shared master secret.

2.4 Integrity
The keyed MAC algorithm, specified by the negotiated cipher suite, provides message
integrity. Two MAC keys are derived: 1) a MAC key to be used when the client is themessage sender and the server is the message receiver (the client write MAC key), and 2)
a second MAC key to be used when the server is the message sender and the client is themessage receiver (the server write MAC key). The sender of a message (client or server)
calculates the MAC for the message using the appropriate MAC key, and encrypts boththe message and the MAC using the appropriate encryption key. The sender thentransmits the encrypted message and MAC to the receiver. The receiver decrypts thereceived message and MAC, and calculates its own version of the MAC using the MAC
algorithm and sender’s MAC key. The receiver verifies that the MAC that it calculates
matches the MAC sent by the sender.
Two types of constructions are used for MAC algorithms in TLS. All versions of TLS
support the use of the Keyed-Hash Message Authentication Code (HMAC) using thehash algorithm specified by the negotiated cipher suite. With HMAC, MACs for serverto-client messages are keyed by the server write MAC key, while MACs client-to-server

6

Guidelines for TLS Implementations

messages are keyed by the client write MAC key. These MAC keys are derived from theshared master secret.
TLS 1.2 added support for AEAD cipher modes, such as Counter with CBC-MAC(CCM) and Galois Counter Mode (GCM), as an alternative way of providing integrity
and confidentiality. In AEAD modes, the sender uses its write key for both encryption
and integrity protection. The client and server write MAC keys are not used. The
recipient decrypts the message and verifies the integrity information. Both the senderand the receiver use the sender’s write key to perform these operations.

2.5 Authentication
Server authentication is performed by the client using the server’s public key certificate,
which the server presents during the handshake. The exact nature of the cryptographic
operation for server authentication is dependent on the negotiated cipher suite andextensions. In most cases (e.g., RSA for key transport, DH and ECDH), authentication is
performed explicitly through verification of digital signatures present in certificates, andimplicitly by the use of the server public key by the client during the establishment of themaster secret. A successful Finished message implies that both parties calculated thesame master secret and thus, the server must have known the private key corresponding
to the public key used for key establishment.
Client authentication is optional, and only occurs at the server’s request. Client
authentication is based on the client’s public key certificate. The exact nature of thecryptographic operation for client authentication depends on the negotiated cipher suite’s
key exchange algorithm and the negotiated extensions. For example, when the client’s
public key certificate contains an RSA public key, the client signs a portion of thehandshake message using the private key corresponding to that public key, and the server
verifies the signature using the public key to authenticate the client.

2.6 Anti-Replay
The integrity-protected envelope of the message contains a monotonically increasing
sequence number. Once the message integrity is verified, the sequence number of thecurrent message is compared with the sequence number of the previous message. The
sequence number of the current message must be greater than the sequence number of theprevious message in order to further process the message.

2.7 Key Management
The server public key certificate and corresponding private key, and optionally the client
public key certificate and corresponding private key, are used in the establishment of thepremaster secret, according to the key exchange algorithm dictated by the selected cipher
suite. The premaster secret, server random, and client random are used to determine themaster secret, which is then used to derive the symmetric session keys.
The security of the server’s private key is critical to the security of TLS. If the server’s
private key is weak or can be obtained by a third party, the third party can masquerade as
the server to all clients. Similarly, if a third party can obtain a public key certificate for a
public key corresponding to his own private key in the name of a legitimate server from a7

Guidelines for TLS Implementations

certification authority (CA) trusted by the clients, the third party can masquerade as theserver to the clients. Requirement and recommendations to mitigate these concerns are
addressed later in these guidelines.
Similar threats exist for clients. If a client’s private key is weak or can be obtained by athird party, the third party can masquerade as the client to the server. Similarly, if a third
party can obtain a public key certificate for a public key corresponding to his own private
key in the name of a client from a CA trusted by the server, the third party canmasquerade as that client to the server. Requirements and recommendations to mitigate
these concerns are addressed later in these guidelines.
Since the random numbers generated by the client and server contribute to therandomness of the session keys, the client and server must be capable of generating
pseudorandom numbers with at least 112 bits of security 6 each. The various TLS session
keys derived from these random values and other data are valid for the duration of thesession. Because the session keys are only used to protect messages exchanged during anactive TLS session, and are not used to protect any data at rest, there is no requirement
for recovering TLS session keys. However, servers and clients may (and often do) cache
the master secret (but not the session keys) to reduce the significant overhead in session
resumption. If both the client and server have the master secret and associated session ID
from a previous session in their caches, an abbreviated handshake can be used to resume
the session. A resumed session uses the same negotiated parameters as the previous
session, but uses new session keys derived from the master secret and new server random
and client random values. After some reasonable timeout period, the master secret should
be destroyed on both the server and the client. All of the state variables, including thesession keys, are destroyed when the session ends. The protocol implementation relieson the operating system to ensure that there is no reuse of the keying material, such as therandom values, premaster secret and session keys.

6 Bits of security provided by Approved algorithms are described in SP 800-57 part 1 [SP800-57p1], Section 5.6.

8

Guidelines for TLS Implementations

3 Minimum Requirements for TLS Servers
This section provides a minimum set of requirements that a server must implement inorder to meet these guidelines. Requirements are organized in the following sections:
TLS protocol version support; server keys and certificates; cryptographic support; TLS
extension support; client authentication; session resumption; compression methods; andoperational considerations.
Specific requirements are stated as either implementation requirements or configuration
requirements. Implementation requirements indicate that Federal agencies shall not
procure TLS server implementations unless they include the required functionality, or canbe augmented with additional commercial products to meet requirements. Configuration
requirements indicate that TLS server administrators are required to verify that particularfeatures are enabled, or in some cases, configured appropriately, if present.

3.1 Protocol Version Support
TLS version 1.1 is required, at a minimum, in order to mitigate various attacks on version
1.0 of the TLS protocol. Support for TLS version 1.2 is strongly recommended.
Servers that support government-only applications shall be configured to support TLS
1.1, and should be configured to support TLS 1.2. These servers shall not support TLS
1.0, SSL 2.0, or SSL 3.0. TLS versions 1.1 and 1.2 are represented by major and minor
number tuples (3, 2) and (3, 3), respectively7. Agencies shall develop migration plans to
support TLS 1.2 by January 1, 2015.
Servers that support citizen or business-facing applications shall be configured to supportversion 1.1 and should be configured to support version 1.2. These servers may also be
configured to support TLS version 1.0 in order to enable interaction with citizens andbusinesses. These servers shall not support SSL version 3.0 or earlier. If TLS 1.0 is
supported, the use of TLS 1.1 and 1.2 shall be preferred over TLS 1.0.
Some server implementations are known to implement version negotiation incorrectly.
For example, there are TLS 1.0 servers that terminate the connection when the client
offers a version newer than TLS 1.0. Servers that incorrectly implement TLS version
negotiation shall not be used.

3.2 Server Keys and Certificates
The TLS server shall be configured with one or more public key certificates and theassociated private keys. TLS server implementations should support multiple server
certificates with their associated private keys to support algorithm and key size agility.
There are six options for TLS server certificates that can satisfy the requirement for
Approved cryptography: an RSA key encipherment certificate; an RSA signature
certificate; an Elliptic Curve Digital Signature Algorithm (ECDSA) signature certificate;

7 Historically TLS 1.0 was assigned major, minor tuple (3,1) to align it as SSL 3.1.

9

Guidelines for TLS Implementations

a Digital Signature Algorithm (DSA) 8 signature certificate; a Diffie-Hellman certificate;
and an ECDH certificate.
At a minimum, TLS servers conforming to this specification shall be configured with an
RSA key encipherment certificate, and also should be configured with an ECDSA
signature certificate or RSA signature certificate. If the server is not configured with an
RSA signature certificate, an ECDSA signature certificate using a Suite B named curvefor the signature and public key in the ECDSA certificate should be used. 9
TLS servers shall be configured with certificates issued by a CA, rather than self-signed
certificates. Furthermore, TLS server certificates shall be issued by a CA that publishes
revocation information in either a Certificate Revocation List (CRL) [RFC5280] or in
Online Certificate Status Protocol (OCSP) [RFC6960] responses. The source for the
revocation information shall be included in the CA-issued certificate in the appropriate
extension to promote interoperability.
A TLS server that has been issued certificates by multiple CAs can select the appropriate
certificate, based on the client specified “Trusted CA Keys” TLS extension, as describedin Section 3.4.1.4. A TLS server that has been issued certificates for multiple names canselect the appropriate certificate, based on the client specified “Server Name” TLS
extension, as described in Section 3.4.1.3. A TLS server may also contain multiple
names in the Subject Alternative Name extension of the server certificate in order tosupport multiple server names of the same name form (e.g., DNS Name) or multiple
server names of multiple name forms (e.g., DNS Names, IP Address, etc.)
Section 3.2.1 specifies a detailed profile for server certificates. Basic guidelines for DSA,
DH, and ECDH certificates are provided; more detailed profiles may be provided if thesealgorithms experience broad use in the future. Section 3.2.2 specifies requirements forrevocation checking. System administrators shall use these sections to identify anappropriate source for certificates. Section 3.5.4 specifies requirements for the “hints
list.”

3.2.1

Server Certificate Profile

The server certificate profile, described in this section, provides requirements andrecommendations for the format of the server certificate. For these guidelines, the TLS
server certificate shall be an X.509 version 3 certificate; both the public key contained in
the certificate and the signature shall have at least 112 bits of security. The certificate
shall be signed with an algorithm consistent with the public key10:•

Certificates containing RSA (key encipherment or signature), ECDSA, or DSA
public keys shall be signed with those same signature algorithms, respectively;

8 In the names for the TLS cipher suites, DSA is referred to as DSS (Digital Signature Standard), for historical reasons.
9 The Suite B curves are known as P-256 and P-384. These curves are defined in [FIPS186-4] and their inclusion inSuite B is documented in [RFC6460].
10 Algorithm-dependent rules exist for the generation of public and private key pairs. For guidance on the generationof DH and ECDH key pairs, see [SP800-56A]. For guidance regarding the generation of RSA key pairs, see[SP800-56B]. For guidance regarding the generation of DSA and ECDSA key pairs, see [FIPS186-4].

10

Guidelines for TLS Implementations•

Certificates containing Diffie-Hellman public keys shall be signed with DSA; and•Certificates containing ECDH public keys shall be signed with ECDSA.

The extended key usage extension limits the operations that keys in a certificate may be
used for. There is an extended key usage extension specifically for server authentication,and the server should be configured to support it. The use of the extended key usage
extension will facilitate successful server authentication, as some clients may require thepresence of an extended key usage extension. The extended key usage extension will also
indicate that the certificate is not intended to be used for other purposes, such as codesigning. The use of the server DNS name in the Subject Alternative Name field ensuresthat any name constraints on the certification path will be properly enforced.
The server certificate profile is listed in Table 3-1. In the absence of agency-specificcertificate profile requirements, this certificate profile should be used for the server
certificate.
Note that for ECDH, the algorithm object identifier (OID) and the signature OID are
identical to those of ECDSA. For interoperability reasons, the algorithm OID is notchanged and the key usage extension determines if the public key is used for key
agreement or signature verification.
Table 3-1: TLS Server Certificate Profile
Field

CriticalValue

Description

VersionN/A

2

Version 3

Serial Number

N/A

Unique positive integer

Issuer Signature Algorithm

N/A

Must be unique
Values by certificate type:sha256WithRSAEncryption {1 2 840
113549 1 1 11}, or stronger

RSA key encipherment certificate, RSA
signature certificate

ecdsa-with-SHA256 {1 2 840 10045 4 32}, or stronger

ECDSA signature certificate, ECDH
certificate
DSA signature certificate, DH certificate

Issuer Distinguished Name
(DN)

N/A

id-dsa-with-sha256 {2 16 840 1 101 3 4 32}, or stronger
Unique X.500 Issuing CA DN

Validity Period

N/A

3 years or lessDates through 2049 expressed in UTCTime

Subject Distinguished Name

N/A

Unique X.500 subject DN per agency
requirements

Single value shall be encoded in each
RDN. All attributes that are of
directoryString type shall be encoded as a
printable string.

Single value shall be encoded in each
Relative Distinguished Name (RDN). All
attributes that are of directoryString typeshall be encoded as a printable string.

CN={Host IP Address | Host DNS Name}

11

Guidelines for TLS Implementations

Field
Subject Public Key
Information

CriticalN/A

Description

Value

Values by certificate type:rsaEncryption {1 2 840 113549 1 1 1}

RSA key encipherment certificate, RSA
signature certificate
2048-bit RSA key modulus, or otherapproved lengths as defined in [SP80056B] and [SP800-57p1]
Parameters: NULL

ecPublicKey {1 2 840 10045 2 1}

ECDSA signature certificate, or ECDH
certificate
Parameters: namedCurve OID for names
curve specified in FIPS 186-4. The curve
shall be P-256 or P-384
SubjectPublic Key: Uncompressed EC
Point.

id-dsa {1 2 840 10040 4 1}

DSA signature certificate
Parameters: p, q, g (2048 bit large prime,i.e., p)dhpublicnumber {1 2 840 10046 2 1}

DH certificate
Parameters: p, g, q (2048 bit large prime,i.e., p)Issuer’s Signature

Values by certificate type:

N/A

sha256WithRSAEncryption {1 2 840
113549 1 1 11}, or stronger

RSA key encipherment certificate, RSA
signature certificate

ecdsa-with-SHA256 {1 2 840 10045 4 32}, or stronger

ECDSA signature certificate, ECDH
certificate

id-dsa-with-sha256 { 2 16 840 1 101 3 4
3 2}, or stronger

DSA signature certificate, DH certificate

Same as subject key identifier in Issuing
CA certificate

Extensions
Authority Key IdentifierNo

Octet String

Subject Key IdentifierNo

Octet String

Key Usage

Yes

Prohibited: Issuer DN, Serial Number tuple

Extended Key Usage

No

Same as in PKCS-10 request or calculatedby the Issuing CA
Values by certificate type:

keyEncipherment

RSA key encipherment certificate

digitalSignature

RSA signature certificate, ECDSA
signature certificate, or DSA signature
certificate

keyAgreement

ECDH certificate, DH certificate

id-kp-serverAuth {1 3 6 1 5 5 7 3 1}

Required

id-kp-clientAuth {1 3 6 1 5 5 7 3 2}

Optional
Prohibited: anyExtendedKeyUsage, allothers unless consistent with key usage
extension

Certificate Policies

No

Per agency X.509 certificate policy

Subject Alternative NameNo

DNS Host Name or IP Address if there is
no DNS name assigned

Multiple SANs are permitted, e.g., for loadbalanced environments.

Authority Information AccessNo

id-ad-caIssuers

Required. Access method entry contains
HTTP URL for certificates issued to
Issuing CA

id-ad-ocsp

Optional. Access method entry contains
HTTP URL for the Issuing CA OCSP
Responder

See comments

Optional. HTTP value in distributionPoint
field pointing to a full and complete CRL.

CRL Distribution Points

No

Prohibited: reasons and cRLIssuer fields,
and nameRelativetoCRLIssuer CHOICE

12

Guidelines for TLS Implementations

3.2.2

Obtaining Revocation Status Information for the Client
Certificate

The server shall perform revocation checking of the client certificate, when client
authentication is used. Revocation information shall be obtained by the server from one
or more of the following locations:
1. Certificate Revocation List (CRL) or OCSP [RFC6960] response in the server’s
local store;
2. OCSP response from a locally configured OCSP Responder;
3. OCSP response from the OCSP Responder location identified in the OCSP fieldin the Authority Information Access extension in the client certificate; or
4. CRL from the CRL Distribution Point extension in the client certificate.
When the local store does not have the current or a cogent 11 CRL or OCSP response, andthe OCSP Responder and the CRL Distribution Point are unavailable or inaccessible atthe time of TLS session establishment, the server will either deny the connection or
accept a potentially revoked or compromised certificate. The decision to accept or reject acertificate in this situation should be made according to agency policy.

3.2.3

Server Public Key Certificate Assurance

After the server public key certificate has been verified by a client, it may be trusted by
the client on the basis of policies, procedures and security controls used to issue theserver public key certificate. The server is required to possess an X.509 version 3 publickey certificate. The policy, procedures and security controls are optionally represented in
the certificate using the certificatePolicies extension, specified in [RFC5280] and updated
in [RFC6818]. When used, one or more certificate policy OIDs are asserted in thisextension. The actual policies and procedures and security controls associated with eachcertificate policy OID are documented in a certificate policy. In the absence of agencyspecific policies, Federal agencies shall use the Common Policy [COMMON].
The use of a certificate policy that is designed with the secure operation of PKI in mind
and adherence to the stipulated certificate policy mitigates the threat that the issuing CA
can be compromised or that the registration system, persons or process can becompromised to obtain an unauthorized certificate in the name of a legitimate entity, andthus compromise the clients. With this in mind, the CA Browser Forum, a private sector
organization, has carried out some efforts in this area. The guideline was first publishedas the Extended Validation guideline [EVGUIDE]. Under another effort, the CA
Browser Forum published requirements for issuing certificates from publicly trusted CAs
in order for those CAs and their trust anchor to remain in browser trust stores[CABBASE].

11 A CRL is considered “cogent” when the “CRL Scope” is appropriate for certificate in question. “CRL Scope” isdefined in [RFC5280].

13

Guidelines for TLS Implementations

It should be noted that there are TLS clients that do not perform X.509 certificate policy
processing as mandated by [RFC5280]. Thus, they are not able to accept or reject a TLS
server certificate based on the assurance level specified by the policy. This may result in
the acceptance of a fraudulent certificate and may expose user data to unintended parties.
The Federal Government and CA Browser Forum hope that the security requirements in[COMMON], [EVGUIDE], and [CABBASE] are adopted by all CAs under theirpurview, mitigating the lack of a policy processing capability.
In order to further mitigate the risk associated with a CA or X.509 certificate registration
system, process or personnel compromise, several concepts are under development.
These emerging concepts are further discussed in Appendix D.

3.3 Cryptographic Support
Cryptographic support in TLS is provided through the use of various cipher suites. A
cipher suite specifies a collection of algorithms for key exchange and for providing
confidentiality and integrity services to application data. The cipher suite negotiation
occurs during the TLS handshake protocol. The client presents cipher suites that itsupports to the server, and the server selects one of them to secure the session data.
Cipher suites have the form:
TLS_KeyExchangeAlg_WITH_EncryptionAlg_MessageAuthenticationAlg
For example, the cipher suite TLS_RSA_WITH_AES_128_CBC_SHA uses RSA for the
key exchange, AES-128 in cipher block chaining mode for encryption, and message
authentication is performed using HMAC_SHA 12. For further information on cipher suiteinterpretation, see Appendix B.

3.3.1

Cipher Suites

The server shall be configured to only use cipher suites that are composed entirely of
Approved algorithms. A complete list of acceptable cipher suites for general use isprovided in this section, grouped by certificate type and TLS protocol version.
In some situations, such as closed environments, it may be appropriate to used pre-sharedkeys. Pre-shared keys are symmetric keys that are already in place prior to the initiationof a TLS session, which are used in the derivation of the premaster secret. For cipher
suites that are acceptable in pre-shared key environments, see Appendix C.
In order to maximize interoperability, TLS server implementations shall support thefollowing cipher suites:• TLS_RSA_WITH_3DES_EDE_CBC_SHA 13• TLS_RSA_WITH_AES_128_CBC_SHA14
In addition, TLS server implementations should support the following cipher suites:
12 SHA indicates the use of the SHA-1 hash algorithm.
13 Support of this cipher suite is mandatory for TLS 1.1 [RFC4346]
14 Support of this cipher suite is mandatory for TLS 1.2 [RFC5246]

14

Guidelines for TLS Implementations

•
•
•
•
•

TLS_RSA_WITH_AES_256_CBC_SHA
TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA 15
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA
TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA

When ephemeral keys are used to establish the master secret, each ephemeral key-pair(i.e., the server ephemeral key-pair and the client ephemeral key-pair) shall have at least
112 bits of security.
TLS version 1.2 adds support for authenticated encryption modes, and support for the
SHA-256 and SHA-384 hash algorithms, which are not supported in prior versions ofTLS. These cipher suites are described in [RFC5288] and [RFC5289]. In addition to
supporting the cipher suites listed above, TLS 1.2 servers shall be configured to support
the following cipher suite:• TLS_RSA_WITH_AES_128_GCM_SHA256
TLS 1.2 servers should be configured to support the following cipher suites:• TLS_RSA_WITH_AES_256_GCM_SHA384• TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256• TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256• TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384• TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256• TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
NIST may define additional mandatory or recommended cipher suites at a later date.
The server shall be configured to only support cipher suites for which it has a valid
certificate containing a signature providing at least 112 bits of security. The following
cipher suite tables are grouped by certificate type and TLS protocol version. The cipher
suites in these tables include the cipher suites that shall and should be supported (asdescribed above), and may be supported. Only cipher suites that are composed ofApproved algorithms are acceptable and are listed in this section. Cipher suites that do
not appear in this section or Appendix C shall not be used.
In the following tables listing recommended cipher suites, cipher suites shown in bold
font shall be supported, cipher suites shown in italics font should be supported, andcipher suites shown in regular font may be supported.
Table 3-2 identifies the three categories (shall, should, and may) of acceptable cipher
suites for a TLS server that has been configured with an RSA private key and a
15 In TLS versions 1.0 and 1.1, DHE and ECDHE cipher suites use SHA-1 for signature generation on the ephemeralparameters (including keys) in the ServerKeyExchange message. [SP800-131A] states that the use of SHA-1 fordigital signature generation is disallowed after 2013. Due to the random nature of the ephemeral keys, third party
cannot cause effective collision. The server and client do not have anything to gain by causing collision for theconnection. Due to client random and server random, the server, the client, or a third party cannot use a colliding
set of messages to masquerade as client or server in future connections. Any modification to the parameters by athird party during the handshake will ultimately result in a failed connection. Because of these reasons, SHA-1 isallowed for generating digital signatures on ephemeral parameters in TLS.

15

Guidelines for TLS Implementations

corresponding RSA certificate. Table 3-3 identifies additional RSA cipher suites for TLS
version 1.2 servers, for the three categories. A server having a RSA certificate may
support any cipher suite that appears in Table 3-2 or Table 3-3. The key usage extension
in the RSA certificate shall specify key encipherment for cipher suites that use RSA key
transport to carry out the key exchange, and the key usage extension shall specify digital
signature for cipher suites using ECDHE for key exchange.
Table 3-2: Cipher Suites for RSA Server Certificates
Cipher Suite NameKey
Exchange

Encryption

Hash
Functionfor HMAC

TLS_RSA_WITH_3DES_EDE_CBC_SHA
TLS_RSA_WITH_AES_128_CBC_SHA
TLS_RSA_WITH_AES_256_CBC_SHA
TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA
TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA

RSA
RSA
RSA
ECDHE
ECDHE
ECDHE

3DES_EDE_CBC
AES_128_CBC
AES_256_CBC
3DES_EDE_CBC
AES_128_CBC
AES_256_CBC

SHA-1
SHA-1
SHA-1
SHA-1
SHA-1
SHA-1

Hash
Functionfor PRF 16
Per RFCPer RFCPer RFCPer RFCPer RFCPer RFC

Table 3-3: Additional TLS 1.2 Cipher Suites for RSA Server Certificates
Cipher Suite NameKey
Exchange

Encryption

TLS_RSA_WITH_AES_128_GCM_SHA256
TLS_RSA_WITH_AES_256_GCM_SHA384
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
TLS_RSA_WITH_AES_128_CBC_SHA256
TLS_RSA_WITH_AES_256_CBC_SHA256
TLS_RSA_WITH_AES_128_CCM 17

RSA
RSA
ECDHE
ECDHE
RSA
RSA
RSA
RSA

TLS_RSA_WITH_AES_256_CCM

AES_128_GCM
AES_256_GCM
AES_128_CBC
AES_128_GCM
AES_128_CBC
AES_256_CBC
AES_128_CCM

Hash
Functionfor HMAC
N/A
N/A
N/A
N/A
SHA-256
SHA-256
N/A

Hash
Functionfor PRF
SHA-256
SHA-384
SHA-256
SHA-256
SHA-256
SHA-256
SHA-256

AES_256_CCM

N/A

SHA-256

Table 3-4 identifies the two categories (should and may) of cipher suites for a TLS serverthat has been configured with an elliptic curve private key and a corresponding ECDSA
certificate. These cipher suites are described in [RFC4492]. Table 3-5 identifies
additional two categories (should and may) of ECDSA cipher suites, described in[RFC5289] for a TLS version 1.2 server. A server that is configured with an ECDSA
certificate may support any of the cipher suites listed in Table 3-4 or Table 3-5.
Table 3-4: Cipher Suites for ECDSA Server Certificates
Cipher Suite NameKey
Exchange

Encryption

TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA
TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA

ECDHE
ECDHE
ECDHE

3DES_EDE_CBC
AES_128_CBC
AES_256_CBC

Hashfunctionfor HMAC
SHA-1
SHA-1
SHA-1

Hash
Functionfor PRFPer RFCPer RFCPer RFC

16 In TLS versions 1.0 and 1.1, the hash function used in the PRF is a parallel application of MD5 and SHA-1, as
defined in [RFC2246] and [RFC4346]. For TLS 1.2, the PRF hash function is SHA-256, unless otherwise stated.
17 AES-CCM cipher suites are defined in [RFC6655].

16

Guidelines for TLS Implementations

Table 3-5: Additional TLS 1.2 Cipher Suites for ECDSA Server Certificates
Cipher Suite NameKey
Exchange

Encryption

TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384
TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384

ECDHE
ECDHE
ECDHE
ECDHE

AES_128_CBC
AES_128_GCM
AES_256_GCM
AES_256_CBC

Hashfunctionfor
HMAC
SHA-256
N/A
N/A
SHA-384

Hash
Functionfor PRF
SHA-256
SHA-256
SHA-384
SHA-384

Table 3-6 identifies cipher suites that may be supported by a server that has been
configured with a DSA private key and a corresponding DSA certificate. Table 3-7
identifies additional DSA cipher suites that may be supported by a TLS version 1.2
server. A server that is configured with a DSA certificate may support any of the cipher
suites listed in Table 3-6 or Table 3-7.
Table 3-6: Cipher Suites for DSA Server Certificates
Cipher Suite Name
TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA
TLS_DHE_DSS_WITH_AES_128_CBC_SHA
TLS_DHE_DSS_WITH_AES_256_CBC_SHA

KeyExchange

Encryption

DHE
DHE
DHE

3DES_EDE_CBC
AES_128_CBC
AES_256_CBC

Hashfunction for
HMAC
SHA-1
SHA-1
SHA-1

Hash
Functionfor PRFPer RFCPer RFCPer RFC

Table 3-7: Additional TLS 1.2 Cipher Suites for DSA Server Certificates
Cipher Suite NameKey
Exchange

Encryption

TLS_DHE_DSS_WITH_AES_128_CBC_SHA256
TLS_DHE_DSS_WITH_AES_256_CBC_SHA256
TLS_DHE_DSS_WITH_AES_128_GCM_SHA256
TLS_DHE_DSS_WITH_AES_256_GCM_SHA384

DHE
DHE
DHE
DHE

AES_128_CBC
AES_256_CBC
AES_128_GCM
AES_256_GCM

Hashfunction for
HMAC
SHA-256
SHA-256
N/A
N/A

Hash
Functionfor PRF
SHA-256
SHA-256
SHA-256
SHA-384

Table 3-8 identifies cipher suites that may be supported by a TLS server that has been
configured with a DH private key and a corresponding DH certificate signed using DSA.
Table 3-9 identifies additional DH cipher suites that may be supported by a TLS version
1.2 server [RFC5246], [RFC5288].
Table 3-8: Cipher Suites for DH Server Certificates
Cipher Suite Name
TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA
TLS_DH_DSS_WITH_AES_128_CBC_SHA
TLS_DH_DSS_WITH_AES_256_CBC_SHA

KeyExchange

Encryption

DH
DH
DH

3DES_EDE_CBC
AES_128_CBC
AES_256_CBC

17

Hash
function for
HMAC
SHA-1
SHA-1
SHA-1

Hash
Functionfor PRFPer RFCPer RFCPer RFC

Guidelines for TLS Implementations

Table 3-9: Additional TLS 1.2 Cipher Suites for DH Server Certificates
Cipher Suite NameKey
Exchange

Encryption

TLS_DH_DSS_WITH_AES_128_CBC_SHA256
TLS_DH_DSS_WITH_AES_256_CBC_SHA256
TLS_DH_DSS_WITH_AES_128_GCM_SHA256
TLS_DH_DSS_WITH_AES_256_GCM_SHA384

DH
DH
DH
DH

AES_128_CBC
AES_256_CBC
AES_128_GCM
AES_256_GCM

Hashfunction for
HMAC
SHA-256
SHA-256
N/A
N/A

Hash
Functionfor PRF
SHA-256
SHA-256
SHA-256
SHA-384

Table 3-10 identifies cipher suites that may be supported by a server that has been
configured with an elliptic curve private key and a corresponding ECDH certificate
signed using ECDSA. Table 3-11 identifies additional ECDH cipher suites that may besupported by a TLS 1.2 server. These cipher suites are defined in [RFC5289].
Table 3-10: Cipher Suites for ECDH Server Certificate
Cipher Suite NameKey
Exchange

Encryption

Hash
Functionfor PRF

3DES_EDE_CBC

Hashfunction
for
HMAC
SHA-1

TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA

ECDH

TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA
TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA

ECDH
ECDH

AES_128_CBC
AES_256_CBC

SHA-1
SHA-1

Per RFCPer RFCPer RFC

Table 3-11: Additional TLS 1.2 Cipher Suites for ECDH Server Certificate
Cipher Suite NameKey
Exchange

Encryption

TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA256
TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384
TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256
TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384

ECDH
ECDH
ECDH
ECDH

AES_128_CBC
AES_256_CBC
AES_128_GCM
AES_256_GCM

Hashfunctionfor
HMAC
SHA-256
SHA-384
N/A
N/A

Hash
Functionfor PRF
SHA-256
SHA-384
SHA-256
SHA-384

Appendix B provides further details on cipher suite name interpretation. While the cipher
suite name is used in descriptions, the actual protocol uses assigned numbers to identify
cipher suites.
When negotiating a cipher suite, the client sends a handshake message with a list of
cipher suites it will accept. The server chooses from the list and sends a handshakemessage back indicating which cipher suite it will accept. Although the client may order
the list with the strongest cipher suites listed first, the server may choose any of thecipher suites proposed by the client. Therefore there is no guarantee that the negotiation
will settle on the strongest suite in common. If no cipher suites are in common theconnection is aborted.
Cipher suites using ephemeral DH and ephemeral ECDH (i.e., those with DHE or
ECDHE in the second mnemonic) provide perfect forward secrecy 18, ensuring long-term

18 Perfect forward secrecy is the condition in which the compromise of a long-term private key used in deriving asession key subsequent to the derivation does not cause the compromise of the session key.

18

Guidelines for TLS Implementations

confidentiality of the session. While support of these cipher suites is not required by theseguidelines, it is strongly recommended.
There is no mechanism to specify the minimum key size for the server or client certificate
or for the CAs that are in the certification path.
3.3.1.1 Implementation Considerations
System administrators need to fully understand the ramifications of selecting cipher
suites and configuring applications to support only those cipher suites. The security
guarantees of the cryptography are limited to the weakest cipher suite supported by theconfiguration. When configuring an implementation, there are several factors that affectsupported cipher suite selection.
[RFC4346] describes timing attacks on CBC cipher suites, as well mitigation techniques.
TLS implementations shall use the bad_record_mac error to indicate a padding error.
Implementations shall compute the MAC regardless of whether padding errors exist.
In addition to the CBC attacks addressed in [RFC4346], the Lucky 13 attack [Lucky13]
demonstrates that a constant-time decryption routine is also needed to prevent timing
attacks. TLS implementations should support constant-time decryption, or near constanttime decryption.
Note that CBC-based attacks can be prevented by using AEAD cipher suites (e.g., GCM,
CCM), supported in TLS 1.2.
3.3.1.1.1 Algorithm Support
Many TLS servers and clients support RC4 [Schneier96] cipher suites. RC4 is not an
Approved algorithm. If the server were configured to support RC4 cipher suites, they
may be chosen over the recommended cipher suites composed of Approved algorithms.
Therefore it is important that the server is configured only to use recommended cipher
suites.
Server implementations may not allow the server administrator to specify preference
order. In such servers, the only way to ensure that a server uses Approved algorithms forencryption is to disable cipher suites that use other encryption algorithms (such as RC4
and Camellia [RFC3713]).
3.3.1.1.2 Cipher Suite Scope
The selection of a cryptographic algorithm may be system-wide and not application
specific for some implementations. For example, disabling an algorithm for one
application on a system might disable that algorithm for all applications on that system.

3.3.2

Validated Cryptography

The cryptographic module used by the server shall be a FIPS 140-validated
cryptographic module. All cryptographic algorithms that are included in the configured
cipher suites shall be within the scope of the validation, as well as the random number
generator. Note that the TLS 1.1 pseudorandom function (PRF) uses MD5 and SHA-1 inparallel so that if one hash function is broken, security is not compromised. While MD5
19

Guidelines for TLS Implementations

is not an Approved algorithm, the TLS 1.1 PRF is specified as acceptable in[FIPS140Impl] and [SP800-135]. Note that in TLS 1.1, use of SHA-1 is foundacceptable for specific cases of signing ephemeral keys and for signing for client
authentication. This is acceptable due the fact that a third party cannot cause a collisionthat is not detected and client and server cannot exploit the collision they can cause as
further explained in a footnote in Section 3.3.1. In TLS 1.2, the default hash function in
the PRF is SHA-256. Other than the SHA-1 exception listed for specific instances above,all cryptography used shall provide at least 112 bits of security. All server and clientcertificates shall contain public keys that offer at least 112 bits of security. All serverand client certificates and certificates in their certification paths shall be signed using key
pairs that offer at least 112 bits of security and SHA-224 or stronger hashing algorithm.
All ephemeral keys used by the client and server shall offer at least 112 bits of security.
All symmetric algorithms used to protect the TLS data shall use keys that offer at least
112 bits of security.
The random number generator shall be tested and validated in accordance with [SP80090A] under the NIST Cryptographic Algorithm Validation Program (CAVP) andsuccessful results of this testing shall be indicated on the cryptographic module’s FIPS
140 validation certificate.
The server random value, sent in the ServerHello message, contains a 4-byte timestamp 19
value and 28-byte random value. The validated random number generator shall be used
to generate the 28-byte random value of the server random value. The validated random
number generator should be used to generate the 4-byte timestamp of the server random
value.

3.4 TLS Extension Support
Several TLS extensions are described in [RFC6066]. Servers are encouraged to support
these extensions, except where discouraged as specified in Section 3.4.3. Additional
extensions are described in [RFC4492], [RFC5246], and [RFC5746]. This section
contains recommendations for a subset of the TLS extensions that the Federal agencies
shall, should, or should not use as they become prevalent in commercially available
TLS servers and clients.
Some servers will refuse the connection if any TLS extensions are included in the
ClientHello message. Interoperability with servers that do not properly handle TLS
extensions may require multiple connection attempts by the client.

3.4.1

Mandatory TLS Extensions

The server shall support the following TLS extensions.
1. Renegotiation Indication
2. Certificate Status Request
3. Server Name Indication
19 The timestamp value does not need to be correct in TLS. It can be any 4-byte value, unless otherwise restricted byhigher-level or application protocols.

20

Guidelines for TLS Implementations

4. Trusted CA Indication
3.4.1.1 Renegotiation Indication
TLS session renegotiation is vulnerable to an attack in which the attacker forms a TLS
connection with the target server, injects content of his choice, and then splices in a new
TLS connection from a legitimate client. The server treats the legitimate client’s initial
TLS handshake as a renegotiation of the attacker’s negotiated session and thus believes
that the initial data transmitted by the attacker is from the legitimate client. The session
renegotiation extension is defined to prevent such a session splicing or sessioninterception. The extension uses the concept of cryptographically binding the initial
session negotiation and session renegotiation.
Servers shall perform initial and subsequent renegotiations in accordance with
[RFC5746].
3.4.1.2 Certificate Status Request
When the client wishes to receive the revocation status of the TLS server certificate fromthe TLS server, the client includes the Certificate Status Request (status_request)
extension in the ClientHello message. Upon receipt of the status_request extension, theserver shall include the certificate status along with its certificate by sending a
CertificateStatus message immediately following the Certificate message. While theextension itself is extensible, only OCSP type certificate status is defined in [RFC6066].
This extension is also called OCSP stapling.
3.4.1.3 Server Name Indication
Multiple virtual servers may exist at the same network address. The server nameindication extension allows the client to specify which of the servers located at theaddress it is trying to connect with. The server shall be able to process and respond to theserver name indication extension received in a ClientHello message as described in[RFC6066].
3.4.1.4 Trusted CA Indication
The trusted CA indication (trusted_ca_keys) extension allows a client to specify which
CA root keys it possesses. This is useful for sessions where the client is memoryconstrained and possesses a small number of root CA keys. The server shall be able toprocess and respond to the trusted CA indication extension received in a ClientHellomessage as described in [RFC6066].

3.4.2

Conditional TLS Extensions

A TLS server may be able to support the following TLS extensions under thecircumstances described in the following paragraphs:
1. The Supported Elliptic Curves TLS extension shall be supported if the server
supports EC cipher suite(s).
2. The EC Point Format TLS extension shall be supported if the server supports EC
cipher suite(s).
21Guidelines for TLS Implementations

3. The Signature Algorithms TLS extension shall be supported when the server isoperating in TLS 1.2.
4. The Multiple Certificate Status extension shall be supported if the extension is
supported by the server implementation.
5. The Truncated HMAC extension may be supported if the server communicateswith constrained device clients and the server implementation does not support
variable-length padding.
3.4.2.1 Supported Elliptic Curves
Servers that support elliptic curve cipher suites shall be able to process the elliptic curves
received in the ClientHello message. The curves P-256 and P-384 shall be supported.
The servers shall process this extension in accordance with Section 5.1 of [RFC4492].
3.4.2.2 EC Point Format
The servers that support EC cipher suites shall be able to process the supported EC point
format received in the ClientHello message by the client. The servers shall process thisextension in accordance with Section 5.1 of [RFC4492].
The servers that support EC cipher suites shall also be able to send the supported EC
point format in the ServerHello message as described in Section 5.2 of [RFC4492].
3.4.2.3 Signature Algorithms
The servers that support TLS 1.2 shall support the processing of the signature algorithms
extension received in a ClientHello message. The extension, its syntax, and processing
rules are described in Sections 7.4.1.4.1, 7.4.2, and 7.4.3 of [RFC5246].
3.4.2.4 Multiple Certificate Status
The multiple certificate status extension improves on the Certificate Status Request
extension described in Section 3.4.1.2 by allowing the client to request the status of allcertificates provided by the server in the TLS handshake. When the server returns therevocation status of all the certificates in the server certificate chain, the client does notneed to query any revocation service providers, such as OCSP responders. This extension
is documented in [RFC6961]. Server implementations that have this capability shall be
configured to support this extension.
3.4.2.5 Truncated HMAC
The Truncated HMAC extension allows a truncation of the HMAC output to 80 bits foruse as a MAC tag. An 80-bit MAC tag complies with the recommendations in [SP800107], but reduces the security provided by the integrity algorithm. Because forging a
MAC tag is an online attack, and the TLS session will terminate immediately when an
invalid MAC tag is encountered, the risk introduced by supporting this extension is low.
However, truncated MAC tags shall not be used in conjunction with variable-length
padding, due to attacks described in [Paterson11].

3.4.3

Discouraged TLS Extensions

The following extensions should not be used:
1. Client Certificate URL
22

Guidelines for TLS Implementations

The Client Certificate URL extension allows a client to send a URL pointing to acertificate, rather than sending a certificate to the server during mutual authentication.
This can be very useful for mutual authentication with constrained clients. However, thisextension can be used for malicious purposes. The URL could belong to an innocent
server on which the client would like to perform a denial of service attack, turning the
TLS server into an attacker. A server that supports this extension also acts as a client
while retrieving a certificate, and therefore becomes subject to additional security
concerns. For these reasons, the Client Certificate URL extension should not be
supported. However, if an agency determines the risks to be minimal, and this extension
is needed for environments where clients are in constrained devices, the extension may besupported. If the client certificate URL extension is supported, the server shall be
configured to mitigate the security concerns described above and in Section 11.3 of[RFC6066].

3.5 Client Authentication
Where strong cryptographic client authentication is required, TLS servers may use the
TLS protocol client authentication option to request a client certificate to
cryptographically authenticate the client. 20 For example, the Personal Identity
Verification (PIV) Authentication Certificate [FIPS201-1] (and the associated privatekey) provides a suitable option for strong authentication of Federal employees andcontractors with on-site access. To ensure that agencies are positioned to take fulladvantage of the PIV card, all TLS servers that perform client authentication shallsupport certificate-based client authentication.
The client authentication option requires the server to implement the X.509 pathvalidation mechanism and a trust anchor store. Requirements for these mechanisms are
specified in Sections 3.5.1 and 3.5.2, respectively. To ensure that cryptographic
authentication actually results in strong authentication, client keys shall contain at least
112 bits of security. Section 3.5.3 describes mechanisms that can contribute, albeit
indirectly, to enforcing this requirement. Section 3.5.4 describes the client’s use of theserver hints list.
The TLS server shall be configurable to terminate the connection with a fatal “handshakefailure” alert when a client certificate is requested, and the client does not have a suitable
certificate.

3.5.1

Path Validation

The client certificate shall be validated in accordance with the certification pathvalidation rules specified in Section 6 of [RFC5280]. In addition, the revocation status of
20 The CertificateVerify message is sent to explicitly verify a client certificate that has signing capability. In TLS 1.1(and TLS 1.0), this message uses SHA-1 to generate a signature on all handshake messages that came before it.
[SP800-131A] states that the use of SHA-1 for digital signature generation is disallowed after 2013. Even if acollision is found, the client must use its private key to authenticate itself by signing the hash. Due to client
random and server random, the server, the client, or a third party cannot use a colliding set of messages tomasquerade as client or server in future connections. Any modification to this message, preceding messages, or
subsequent messages will ultimately result in a failed connection. Because of these reasons, SHA-1 is allowed forgenerating digital signatures in the TLS CertificateVerify message.

23

Guidelines for TLS Implementations

each certificate in the certification path shall be validated using a Certificate Revocation
List (CRL) or Online Certificate Status Protocol (OCSP). OCSP checking shall be incompliance with [RFC6960] and should use only one of the following options:•••

The OCSP Responder is trusted by the server, i.e., the OCSP Responder public
key is the same as that of one of the public keys in the server’s trust anchor store;or
The OCSP Response is signed using the same key as for the certificate whose
status is being checked; or
The OCSP Response is signed by a designated/delegated OCSP Responder as
described in [RFC6960], and the OCSP Responder certificate is signed using thesame key as for the certificate whose status is being checked.

Revocation information shall be obtained as described in Section 3.2.2.
The server shall be able to determine the certificate policies that the client certificate istrusted for by using the certification path validation rules specified in Section 6 of
[RFC5280]. Server and backend applications may use this determination to accept or
reject the certificate. Checking certificate policies assures the server that only clientcertificates that have been issued with acceptable assurance, in terms of CA andregistration system and process security, are accepted.
Not all commercial products may support the public key certification path validation andcertificate policy processing rules listed and cited above. When implementing client
authentication, the Federal agencies shall either use the commercial products that meetthese requirements or augment commercial products to meet these requirements.
The server shall be able to provide the client certificate, and the certificate policies forwhich the client certification path is valid, to the applications in order to support access
control decisions.

3.5.2

Trust Anchor StoreHaving an excessive number of trust anchors installed in the TLS application can expose
the application to all the PKIs emanating from these trust anchors. The best way tominimize the exposure is to only include the trust anchors in the trust anchor store thatare absolutely necessary for client public key certificate authentication.
The server shall be configured with only the trust anchors that the server trusts, and of
those, only the ones that are required to authenticate the clients, in the case where theserver supports client authentication in TLS. These trust anchors are typically a small
subset of the trust anchors that may be included on the server by default. Also note that
this trust anchor store is distinct from the machine trust anchor store. Thus, the default
set of trust anchors shall be examined to determine if any of them are required for client
authentication. Some specific enterprise and/or PKI service provider trust anchor may
need to be added.
In the U.S. Federal environment, in most situations, the Federal Common Policy Root or
the Agency Root (if cross certified with the Federal Bridge Certification Authority)
should be sufficient to build a certification path to the client certificates.
24

Guidelines for TLS Implementations

System administrators of a TLS server that supports certificate-based client
authentication shall perform an analysis of the client certificate issuers and use that
information to determine the minimum set of trust anchors required for the server. The
server shall be configured only to include those trust anchors.

3.5.3

Checking the Client Key Size

The only direct mechanism for a server to check whether the key size and algorithms
presented in a client public certificate are acceptable is for the server to examine thepublic key and algorithm in the client’s certificate. An indirect mechanism is to checkthat the certificate policies extension in the client public key certificate indicates theminimum cryptographic strength of the signature and hashing algorithms used, and forthe server to perform certificate policy processing and checking. A more scalable andmore robust alternative that is standards-based, but has not gained widespreadcommercial deployment, is described in Appendix D. The server shall check the client
key length if client authentication is performed, and the server implementation provides amechanism to do so. The server shall also check the client public key length if the client
uses ephemeral keys for the creation of the master secret, and the server implementation
provides a mechanism to do so. Federal Agencies shall use the key size guidelines
provided in [SP800-131A] to check the client key size.

3.5.4

Server Hints List

Clients may use the list of trust anchors sent by the server in the CertificateRequest
message to determine if the client’s certification path terminates at one of these trust
anchors. The list sent by the server is known as a “hints list.” When the server and client
are in different PKI domains, and the trust is established via direct cross certification
between the two PKI domains (i.e., the server PKI domain and the client PKI domain) or
via transitive cross certification (i.e., through cross certifications among multiple PKI
domains), the client may erroneously decide that its certificate will not be accepted by theserver, since the client’s trust anchor is not sent in the hints list. To mitigate this failure,
the server shall maintain the trust anchors of the various PKIs whose subscribers are thepotential clients for the server, and include them in the hints list. Alternatively, the server
should be configured to send an empty hints list so that the client can always provide a
certificate it possesses. However, this list shall be distinct from the server’s trust anchor
store 21. In other words, the server shall continue to only populate its trust anchor store
with the trust anchor of the server’s PKI domain and the domains it needs to trust directlyfor client authentication. Note that the distinction between the server hints list and theserver’s own trust store is as follows: 1) the hints list is the list of trust anchors a potential
client might trust; and 2) the server’s trust store is the list of trust anchors the server
explicitly trusts.

21 Depending on the server and client trust anchors, the two lists could be identical, could have some trust anchors incommon, or have no trust anchors in common.

25

Guidelines for TLS Implementations

3.6 Session Resumption
During the initial handshake between the client and server, the server generates a session
identifier (ID) and passes this value to the client during the handshake. Both the serverand client store the session ID (along with the keying material and cipher suite) after
completion of the handshake for later use. If the server is willing to resume a session at
the request of a client, the server responds with the original session ID and cipher suite at
the start of the handshake. In the event that the server is unwilling to resume the session,
the server generates and responds with a new session ID.
Typical server implementations are agreeable to resuming a previous session. This is asecure mode of operation, as the master secret is known only to the client and server, andis coupled with the initial client authentication, if client authentication was required.
However, if there is a requirement to authenticate each client as it initiates a connection
session, the server shall be configured to ignore requests to resume a session, andgenerate a new session ID, which forces the entire handshake procedure (including client
authentication) to proceed.

3.7 Compression Methods
The use of compression may enable attackers to perform attacks using compressionbased side channels. Because of this, only the null compression method, which disables
TLS compression, should be used. If compression is used, the methods defined in[RFC3749] shall be used. If the client population served is known to support thecompression method in [RFC3943], that method may be used instead. Other
compression methods shall not be used. Compression method recommendations are
based on the TLS standards. Limitations are recommended to ensure interoperability.

3.8 Operational Considerations
The sections above specify TLS-specific functionality. This functionality is necessary,
but is not sufficient, to achieve security in an operational environment.
Federal agencies shall ensure that TLS servers include appropriate network security
protections as specified in other NIST guidelines, such as [SP800-53].
The server shall operate on a secure operating system 22. Where the server relies on a
FIPS 140 Level 1 cryptographic module, the software and private key shall be protectedusing the operating system identification, authentication and access control mechanisms.
In some highly sensitive applications, server private keys may require protection using a
FIPS 140 Level 2 or higher hardware cryptographic module.
The server and associated platform shall be kept up-to-date in terms of security patches.
This is critical to various aspects of security, including the black list of certificatespushed by the product vendors. The black list of certificates is useful when an upstream
CA certificate or client certificate is declared to be invalid or not operating with
22 A secure operating system contains and uses the following features: operating system protection from applications
and processes; operating system mediated isolation among applications and processes; user identification andauthentication; access control based on authenticated user identity, and event log of security relevant activities.

26

Guidelines for TLS Implementations

appropriate security measures, and the server does not perform revocation checking, does
not have access to the latest revocation information, or the certificate has not been
revoked.

3.9 Server Recommendations
This section contains summarized recommendations from Section 3.1 through Section 3.8
for the selection, configuration, and maintenance of a TLS server.

3.9.1

Recommendations for Server Selection

The following summary of recommendations is for individuals tasked with selecting a
TLS server implementation for procurement. TLS server implementations shall not be
procured unless they include the required functionality. Recommendations for server
selection are:
1.
2.
3.
4.

Server implementations shall support TLS version 1.1.
Server implementations should support TLS version 1.2.
Server implementations may support TLS version 1.0.
Server implementations that incorrectly implement TLS version negotiation shall not
be selected.
5. Server implementations shall use the bad_record_mac error to indicate a padding
error.
6. Server implementations shall compute the MAC regardless of whether padding errors
exist.
7. Server implementations should support constant-time decryption, or near constanttime decryption.
8. Server implementations should support multiple server certificates with their private
keys to support algorithm and key size agility.
9. Server implementations shall use an Approved random bit generator specified in[SP800-90A].
10. Server implementations shall be able to terminate the connection with a “fatalhandshake failure” alert when the client does not have a certificate or an acceptable
certificate.
11. Server implementations shall be configurable to support Certificate Revocation List
(CRL) or Online Certificate Status Protocol (OCSP), or both.
12. Server implementations shall either support the path validation recommendations in
Section 3.5.1 or be augmented to support them.
13. The server shall be able to provide the client certificate, and the certificate policies
for which the client certification path is valid, to the applications in order to support
access control decisions.

3.9.2

Recommendations for Server Installation and Configuration

The following summary of recommendations is for individuals tasked with the
installation and initial configuration of a TLS server implementation. Recommendations
for TLS server configuration are:
1. Version Support
a. The server shall be configured to support TLS version 1.1.
27

Guidelines for TLS Implementations

b. The server should be configured to support TLS version 1.2.
c. If the server supports government-only applications, it shall not be configured
to support TLS version 1.0.
d. If the server supports citizen or business facing applications, it may be
configured to support TLS version 1.0.
e. If TLS 1.0 is supported, TLS 1.1 and 1.2 shall be preferred over TLS 1.0.
f. The server shall not be configured to support SSL 2.0 or SSL 3.0.
2. Certificatesa. The server shall be configured with one or more public key certificates andthe associated private keys.
b. The server shall be configured with an RSA key encipherment certificate.
c. The server should be configured with an ECDSA signature certificate or RSA
signature certificate.
d. If the server is not configured with an RSA signature certificate, an ECDSA
signature certificate using a Suite B named curve for the signature and public
key in the ECDSA certificate should be used.
e. The server shall be configured with certificates issued by a CA, rather thanself-signed certificates.
f. Server certificates shall be issued by a CA that publishes revocation
information in either CRLs or OCSP responses.
g. The source for the revocation information shall be included in the certificate
in the appropriate extension to promote interoperability.
h. All server certificates shall be X.509 version 3 certificates.
i. Both the public key contained in the certificate and the signature shall have atleast 112 bits of security. In addition, ephemeral keys, when used to establish
the master secret, shall have at least 112 bits of security.
j. The certificate shall be signed with an algorithm consistent with the public
key, as described in Section 3.2.1.
k. The server should be configured to support the server authentication extended
key usage extension.
l. In the absence of agency-specific server certificate profile requirements, thecertificate profile of Table 3-1 should be used for the server certificate.
m. The server shall perform revocation checking of the client certificate, whenclient authentication is used.
i. Revocation information shall be obtained by the server from one ormore of the locations described in Section 3.2.2.
ii. When the server cannot obtain current revocation information, thedecision to accept or reject a certificate should be made according toagency policy.
n. In the absence of agency-specific policies, Federal agencies shall use theCommon Policy.
3. Cryptographic Support
a. The server shall be configured for data confidentiality and integrity services.
b. The server shall be configured to only support cipher suites that are composed
entirely of Approved algorithms.
c. The server shall be configured to support the following cipher suites:28

Guidelines for TLS Implementations

TLS_RSA_WITH_3DES_EDE_CBC_SHA
TLS_RSA_WITH_AES_128_CBC_SHA
d. The server should be configured to support the following cipher suites:
TLS_RSA_WITH_AES_256_CBC_SHA
TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA
TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA
e. If the server is configured to support TLS version 1.2, then the server shall be
configured to support the following cipher suite:
TLS_RSA_WITH_AES_128_GCM_SHA256
f. If the server is configured to support TLS version 1.2, then the server should
be configured to support the following cipher suites:
TLS_RSA_WITH_AES_256_GCM_SHA384
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
g. The server may be configured to support other acceptable cipher suites, as
described in Section 3.3.1.
h. The server shall only support cipher suites for which it has a valid certificate
containing a signature providing at least 112 bits of security.
i. The server shall not be configured to use cipher suites that do not appear in
Section 3.3.1or Appendix C.
j. For the RSA certificates, the key usage extension shall specify key
encipherment for cipher suites that carry out the key exchange with RSA, andthe key usage extension shall specify digital signature for cipher suites using
ECDHE key exchange.
k. The cryptographic module used by the server shall be a FIPS 140-validated
cryptographic module.
l. All cryptographic algorithms that are included in the cipher suites shall bewithin the scope of the validation, as well as the random number generator.
m. The random number generator shall be tested and validated in accordancewith [SP800-90A] under the NIST Cryptographic Algorithm Validation
Program (CAVP) and successful results of this testing shall be indicated on
the cryptographic module’s FIPS 140 validation certificate.
n. The validated random number generator shall be used to generate the 28-byte
random value of the server random value.
o. The validated random number generator should be used to generate the 4-bytetimestamp of the server random value.
4. Extensions
a. The TLS server shall support the following TLS extensions, as described in
Section 3.4.1:
Renegotiation Indication
Certificate Status Request
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Guidelines for TLS Implementations

Server Name Indication
Trusted CA Indicationb. The TLS server shall support the following TLS extensions, as described in
Section 3.4.2, when the conditions stated in Section 3.4.2 are met:
Supported Elliptic Curves
EC Point Format
Signature Algorithms
Multiple Certificate Status
c. If the Supported Elliptic Curves extension is supported, the curves P-256 andP-384 shall be supported.
d. The TLS server may support the Truncated HMAC extension, as described in
Section 3.4.2.5, when the conditions stated in Section 3.4.2 are met.
e. The TLS server should not support the Client Certificate URL extension.
f. If the Client Certificate URL extension is supported, the server shall be
configured to mitigate attacks described in Section 3.4.3.
g. If possible, the server shall check the client ephemeral public key size used
for establishing the master secret to ensure that the client ephemeral publickey can offer at least 112 bits of security.
5. Client Authentication
a. If the server supports client authentication, it shall support certificate-basedclient authentication.
b. If possible, the server shall check the client public key size to ensure that theclient public key can offer at least 112 bits of security.
c. The server shall be configured to terminate the connection with a fatal“handshake failure” alert when a client certificate is requested, and the client
does not have a suitable certificate.
d. If client authentication is performed, the server shall validate the clientcertificate in accordance with the certification path validation rules specified
in Section 6 of [RFC5280].
e. The server shall be configured such that each certificate in the certification
path shall be validated using a Certificate Revocation List (CRL) or Online
Certificate Status Protocol (OCSP).
f. If the server supports OCSP, then OCSP checking shall be in compliance with
[RFC6960] and should use only one of the options described in Section 3.5.1of this document.
g. The server shall be able to determine the certificate policies that the clientcertificate is trusted for by using the certification path validation rules
specified in Section 6 of [RFC5280].
h. The server shall be configured with only the trust anchors the server trusts,
and of those, only the ones that are required to authenticate the clients, in thecase where the server supports client authentication in TLS.
i. The default set of trust anchors for the server shall be examined to determine
if any of them are required for client authentication.
j. The server shall check the client key length if client authentication is
performed, and the server implementation provides a mechanism to do so.

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Guidelines for TLS Implementations

i. Federal Agencies shall use the key size guidelines provided in [SP800131A] to check the client key size.
k. The server shall be configured to maintain the trust anchors of the various
PKIs whose subscribers are the potential clients for the server, and include
them in the hints list.
i. Alternatively, the server should be configured to send an empty hints
list so that the client can always provide a certificate it possesses.
l. The server hints list shall be distinct from the server’s trust anchor store.
m. The server shall continue to only populate its trust anchor store with the trust
anchor of the server PKI domain and the domains it needs to trust directly forclient authentication.
6. Session Resumption
a. If there is a requirement to authenticate each client as it initiates a connection
session, the server shall be configured to ignore requests to resume a session,and generate a new session ID, which forces the entire handshake procedure(including client authentication) to proceed.
7. Compression Methods
a. The server should be configured to only support the null compression method,which disables TLS compression.
b. If compression is used, the server shall be configured to only support themethods defined in [RFC3749].
i. If the client population served is known to support the compression
method in [RFC3943], that method may be used instead.
c. The server shall not be configured to support other compression methods.
8. Operational Considerations
a. The server shall operate on a secure operating system.
b. Where the server relies on a FIPS 140 Level 1 cryptographic module, thesoftware and private key shall be protected using the operating system
identification, authentication and access control mechanisms.

3.9.3

Recommendations for Server System Administrators

A Server System Administrator is an individual who is responsible for maintaining the
TLS server on a day-to-day basis.
1. Version Support
a. System administrators shall develop migration plans to support TLS 1.2 byJanuary 1, 2015.
2. Certificatesa. System administrators shall use Sections 3.2.1 and 3.2.2 to identify anappropriate source for certificates.
b. System administrators shall install, maintain, and update certificates inaccordance with the certificate recommendations of Section 3.9.2.
3. Cryptographic Support
a. System administrators shall maintain confidentiality and integrity service
configurations in accordance with the recommendations of Section 3.9.2.
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Guidelines for TLS Implementations

4. Client Authentication
a. System administrators of a TLS server that supports certificate-based client
authentication shall perform an analysis of the client certificate issuers anduse that information to determine the minimum set of trust anchors required
for the server.
i. The server shall be configured only to include only the minimum set oftrust anchors needed.
5. Operational Considerations
a. System administrators shall ensure that TLS servers include appropriate
network security protections as specified in other NIST guidelines.
b. The server shall operate on a secure operating system.
c. Where the server relies on a FIPS 140 Level 1 cryptographic module, thesystem administrator shall ensure that the software and private key areprotected using the operating system identification, authentication and access
control mechanisms.
d. The system administrator shall ensure that the server and associated platform
are kept up-to-date in terms of security patches.

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Guidelines for TLS Implementations

4 Minimum Requirements for TLS Clients
This section provides a minimum set of requirements that a TLS client must meet inorder to adhere to these guidelines. Requirements are organized in the following
sections: TLS protocol version support; client keys and certificates; cryptographic
support; TLS extension support; server authentication; session resumption; compression
methods; and operational considerations.
Specific requirements are stated as either implementation requirements or configuration
requirements. Implementation requirements indicate that Federal agencies shall not
procure TLS client implementations unless they include the required functionality.
Configuration requirements indicate that system administrators are required to verify thatparticular features are enabled, or in some cases, configured appropriately if present.

4.1 Protocol Version Support
The client shall be configured to support TLS 1.1, and should be configured to support
TLS 1.2. The client may be configured to support TLS 1.0 to facilitate communication
with private sector servers, where necessary. If TLS 1.0 is supported, the use of TLS 1.1
and 1.2 shall be preferred over TLS 1.0. The client shall not support SSL version 3.0 or
earlier. Agencies shall develop migration plans to support TLS 1.2 by January 1, 2015.

4.2 Client Keys and Certificates
4.2.1

Client Certificate Profile

When client authentication is needed, the client shall be configured with a certificate thatadheres to the recommendations presented in this section. A client certificate may be
configured on the system, or located on an external device (e.g., a PIV card). For thisspecification, the TLS client certificate shall be an X.509 version 3 certificate; both the
public key contained in the certificate and the signature shall have at least 112 bits of
security. The certificate shall be signed with an algorithm consistent with the public key:•••

Certificates containing RSA (signature), ECDSA, or DSA public keys shall besigned with those same signature algorithms, respectively;
Certificates containing Diffie-Hellman certificates shall be signed with DSA; andCertificates containing ECDH public keys shall be signed with ECDSA.

The extended key usage extension limits the operations that keys in a certificate may be
used for. There is a key usage extension specifically for client authentication. The use of
the extended key usage extension will ensure that the servers accept the certificate as aclient certificate. The extended usage extension can also indicate that the certificate is not
to be used for other purposes, such as code signing. The client certificates should
include an extended key usage extension that specifies the client authentication key
purpose object identifier 23.
23 Absence of extended key usage extension in some implementation is known to be interpreted as having special
permission such as code signing, even though not specifically indicated in the certificate.

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Guidelines for TLS Implementations

The client certificate profile is listed in Table 4-1: TLS Client Certificate Profile. In theabsence of an agency-specific client certificate profile, this profile should be used forclient certificates.
Note that for ECDH, the algorithm OID and the signature OID are identical to those of
ECDSA. For interoperability reasons, algorithm OID is not changed and the key usage
extension determines if the public key is used for key agreement or signature verification.
Table 4-1: TLS Client Certificate Profile
Field

CriticalValue

Description

VersionN/A

2

Version 3

Serial Number

N/A

Unique positive integer

Issuer Signature Algorithm

N/A

Must be unique
Values by certificate type:sha256WithRSAEncryption {1 2 840
113549 1 1 11}, or stronger
ecdsa-with-SHA256 {1 2 840 10045 4 32}, or stronger

RSA signature certificate

DSA signature certificate, DH certificate

ECDSA signature certificate, ECDH
certificate

Issuer Distinguished Name

N/A

id-dsa-with-sha256 {2 16 840 1 101 3 4 32}, or stronger
Unique X.500 Issuing CA DN

Validity Period

N/A

3 years or lessDates through 2049 expressed in UTCTime

Subject Distinguished Name

N/A

Unique X.500 subject DN per agency
requirements

Single value shall be encoded in each
RDN. All attributes that are of
directoryString type shall be encoded as a
printable string.

Subject Public Key
Information

N/A

Single value shall be encoded in each
RDN. All attributes that are of
directoryString type shall be encoded as a
printable string.

Values by certificate type:rsaEncryption {1 2 840 113549 1 1 1}

RSA key encipherment certificate, RSA
signature certificate
2048-bit RSA key modulus, or otherapproved lengths as defined in [FIPS168-4]
and [SP800-57p1]
Parameters: NULL

ecPublicKey {1 2 840 10045 2 1}

ECDSA signature certificate, or ECDH
certificate
Parameters: namedCurve OID for names
curve specified in FIPS 186-4. The curve
shall be P-256 or P-384
SubjectPublic Key: Uncompressed EC
Point.

id-dsa {1 2 840 10040 4 1}

DSA signature certificate
Parameters: p, q, g

dhpublicnumber {1 2 840 10046 2 1}

DH certificate
Parameters: p, g, q

Issuer’s Signature

Values by certificate type:

N/A

sha256WithRSAEncryption {1 2 840
113549 1 1 11}, or stronger

RSA key encipherment certificate, RSA
signature certificate

ecdsa-with-SHA256 {1 2 840 10045 4 32}, or stronger

ECDSA signature certificate, ECDH
certificate

id-dsa-with-sha256 { 2 16 840 1 101 3 4
3 2}, or stronger

DSA signature certificate, DH certificate

Octet String

Same as subject key identifier in Issuing
CA certificate

Extensions
Authority Key IdentifierNo

Prohibited: Issuer DN, Serial Number tuple

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Guidelines for TLS Implementations

CriticalNo

Value
Octet String

Description

Subject Key Identifier
Key Usage

Yes

digitalSignature

RSA certificate, DSA certificate, ECDSA
certificate

keyAgreement

ECDH certificate, DH certificate

Extended Key UsageNo

id-kp-clientAuth {1 3 6 1 5 5 7 3 2}

RequiredanyExtendedKeyUsage {2 5 29 37 0}

Prohibited 24

Field

Same as in PKCS-10 request or calculatedby the Issuing CA

Prohibited: all others unless consistent withkey usage extension

Certificate Policies

No

Per agency X.509 certificate policy

Subject Alternative Name

No

Optional

Authority Information AccessNo

RFC 822 e-mail address, Universal
Principal Name (UPN), DNS Name,
and/or others
id-ad-caIssuers

id-ad-ocsp

Optional. Access method entry contains
HTTP URL for the Issuing CA OCSP
Responder

See comments

Optional: HTTP value in distributionPoint
field pointing to a full and complete CRL.

CRL Distribution Points

No

Required. Access method entry contains
HTTP URL for certificates issued to
Issuing CA

Prohibited: reasons and cRLIssuer fields,
and nameRelativetoCRLIssuer CHOICE

Multiple client certificates may be present that meet the requirements of the TLS server.
The TLS client (e.g., a browser) may ask users to select from a list of certificates. The
use of the Extended Key Usage (EKU) extension may eliminate this request.
Client certificates are also filtered by TLS clients on the basis of an ability to build a path
to one of the trust anchors in the hints list sent by the server, as described in Section
3.5.4.

4.2.2

Obtaining Revocation Status Information for the Server
Certificate

The client shall perform revocation checking of the server certificate. Revocation
information can be obtained by the client from one of the following locations:
1. OCSP response or responses in the server’s CertificateStatus message [RFC6066],[RFC6961].
2. Certificate Revocation List (CRL) or OCSP [RFC6960] response in the client’s
local certificate store;
3. OCSP response from a locally configured OCSP responder;
4. OCSP response from the OCSP responder location identified in the OCSP field in
the Authority Information Access extension in the server certificate; or
5. CRL from the CRL Distribution Point extension in the server certificate.
24 The presence of anyExtendedKeyUsage {2 5 29 37 0} in some implementation is known to be interpreted as having
special permission such as code signing, even though not specifically indicated in the certificate.

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Guidelines for TLS Implementations

When the server does not provide the revocation status, the local certificate store does nothave the current or a cogent CRL or OCSP response, and the OCSP Responder and theCRL Distribution Point are unavailable or inaccessible at the time of TLS session
establishment, the client will either terminate the connection or accept a potentially
revoked or compromised certificate. The decision to accept or reject a certificate in thissituation should be made according to agency policy.
Other emerging concepts that can be useful in lieu of revocation checking are further
discussed in Appendix D.

4.2.3

Client Public Key Certificate Assurance

The client public key certificate may be trusted by the servers on the basis of the policies,
procedures and security controls used to issue the client public key certificate as
described in Section 3.5.1. For example, as the implementation of Personal Identify
Verification (PIV) [FIPS201-1] becomes more established in Federal Agencies, theseguidelines recommend that the PIV Authentication certificate be the norm forauthentication of Federal employees and long-term contractors. For users who do nothave PIV Cards, such as external users, the set of certificate policies to accept should bedetermined as specified in Appendix B of [SP800-63], based on the level of assurance
required by the application. PIV Authentication certificate policy is defined in[COMMON] and PIV-I Authentication certificate policy is defined in [FBCACP].
Depending on the requirements of the server-side application, other certificate policies
defined in [COMMON] may also be acceptable. Guidance regarding the acceptable
certificate policies is outside the scope of these guidelines.

4.3 Cryptographic Support
4.3.1

Cipher Suites

The acceptable cipher suites for a TLS client are the same as those for a TLS server.
General-purpose cipher suites are listed in Section 3.3.1, and cipher suites appropriate forpre-shared key environments are listed in Appendix C. When ephemeral keys are used toestablish the master secret, each ephemeral key-pair (i.e., the server ephemeral key-pairand the client ephemeral key-pair) shall have at least 112 bits of security.
The client should not be configured to use cipher suites other than those listed in Section
3.3.1 or Appendix C.
To mitigate attacks against CBC mode, TLS implementations shall use thebad_record_mac error to indicate a padding error. Implementations shall compute theMAC regardless of whether padding errors exist. TLS implementations should support
constant-time decryption, or near constant-time decryption.

4.3.2

Validated Cryptography

The client shall use validated cryptography, as described for the server in Section 3.3.2.
The validated random number generator shall be used to generate the 28-byte random
value of the client random value. The validated random number generator should be used
to generate the 4-byte timestamp of the client random value.
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Guidelines for TLS Implementations

4.4 TLS Extension Support
4.4.1

Mandatory TLS Extensions

The client shall support the following extensions:
1. Renegotiation Indication
2. Server Name Indication
4.4.1.1 Renegotiation Indication
The Renegotiation Indication extension is required by these guidelines as described in
Section 3.4.1.1. Clients shall perform initial and subsequent renegotiations in accordancewith [RFC5746].
4.4.1.2 Server Name Indication
The server name indication extension is described in Section 3.4.1.3. The client shall becapable of including this extension in a ClientHello message, as described in [RFC6066].

4.4.2

Conditional TLS Extensions

A TLS client supports the following TLS extensions under the circumstances described:
1. The Supported Elliptic Curves TLS extension shall be supported if the client
supports EC cipher suite(s).
2. The EC Point Format TLS extension shall be supported if the client supports EC
cipher suite(s).
3. The Signature Algorithms TLS extension shall be supported when the client is
operating in TLS 1.2.
4. The Certificate Status Request extension shall be supported when the client is notable to obtain revocation information.
5. The Multiple Certificate Status extension shall be supported if the extension is
supported by the client implementation.
6. The Trusted CA Indication extension should be supported by clients that run on
memory-constrained devices where only a small number of CA root keys are
stored.
7. The Truncated HMAC extension may be supported by clients that run on
constrained devices when variable-length padding is not supported.
4.4.2.1 Supported Elliptic Curves
The clients that support EC cipher suites shall be capable of listing the elliptic curves
supported in the ClientHello message, in accordance with Section 5.1 of [RFC4492].
4.4.2.2 EC Point Format
The clients that support EC cipher suites shall be capable of specifying the supported EC
point format in the ClientHello message, in accordance with Section 5.1 of [RFC4492].

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Guidelines for TLS Implementations

Clients that support EC cipher suites shall support the processing of at least one 25 of the
EC point formats received in the ServerHello message, as described in Section 5.2 of[RFC4492].
4.4.2.3 Signature Algorithms
The clients that support TLS 1.2 shall be able to assert acceptable hashing and signaturealgorithm pairs in this extension in a ClientHello message. The extension, its syntax, andprocessing rules are described in Sections 7.4.1.4.1, 7.4.4, 7.4.6 and 7.4.8 of [RFC5246].
4.4.2.4 Certificate Status Request
When the client wishes to receive the revocation status of the TLS server certificate fromthe TLS server, the client shall include the “status_request” extension in the ClientHellomessage.
4.4.2.5 Multiple Certificate Status
The multiple certificate status extension is described in Section 3.4.2.4. This extension
improves on the Certificate Status Request extension described in Section 3.4.1.2 byallowing the client to request the status of all certificates provided by the Server in the
TLS handshake. This extension is documented in [RFC6961]. Client implementations
that have this capability shall be configured to support this extension.
4.4.2.6 Trusted CA Indication
The client should be capable of including the trusted CA indication (trusted_ca_keys)
extension in a ClientHello message as described in [RFC6066].
4.4.2.7 Truncated HMAC
The Truncated HMAC extension is described in Section 3.4.2.5. Clients running on
constrained devices may support this extension. The Truncated HMAC extension shallnot be used in conjunction with variable-length padding, due to attacks described in[Paterson11].

4.4.3

Discouraged TLS Extensions

The following extension should not be used:
1. Client Certificate URL
The reasons for discouraging the use of this extension can be found in Section 3.4.3.

4.5 Server Authentication
The client shall be able to build the certification path for the server certificate presented
in the TLS handshake with at least one of the trust anchors in the client trust store, if anappropriate trust anchor is present in the store. The client may use all or a subset of thefollowing resources to build the certification path: local certificate store, certificatesreceived from the server during the handshake, LDAP, resources declared in CA
25 The uncompressed point format must be supported, as described in Sections 5.1.2 and 5.2 of [RFC4492].

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Guidelines for TLS Implementations

Repository field of the Subject Information Access extension in various CA certificates,
and resources declared in the CA Issuers field of the Authority Information Accessextension in various certificates.

4.5.1

Path Validation

The client shall validate the server certificate in accordance with the certification pathvalidation rules specified in Section 6 of [RFC5280]. In addition, the revocation status ofeach certificate in the certification path shall be checked using the Certificate Revocation
List (CRL) or Online Certificate Status Protocol (OCSP). OCSP checking shall be incompliance with [RFC6960] and should use only one of the following options:•••

The OCSP Responder is trusted by the client, i.e., the OCSP Responder public
key is the same as that of one of the public keys in the client’s trust anchor store;or
The OCSP Response is signed using the same key as that of the certificate whose
status is being checked; or
The OCSP Response is signed by a designated/delegated OCSP Responder as
described in [RFC6960], and the OCSP Responder certificate is signed using thesame key as that of the certificate whose status is being checked.

Revocation information shall be obtained as described in Section 4.2.2.
Not all commercial products support the public key certification path validation andcertificate policy processing rules listed and cited above. Specifically, revocation
checking in some instances may not be available, or the client could accept a server
public key certificate if the latest revocation information is inaccessible. Similarly, someclients are not able to provide inputs related to acceptable certificate policy or initialvalues for requiring policies, and inhibiting policy mapping. In the absence of clientsthat are fully certificate policy aware, Federal agencies may use other mechanisms todecide if a server certificate has been issued with due diligence.
Not all clients support checking name constraints. The Federal agencies shall only
procure clients that perform name constraint checking in order to obtain assurance thatunauthorized certificates are properly rejected. As an alternative, the Federal agency may
procure clients that use one or more of the features discussed in Appendix D.
The client shall terminate the TLS connection if path validation fails.
Federal agencies shall only use clients that check that the DNS name or IP address,
whichever is presented in the client TLS request, matches a DNS name or IP addresscontained in the server certificate’s subject alternative name extension. If the namepresented in the client TLS request is absent from the server certificate’s subject
alternative name extension, only then the client shall check the server certificate’s subject
distinguished name field to determine if the subject distinguished name (specifically, thecommon name attribute type) contains the requested name. The client shall terminate
the TLS connection if the name check fails.

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Guidelines for TLS Implementations

4.5.2

Trust Anchor StoreHaving an excessive number of trust anchors installed in the TLS client can increase thechances for the client to be spoofed. As the number of trust anchors increase, the number
of CAs that the client trusts increases, and the chances that one of these CAs or theirregistration system or process will be compromised to issue TLS server certificates also
increases. In the minimal case, a Federal Agency relying party client can have a single
trust anchor: an agency legacy trust anchor or the Common Policy trust anchor.
Federal Agencies shall perform a trade-off between the risk associated with and need toaccess commercial web sites to determine the trust anchor store in the various clientmachines. Federal agencies shall administer this trust anchor store through centralized
management applications. Federal agency systems and clients shall be configured suchthat an update to the trust anchor store is a privileged system administrative function
requiring appropriate agency security approval.
To mitigate the client certificate selection and path-building problem at the client end
described in Section 3.5.4, clients shall not overpopulate their trust stores with various
CA certificates that can be verified via cross-certification. Direct trust of thesecertificates can expose the clients unduly to a variety of situations, including but notlimited to, revocation or compromise of these trust anchors. Direct trust also increases
the operational and security burden on the clients to promulgate addition and deletion oftrust anchors. Instead, the client shall rely on the server overpopulating or not providing
the hints listed as discussed in Section 3.5.4.

4.5.3

Checking the Server Key Size

The only direct mechanism for a client to check if the key size presented in a server
public certificate is acceptable is for the client to examine the server public key in thecertificate. An indirect mechanism is to check that the certificate policies extension in theserver public key certificate indicates the minimum cryptographic strength of thesignature and hashing algorithms used and for the client to perform certificate policy
processing and checking. A more scalable and more robust alternative that is standardsbased is described in Appendix D. The client shall check the server public key length if
the client implementation provides a mechanism to do so. The client shall also check theserver public key length if the server uses ephemeral keys for the creation of the mastersecret, and the client implementation provides a mechanism to do so.
The length of each write key is determined by the negotiated cipher suite. Restrictions on
the length of the shared session keys can be enforced by configuring the client to only
support cipher suites that meet the key length requirements.

4.5.4

User Interface

When the TLS client is a browser, the browser interface can be used to determine if a
TLS session is in effect. The indication that a TLS session is in effect varies by browser.
Examples of indicators include a padlock in the URL bar, or a different color for the URL
bar. Some clients, such as browsers, may allow further investigation of the server
certificate and negotiated session parameters by clicking on the lock (or other indicator).
Users should examine the interface for the presence of the indicator to ensure that the40

Guidelines for TLS Implementations

TLS session is in force and should also visually examine the web site URL to ensure thatthe user intended to visit the indicated web site. Users should be aware that URLs canappear to be legitimate, but still not be valid. For example, the numeric “1” and the letter“l” appear quite similar or the same to the human eye. If the user navigates to a URL thatappears to be correct, the browser software could defeat these threats by matching therequested URL with the DNS name in the server certificate.
Client authentication keys may be located outside of the client (e.g., PIV cards). Users
shall follow the policies and procedures for protecting client authentication keys outsideof the client.

4.6 Session Resumption
The client shall follow the same session resumption recommendations as the server,which are described in Section 3.6.

4.7 Compression Methods
The client shall follow the same compression recommendations as the server, which aredescribed in Section 3.7.

4.8 Operational Considerations
The client and associated platform shall be kept up-to-date in terms of security patches.
This is critical to various aspects of security, including the black list of certificatespushed by the product vendors. The black list of certificates is useful when an upstream
CA certificate or server certificate is declared to be invalid or not operating withappropriate security measures, and the client does not perform revocation checking, does
not have access to the latest revocation information, or the certificate has not been
revoked.
Once the TLS-protected data is received at the client, and decrypted and authenticated by
the TLS layer of the client system, the unencrypted data is available to the applications on
the client platform.
These guidelines also do not mitigate the threats against the misuse or exposure of theclient credential that resides on the client machine. These credentials could contain theprivate key used for client authentication or other credentials (e.g., one-time password(OTP) or user ID and password) for authenticating to server side application.
For these reasons, the use of TLS does not obviate the need for the client to use
appropriate security measures, as described in applicable Federal Information Processing
Standards and NIST Special Publications, to protect computer systems and applications.
Users shall operate client systems in accordance with agency and administrator
instructions.

4.9 Client Recommendations
This section contains summarized recommendations from Section 4.1 through Section 4.8
for the selection, configuration, maintenance, and use of a TLS client.
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Guidelines for TLS Implementations

4.9.1

Recommendations for Client Selection

The following summary of recommendations is for individuals tasked with selecting a
TLS client implementation for procurement. TLS clients shall not be procured unlessthey include the required functionality. Recommendations for client selection are:
1.
2.
3.
4.

Client implementations shall support TLS version 1.1.
Client implementations should support TLS version 1.2.
Client implementations may support TLS version 1.0.
Client implementations shall be configurable to prefer TLS 1.1 and TLS 1.2 overTLS 1.0.
5. Client implementations shall use the bad_record_mac error to indicate a padding
error.
6. Client implementations shall compute the MAC regardless of whether padding errors
exist.
7. Client implementations should support constant-time decryption, or near constanttime decryption.
8. Client implementations shall support the client authentication extended key usage
extension.
9. Client implementations shall support name constraint checking in order to ensure thatunauthorized certificates are properly rejected.
10. Client implementations shall check that the DNS name or IP addresses presented in
the client TLS request matches a name or IP address contained in the server
certificate’s subject distinguished name field or subject alternative name extension.
11. Client implementations shall terminate the TLS connection if the path validation
fails.

4.9.2

Recommendations for Client Installation and Configuration

The following summary of recommendations is for individuals tasked with the
installation and initial configuration of a TLS client implementation. Recommendations
for TLS client configuration are:
1. Version Support
a. The client shall be configured to support TLS version 1.1.
b. The client should be configured to support TLS version 1.2.
c. The client may be configured to support TLS version 1.0.
d. If TLS version 1.0 is supported, the client shall be configured to prefer TLS
1.1 and TLS 1.2 over TLS 1.0.
e. The client shall not be configured to support SSL version 3.0 or earlier.
2. Certificatesa. All client certificates shall be X.509 version 3 certificates.
b. Both the public key contained in the certificate and the signature shall have atleast 112 bits of security. In addition, ephemeral keys, when used to establish
the master secret, shall have at least 112 bits of security.
c. The certificate shall be signed with an algorithm consistent with the public
key, as described in Section 4.2.1.
d. The client certificate should include an extended key usage extension thatspecifies the client authentication key purpose object identifier.
42

Guidelines for TLS Implementations

e. In the absence of an agency-specific client certificate profile, the profile in
Table 4-1 should be used for client certificates.
f. The client shall perform revocation checking of the server certificate, as
described in Section 4.2.2.
i. When the client cannot obtain current revocation information, thedecision to accept or reject a certificate should be made according toagency policy.
3. Cryptographic support
a. The client shall be configured to support the following cipher suites:
TLS_RSA_WITH_3DES_EDE_CBC_SHA
TLS_RSA_WITH_AES_128_CBC_SHA
b. The client should be configured to support the following cipher suites:
TLS_RSA_WITH_AES_256_CBC_SHA
TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA
TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA
c. If the client is configured to support TLS 1.2, then the client shall be
configured to support the following cipher suites:
TLS_RSA_WITH_AES_128_GCM_SHA256
d. If the client is configured to support TLS 1.2, then the client should be
configured to support the following cipher suites:
TLS_RSA_WITH_AES_256_GCM_SHA384
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
e. The client should not be configured to support cipher suites other than thoselisted above and in Section 3.3.1 or Appendix C.
f. The cryptographic module used by the client shall be a FIPS 140-validated
cryptographic module.
g. All cryptographic algorithms that are included in the cipher suites shall bewithin the scope of the validation, as well as the random number generator.
h. The random number generator shall be tested and validated in accordancewith [SP800-90A] under the NIST Cryptographic Algorithm Validation
Program (CAVP) and successful results of this testing shall be indicated on
the cryptographic module’s FIPS 140 validation certificate.
i. The validated random number generator shall be used to generate the 28-byte
random value of the client random value.
j. The validated random number generator should be used to generate the 4-bytetimestamp of the client random value.
4. Extensions
a. The TLS client shall support the following TLS extensions, as described in
Section 4.4.1:
Renegotiation Indication
43

Guidelines for TLS Implementations

Server Name Indicationb. The TLS client shall support the following TLS extensions, as described in
Section 4.4.2, when the conditions stated in Section 4.4.2 are met:
Supported Elliptic Curves
EC Point Format
Signature Algorithms
Certificate Status Request
Multiple Certificate Statusc. The TLS client should support the Trusted CA Indication extension, as
described in Section 4.4.2.6, when the conditions stated in Section 4.4.2 aremet.
d. The TLS client may support the Truncated HMAC extension, as described in
Section 4.4.2.7, when the conditions stated in Section 4.4.2 are met.
e. The TLS client should not support the following TLS extension:
Client Certificate URL
5. Server Authenticationa. The client shall be able to build the certification path for the server certificate
presented in the TLS handshake with at least one of the trust anchors in theclient trust store, if an appropriate trust anchor is present in the store.
b. The client may use all or a subset of the following resources to build thecertification path: local certificate store, certificates received from the server
during the handshake, LDAP, resources declared in CA Repository field of
the Subject Information Access extension in various CA certificates, andresources declared in the CA Issuers field of the Authority Information Accessextension in various certificates.
c. The client shall validate the server certificate in accordance with thecertification path validation rules specified in Section 6 of [RFC5280].
d. The client shall be configured such that the revocation status of eachcertificate in the certification path shall be checked using the Certificate
Revocation List (CRL) or Online Certificate Status Protocol (OCSP).
e. If the client supports OCSP, then OCSP checking shall be in compliance with
[RFC6960] and should use only one of the options described in Section 4.5.1of this document.
f. The client shall terminate the TLS connection if path validation fails.
g. The client shall check that the DNS name or IP addresses presented in theclient TLS request matches a name or IP address contained in the server
certificate’s subject alternative name extension.
h. If the name presented in the client TLS request is absent from the server
certificate’s subject alternative name extension, then the client shall check theserver certificate’s subject distinguished name field to determine if the subject
distinguished name contains the requested name.
i. The client shall terminate the TLS connection if the name check fails.
j. Clients shall not overpopulate their trust stores with various CA certificatesthat can be verified via cross-certification.
k. The client shall rely on server trust store overpopulating or not providing thehints list as discussed in Section 3.5.4.
44

Guidelines for TLS Implementations

l. The client shall check the server public key length if the client
implementation provides a mechanism to do so. This is applicable to both the
public keys in the server certificate and the ephemeral server public keys used
for establishing the master secret.
6. Session Resumption
a. If there is a requirement to authenticate the server for each connection session,
the client shall generate a new session ID, which forces the entire handshakeprocedure (including server authentication) to proceed.
7. Compression Methods
a. The client should support the null compression method, which disables TLS
compression.
b. If compression is used, the client shall support the methods defined in[RFC3749].
i. If the server population served is known to support the compression
method in [RFC3943], that method may be used instead.
c. The client shall not support other compression methods.

4.9.3

Recommendations for Client System Administrators

A Client System Administrator is an individual who is responsible for maintaining the
TLS client on a day-to-day basis.
1. Version support
a. System administrators shall develop migration plans to support TLS 1.2 byJanuary 1, 2015.
2. Certificatesa. System administrators shall install, maintain, and update certificates inaccordance with the certificate recommendations of Section 4.9.2.
3. Server Authentication
a. System administrators shall perform a trade-off between risk associated withand need to access commercial web sites to determine the trust anchor store in
the various client machines.
b. System administrators shall administer the trust anchor store throughcentralized management applications.
c. System administrators shall configure clients such that an update to the trust
anchor store is a privileged system administrative function requiring
appropriate agency security approval.
d. Administrators shall ensure that client trust stores are not overpopulated withvarious CA certificates that are otherwise to be trusted via cross-certification.
i. Instead, the client shall rely on the server overpopulating or notproviding the hints list as discussed in Section 3.5.4.
4. Operational Considerations
a. The client and associated platform shall be kept up-to-date in terms of
security patches.

4.9.4

Recommendations for End Users

An end user is an individual using a client to establish a TLS connection.
Recommendations for end users are:45

Guidelines for TLS Implementations

1. If the client is a browser, users should examine the interface to ensure that the TLS
session is in force and should also visually examine the web site URL to ensure thatthe user intended to visit the indicated web site.
2. Users should be aware that URLs can appear to be legitimate, but still not be valid.
3. Users shall operate client systems in accordance with agency and administrator
instructions.
4. Users shall follow appropriate policies and procedures for protecting client
authentication keys outside of the client (e.g., PIV cards).

46

Guidelines for TLS Implementations

Appendix A

Acronyms

Selected acronyms and abbreviations used in these guidelines are defined below.
3DES
AEAD
AES
CA
CBC
CCM
CRL
DES
DH
DHE
DNS
DNSSEC
DSA
DSS
EC
ECDHE
ECDSA
FIPS
GCM
IETF
MAC
OCSP
OID
PIV
PKI
PRF
PSK
RFC
SHA
SSL
TLS
URL

Triple DES (TDEA)
Authenticated Encryption with Associated Data
Advanced Encryption Standard
Certification Authority
Cipher Block Chaining
Counter with CBC-MAC
Certificate Revocation List
Data Encryption Standard
Diffie-Hellman key exchange
Ephemeral Diffie-Hellman key exchange
Domain Name System
DNS Security Extensions
Digital Signature Algorithm
Digital Signature Standard (implies DSA)
Elliptic Curve
Ephemeral Elliptic Curve Diffie-Hellman
Elliptic Curve Digital Signature Algorithm
Federal Information Processing Standard
Galois Counter Mode
Internet Engineering Task Force
Message Authentication CodeOnline Certificate Status Protocol
Object Identifier
Personal Identity Verification
Public Key Infrastructure
Pseudo-random Function
Pre-shared Key
Request for Comments
Secure Hash Algorithm
Secure Sockets Layer
Transport Layer Security
Uniform Resource Locator

47

Guidelines for TLS Implementations

Appendix B

Interpreting Cipher Suite Names

The cipher suite name consists of a set of mnemonics separated by underscores (i.e., “_”).
The first mnemonic is the protocol name, i.e., TLS. This section provides guidance forinterpreting the names of cipher suites that are recommended in these guidelines. Future
cipher suites may not follow these conventions.
One or two mnemonics follow the protocol name, indicating the key exchange algorithm.
If there is only one mnemonic, it must be RSA or PSK, based on the recommendations inthese guidelines. The single mnemonic RSA signifies that the public key in the server
certificate is an RSA key transport public key that should be used by the client for
sending the premaster secret to the server. The single mnemonic PSK indicates that thepremaster secret is established using only symmetric algorithms with pre-shared keys, as
described in [RFC4279]. Pre-shared key cipher suites that are approved for use are listed
in Appendix C. If there are two mnemonics following the protocol name, the first key
exchange mnemonic should be DH, ECDH, DHE, or ECDHE. When the first key
exchange mnemonic is DH or ECDH, it indicates that the server’s public key in its
certificate is for either DH or ECDH key exchange, and the second mnemonic indicatesthe signature algorithm that was used by the issuing CA to sign the server certificate.
When the first key exchange mnemonic is DHE or ECDHE, it indicates that ephemeral
DH or ECDH will be used for key exchange, with the second mnemonic indicating theserver signature public key type 26 that will be used to authenticate the server’s ephemeral
public key.
Next are the word WITH and the mnemonic for the symmetric encryption algorithm andassociated mode of operations.
The last mnemonic is generally the hashing algorithm to be used for HMAC, if
applicable 27. In cases where HMAC is not applicable (e.g., AES-GCM), and the cipher
suite is defined after the release of the TLS 1.2 RFC, this mnemonic represents thehashing algorithm for the PRF.
The following examples illustrate how to interpret the cipher suite names:•

TLS_RSA_WITH_3DES_EDE_CBC_SHA: The server is using an RSA public
key that the client would use for key exchange. The CA signature algorithm is not
specified. Once the handshake is completed, the messages are encrypted using
triple DES in CBC mode. In TLS versions 1.0 and 1.1, a combination of SHA-1
and MD5 is used in the PRF, and SHA-1 is used for HMAC computations on themessages. In TLS 1.2, SHA-256 is used for the PRF, and SHA-1 is used for
HMAC computations on the messages.

•

TLS_DH_DSS_WITH_AES_256_CBC_SHA256: The server is using a DH
certificate. If the connection uses TLS version 1.2, and the signature algorithms

26 In this case, the signature algorithm used by the CA to sign the certificate is not articulated in the cipher suite.
27 HMAC is not applicable when the symmetric encryption mode of operation is authenticated encryption, i.e., CCM
or GCM. Separately, note that the CCM mode cipher suites do not specify the last mnemonic and require that
SHA-256 be used for the PRF.

48

Guidelines for TLS Implementations

extension is provided by the client, then the certificate is signed using thealgorithm specified by the extension. Otherwise, the certificate is signed using
DSA. Once the handshake is completed, the messages are encrypted using AES256 in CBC mode. SHA-256 is used for both the PRF and HMAC computations.
Cipher suites that specify secure hash algorithms other than SHA-1 are notsupported prior to TLS 1.2.
•

TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384: Ephemeral ECDH isused for key exchange. The server’s ephemeral public key is authenticated using
the server’s ECDSA public key. The CA signature algorithm used to certify theserver’s ECDSA public key is not specified. Once the handshake is completed,the messages are encrypted and authenticated using AES-256 in GCM mode, andSHA-384 is used for the PRF. Since an authenticated encryption mode is used,messages neither have nor require an HMAC message authentication code.

49

Guidelines for TLS Implementations

Appendix C

Pre-shared Keys

Pre-shared keys (PSK) are symmetric keys that are already in place prior to the initiationof a TLS session (e.g., as the result of a manual distribution). The use of PSKs in the TLS
protocol is described in [RFC4279], [RFC5487], and [RFC5489]. In general, pre-sharedkeys should not be used. However, the use of pre-shared keys may be appropriate forsome closed environments that have adequate key management support. For example,
they might be appropriate for constrained environments with limited processing, memory,or power. If PSKs are appropriate and supported, then the following additional guidelines
shall be followed.
Recommended pre-shared key (PSK) cipher suites are listed in Table C-1; pre-sharedkeys shall be distributed in a secure manner, such as a secure manual distribution or
using a key establishment certificate. These cipher suites employ a pre-shared key forentity authentication (for both the server and the client) and may also use RSA or
ephemeral Diffie-Hellman (DHE) algorithms for key establishment. For example, when
DHE is used, the result of the Diffie-Hellman computation is combined with the preshared key and other input to determine the premaster secret.
The pre-shared key shall have a minimum security strength of 112 bits. Because thesecipher suites require pre-shared keys, these suites are not generally applicable to classic
secure web site applications and are not expected to be widely supported in TLS clientsor TLS servers. NIST suggests that these suites be considered in particular for
infrastructure applications, particularly if frequent authentication of the network entities
is required. These cipher suites may be used with TLS versions 1.1 or 1.2. Note that
cipher suites using GCM, SHA-256, or SHA-384 are only available in TLS 1.2.
Pre-shared key cipher suites may only be used in networks where both the client andserver are government systems. Cipher suites using pre-shared keys shall not be
supported when TLS 1.0 is supported, and shall not be supported where the client orserver communicates with non-government systems.
Table C-1: Pre-shared Key Cipher Suites
Cipher Suite NameKey
Exchange

Encryption

TLS_PSK_WITH_3DES_EDE_CBC_SHA
TLS_PSK_WITH_AES_128_CBC_SHA
TLS_PSK_WITH_AES_256_CBC_SHA
TLS_PSK_WITH_AES_128_GCM_SHA256
TLS_PSK_WITH_AES_256_GCM_SHA384
TLS_DHE_PSK_WITH_3DES_EDE_CBC_SHA
TLS_DHE_PSK_WITH_AES_128_CBC_SHA
TLS_DHE_PSK_WITH_AES_256_CBC_SHA
TLS_DHE_PSK_WITH_AES_128_GCM_SHA256
TLS_DHE_PSK_WITH_AES_256_GCM_SHA384
TLS_RSA_PSK_WITH_3DES_EDE_CBC_SHA
TLS_RSA_PSK_WITH_AES_128_CBC_SHA
TLS_RSA_PSK_WITH_AES_256_CBC_SHA
TLS_RSA_PSK_WITH_AES_128_GCM_SHA256
TLS_RSA_PSK_WITH_AES_256_GCM_SHA384
TLS_ECDHE_PSK_WITH_3DES_EDE_CBC_SHA

PSK
PSK
PSK
PSK
PSK
DHE_PSK
DHE_PSK
DHE_PSK
DHE_PSK
DHE_PSKRSA_PSKRSA_PSKRSA_PSKRSA_PSKRSA_PSKECDHE_PSK

3DES_EDE_CBC
AES_128_CBC
AES_256_CBC
AES_128_GCM
AES_256_GCM
3DES_EDE_CBC
AES_128_CBC
AES_256_CBC
AES_128_GCM
AES_256_GCM
3DES_EDE_CBC
AES_128_CBC
AES_256_CBC
AES_128_GCM
AES_256_GCM
3DES_EDE_CBC

50

Hash
function
for
HMAC
SHA-1
SHA-1
SHA-1
N/A
N/A
SHA-1
SHA-1
SHA-1
N/A
N/A
SHA-1
SHA-1
SHA-1
N/A
N/A
SHA-1

Hash
Functionfor PRFPer RFCPer RFCPer RFC
SHA-256
SHA-384
Per RFCPer RFCPer RFC
SHA-256
SHA-384
Per RFCPer RFCPer RFC
SHA-256
SHA-384
Per RFC

Guidelines for TLS Implementations

Cipher Suite NameKey
Exchange

TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA
TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA
TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256
TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA384

ECDHE_PSKECDHE_PSKECDHE_PSKECDHE_PSK

51

Encryption

AES_128_CBC
AES_256_CBC
AES_128_CBC
AES_256_CBC

Hashfunction
for
HMAC
SHA-1
SHA-1
SHA-256
SHA-384

Hash
Functionfor PRFPer RFCPer RFC
SHA-256
SHA-384

Guidelines for TLS Implementations

Appendix D

Future Capabilities

This section identifies emerging concepts and capabilities that are applicable to TLS. As
these concepts mature, and commercial products are available to support them, theseguidelines will be revised to provide specific recommendations.

D.1 Additional/Alternate Web Server Certificate Validation
Mechanisms
In order to deal with the threat associated with the compromise of a CA, registration
system, or process, new ideas about how to gain assurance of the legitimacy of the server
certificate presented in a TLS session have been developed.
In addition, new standards are emerging in the use of public key technology to secure theDNS. These DNSSEC standards can be used to replace or augment the traditional PKI
approach to establishing trust in the server certificate.
The following sections describe these concepts. In some cases, these concepts are notfully standardized, and in most cases, they are not widely available in commercial
products. As these concepts mature and become widely available, these guidelines will
be revised to describe them further and to recommend how they can be used to augment
or replace traditional mechanisms to establish trust in the server certificate and associatedrevocation checking.

D.1.1

Sovereign Keys

The sovereign key approach has been developed by the Electronic Frontier Foundation.
Under this approach, the server public key certificates and, optionally, intermediate CA
certificates are claimed by the server domain holder, and these claims are countersignedby one or more trusted third parties. When client systems are shipped with these trusted
third-party public keys, clients can query the records and obtain the claims to verify thatthe server certificate being presented in the TLS handshake is legitimate (i.e., has beensigned by a trusted third party). The concept is further described in [SOVER]. While theconcept is still in the development stage, its use can obviate the need for public key
certification path development, validation and revocation checking, and replace the server
authentication requirements listed in Section 4.5.

D.1.2

Certificate Transparency

Google’s Certificate Transparency project [RFC6962] strives to reduce the impact ofcertificate-based threats by making the issuance of CA-signed certificates moretransparent. This is done through the use of public logs of certificates, public log
monitoring, and public certificate auditing. Certificate logs are cryptographically assured
records of certificates that are open to public scrutiny. Certificates may be appended tologs, but they cannot be removed, modified, or inserted into the middle of a log. Monitors
watch certificate logs for suspicious certificates, such as those that were not authorized by
the domain they claim to represent. Auditors have the ability to check the membership ofa particular certificate in a log, as well as verify the integrity and consistency of logs.
52

Guidelines for TLS Implementations

D.1.3

Perspectives and Convergence

Perspectives is a project undertaken at Carnegie Mellon University [PERSP].
Perspectives takes a different approach to establish trust in a TLS server public key
certificate than using trust in certification authorities and the public key certificate trustmodel in X.509 and [RFC5280]. Perspectives has a decentralized model that uses
“network notary servers.” A network notary server is connected to the Internet andregularly monitors websites to build a history of the TLS certificate used by each site.
Rather than validating a TLS server certificate as described in [RFC5280] and in Section
4.5, with Perspectives, the TLS client validates a certificate by checking for consistency
with the certificates observed by the network notaries over time. A client has the network
notaries’ public keys embedded in it and decides which and how many notary servers totrust. Clients can also decide how many notaries must provide a positive response before
trusting a TLS server public key certificate and can augment the decision with trust
history and user input. [PERSP] further describes Perspectives. The decentralized model
used by Perspectives provides a high degree of reliability and availability, while
protecting against single or even a few compromised “network notaries”.
Implementations of Perspectives are available at [Perspectives].
Convergence [Convergence] is another effort to implement concepts from the
Perspectives project, as well as to augment those ideas to form a comprehensive solution.
In particular, it addresses the problems of completeness, privacy, and responsiveness thatexisted in the original Perspectives work. Convergence notaries can also employ
additional methods beyond network perspectives to decide whether a certificate should betrusted.
The Perspectives/Convergence approach can be used to establish confidence in a selfsigned TLS server certificate, and in doing so, reduce the amount of certificate warnings
that are presented to users.

D.1.4

DANE

Standards and products are still emerging in the area of DNS-based Authentication ofNamed Entities (DANE), and some of the standards are informational [RFC6394].
However, one of the following mechanisms can aid in the security of TLS server
authentication and protect the clients from accepting unauthorized certificates issued dueto the errors or compromise in CA or registration system and processes:
1. In addition to the server public key certificate validation as specified in Section
4.5, the client verifies that the TLS server certificate matches the one provided in
the DNS records. Digital signatures on the DNS records are verified inaccordance with the DNS Security Extensions (DNSSEC), as described in[RFC4033].
2. The client forgoes server public key certificate validation as specified in Section
4.5. Instead, the client verifies that the TLS server certificate matches the one
provided in the DNS Records. Digital signatures on the DNS records are verified
in accordance with the DNS Security Extensions (DNSSEC), as described in[RFC4033].
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Guidelines for TLS Implementations

3. In addition to the server public key certificate validation, as specified in Section
4.5, the client verifies that the CA certificate in the certificate list provided by theserver during a handshake matches the certificate provided in the DNS records
and is part of the certification path verified as specified in Section 4.5. Digital
signatures on the DNS Records are verified in accordance with the DNS Security
Extensions (DNSSEC), as described in [RFC4033].
4. The client verifies that the TLS server certificate can be validated by the trust
anchor provided in the DNS records. Digital signatures on the DNS records areverified in accordance with the DNS Security Extensions (DNSSEC), as
described in [RFC4033].

D.2 Checking Server/Client Key Size
If the clients or servers wish to require certain key sizes or algorithms, they can
implement cryptographic algorithm policy using the concept defined in [RFC5698]. The
specification and processing of cryptographic algorithms policy as described in[RFC5698] can ensure that, regardless of the cipher suite specification in the TLS
handshake, unacceptable algorithms and key sizes are not accepted by the entity (client or
the server) who implements the cryptographic algorithms policy.

D.3 Encrypt-then-MAC Extension
The TLS working group is working towards the addition of an Encrypt-then-MAC
construction to TLS, as an extension [ETM]. This is a departure from the MAC-thenEncrypt construction specified in [RFC2246], [RFC4346], and [RFC5246]. If the
Encrypt-then-MAC extension is standardized, it will mitigate or prevent several known
attacks on CBC cipher suites.

54

Guidelines for TLS Implementations

Appendix E

References

The following list of documents, publications, and organizations provide a wide variety
of information on varying aspects of Transport Layer Security.
[Adams99] Adams, C. and Lloyd, S., Understanding PKI: Concepts, Standard, andDeployment Considerations, (Macmillan Technology Publishing, Indianapolis, IN,
ISBN 1-57870-166-X, 1999).
[CABBASE] Baseline Requirements for the Issuance and Management of PubliclyTrusted Certificates, CA Browser Forum, Version 1.1.6, 29 July 2013.
https://cabforum.org/wp-content/uploads/Baseline_Requirements_V1_1_6.pdf[Comer00] Comer, D. E., Internetworking with TCP/IP, Principles, Protocols, andArchitectures, Fourth Edition, (Prentice Hall, Upper Saddle River, NJ 07458, ISBN:
0-13- 018380-6, 2000).
[COMMON] X.509 Certificate Policy for the U.S. Federal PKI Common Policy
Framework, Version 1.21, 18 December 2012.
http://idmanagement.gov/documents/common-policy-framework-certificate-policy[Convergence] Thoughtcrime Labs, Convergence, http://convergence.io/[ETM] Gutmann, P., Encrypt-then-MAC for TLS and DTLS, Internet Engineering Task
Force, December 2013, http://tools.ietf.org/html/draft-gutmann-tls-encrypt-thenmac-05[EVGUIDE] Guidelines For The Issuance and Management of Extended Validation
Certificates, CA Browser Forum, Version 1.4.3, 9 July 2013.
https://cabforum.org/wp-content/uploads/Guidelines_v1_4_3.pdf[FBCACP] X.509 Certificate Policy for the Federal Bridge Certification Authority,
Version 2.26, 26 April 2012.
http://www.idmanagement.gov/sites/default/files/documents/FBCA%20Certificate%20Policy%20v2.26_s.pdf[FIPS140-2] FIPS 140-2, Security Requirements For Cryptographic Modules,
http://csrc.nist.gov/publications/fips/fips140-2/fips1402.pdf[FIPS140Impl] National Institute of Standards and Technology, Implementation
Guidance for FIPS PUB 140-2 and the Cryptographic Module Validation Program,
25 July 2013, http://csrc.nist.gov/groups/STM/cmvp/documents/fips1402/FIPS1402IG.pdf[FIPS180-4] National Institute of Standards and Technology, Secure Hash Standard,Federal Information Processing Standards Publication 180-4, March 2012,
http://csrc.nist.gov/publications/fips/fips180-4/fips-180-4.pdf[FIPS186-4] National Institute of Standards and Technology, Digital Signature Standard,Federal Information Processing Standard 186-4, July 2013,
http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-4.pdf

55

Guidelines for TLS Implementations[FIPS197] National Institute of Standards and Technology, Advanced Encryption
Standard (AES), Federal Information Processing Standard 197, November 26, 2001
http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf[FIPS198-1] National Institute of Standards and Technology, The Keyed-Hash Message
Authentication Code (HMAC), Federal Information Processing Standard 198-1, July
2008, http://csrc.nist.gov/publications/fips/fips198-1/FIPS-198-1_final.pdf[FIPS201-1] National Institute of Standards and Technology, Personal Identification
Verification (PIV) of Federal Employees and Contractors, Federal Information
Processing Standard 201-1, March 2006,
http://csrc.nist.gov/publications/fips/fips201-1/FIPS-201-1-chng1.pdf[Hall00] Hall, E. A., Internet Core Protocols, The Definitive Guide, (O'Reilly &Associates, ISBN: 1-56592-572-6, February 2000).
[Housley01] Housley, R. and Polk, T., Planning for PKI, Best Practices Guide for
Deploying Public Key Infrastructure, (John Wiley & Sons, New York, NY, ISBN
0-471-39702-4, 2001).
[Lucky13] AlFardan, N. J., and Paterson, K. G., Lucky Thirteen: Breaking the TLS and
DTLS Record Protocols, IEEE Symposium on Security and Privacy 2013, pages
526-540, full version at http://www.isg.rhul.ac.uk/tls/TLStiming.pdf[Paterson11] Paterson, K. G., Ristenpart, T., and Shrimpton, T., Tag Size Does Matter:
Attacks and Proofs for the TLS Record Protocol, in ASIACRYPT 2011, (Springer
Lecture Notes in Computer Science, volume 7073, ISBN 978-3-642-25384-3).
[PERSP] Wendlandt D., Andersen D.G. and Perrig A., Perspectives: Improving SSHstyle Host Authentication with Multi-Path Probing, 2011 USENIX.
http://perspectivessecurity.files.wordpress.com/2011/07/perspectives_usenix08.pdf.
[Perspectives] Perspectives Project, http://perspectives-project.org/[Polk03] Polk, W., Hastings, N., and Malani, A., Public Key Infrastructures that Satisfy
Security Goals, IEEE Internet Computing, Volume 7, Number 4, July-August,
2003.
[Rescorla01] Rescorla, E., SSL and TLS – Designing and Building Secure Systems,(Addison- Wesley, Upper Saddle River NJ, 07458, ISBN 0-201-61598, March
2001).
[RFC2119] Bradner, S., Key words for use in RFCs to Indicate Requirement Levels,Internet Engineering Task Force, Request for Comments 2119, March 1997,
http://www.ietf.org/rfc/rfc2119.txt[RFC2246] Dierks, T. and Allen, C., The TLS Protocol Version 1.0, Internet Engineering
Task Force, Request for Comments 2246, January 1999,
http://www.ietf.org/rfc/rfc2246.txt[RFC3279] Polk, W., et al., Algorithms and Identifiers for the Internet X.509 Public Key
Infrastructure Certificate Revocation List (CRL) Profile, Internet Engineering Task
Force, Request for Comments 3279, April 2002, http://www.ietf.org/rfc/rfc3279.txt
56

Guidelines for TLS Implementations[RFC3447] Jonsson, J., and Kaliski, B., Public-Key Cryptography Standards (PKCS) #1:
RSA Cryptography Specifications Version 2.1, Request for Comments 3447,
February 2003, http://www.ietf.org/rfc/rfc3447.txt[RFC3713] Matsui, M., et al. A Description of the Camellia Encryption Algorithm,
Internet Engineering Task Force, Request for Comments 3713, April 2004,
http://www.ietf.org/rfc/rfc3713.txt[RFC3749] Hollenbeck, S., Transport Layer Security Protocol Compression Methods,
Internet Engineering Task Force, Request for Comments 3749, May 2004,
http://www.ietf.org/rfc/rfc3749.txt[RFC3943] Friend, R., Transport Layer Security (TLS) Protocol Compression Using
Lempel-Ziv-Stac (LZS), Internet Engineering Task Force, Request for Comments
3943, November 2004, http://www.ietf.org/rfc/rfc3943.txt[RFC4033] Arends, R. et al., DNS Security Introduction and Requirements, Internet
Engineering Task Force, Request for Comments 4033, March 2005,
http://www.ietf.org/rfc/rfc4033.txt[RFC4055] Shaad, J. et al., Additional Algorithms and Identifiers for RSA Cryptography
for use in the Internet X.509 Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile, Internet Engineering Task Force, Request forComments 4055, June 2005, http://www.ietf.org/rfc/rfc4055.txt[RFC4279] Eronen, P. and Tschofenig, H. Pre-Shared Key Ciphersuites for Transport
Layer Security (TLS), Internet Engineering Task Force, Request for Comments
4279, December 2005, http://www.ietf.org/rfc/rfc4279.txt[RFC4346] Dierks, T. and Rescorla, E., The Transport Layer Security (TLS) Protocol
Version 1.1, Internet Engineering Task Force, Request for Comments 4346, April
2006, http://www.ietf.org/rfc/rfc4346.txt[RFC4492] Blake-Wilson, S., et al., Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS), Internet Engineering Task Force, Request forComments 4492, May 2006, http://www.ietf.org/rfc/rfc4492.txt[RFC5246] Dierks, T. and Rescorla, E., The Transport Layer Security (TLS) Protocol
Version 1.2, Internet Engineering Task Force, Request for Comments 5246, August
2008, http://www.ietf.org/rfc/rfc5246.txt[RFC5280] Cooper, D., et al., Internet X.509 Public Key Infrastructure Certificate andCertificate Revocation List (CRL) Profile, Internet Engineering Task Force,
Request for Comments 5280, May 2008, http://www.ietf.org/rfc/rfc5280.txt[RFC5288] Salowey, J., Choudhury, A., and McGrew, D., AES Galois Counter Mode
(GCM) Cipher Suites for TLS, Internet Engineering Task Force, Request forComments 5288, August 2008, http://www.ietf.org/rfc/rfc5288.txt[RFC5289] Rescorla, E., TLS Elliptic Curve Cipher Suites with SHA-256/384 and AES
Galois Counter Mode (GCM), Internet Engineering Task Force, Request forComments 5289, August 2008, http://www.ietf.org/rfc/rfc5289.txt

57

Guidelines for TLS Implementations[RFC5487] Badra, M., Pre-Shared Key Cipher Suites for TLS with SHA-256/384 and
AES Galois Counter Mode, Internet Engineering Task Force, Request forComments 5487, March 2009, http://www.ietf.org/rfc/rfc5487.txt[RFC5489] Badra, M. and Hajjeh, I., ECDHE_PSK Cipher Suites for Transport Layer
Security (TLS), Internet Engineering Task Force, Request for Comments 5489,
March 2009, http://www.ietf.org/rfc/rfc5489.txt[RFC5698] Kunz, T., Okunick, S., and Pordesch U., Data Structure for the Security
Suitability of Cryptographic Algorithms (DSSC), Internet Engineering Task Force,
Request for Comments 5698, November 2009, http://www.ietf.org/rfc/rfc5698.txt[RFC5746] Rescorla E. et al., Transport Layer Security (TLS) Renegotiation Indication
Extension, Internet Engineering Task Force, Request for Comments 5746, February
2010, http://www.ietf.org/rfc/rfc5746.txt[RFC5758] Dang, Q., et al., Internet X.509 Public Key Infrastructure: Additional
Algorithms and Identifiers for DSA and ECDSA, Internet Engineering Task Force,
Request for Comments 5758, January 2010, http://www.ietf.org/rfc/rfc5758.txt[RFC6066] Eastlake, D., Transport Layer Security (TLS) Extensions: Extension
Definitions, Internet Engineering Task Force, Request for Comments 6066, January
2011, http://www.ietf.org/rfc/rfc6066.txt[RFC6101] Freier, A. e al., The Secure Sockets Layer (SSL) Protocol Version 3.0,
Internet Engineering Task Force, Request for Comments 6101, August 2011,
http://www.ietf.org/rfc/rfc6101.txt[RFC6394] Barnes, R., Use Cases and Requirements for DNS-Based Authentication ofNamed Entities (DANE), Internet Engineering Task Force, Request for Comments
6394, October 2011, http://www.ietf.org/rfc/rfc6394.txt[RFC6460] Salter, M. and Housley, R., Suite B Profile for Transport Layer Security(TLS), Internet Engineering Task Force, Request for Comments 6460, January
2012, http://www.ietf.org/rfc/rfc6460.txt[RFC6655] McGrew, D. and Bailey, D., AES-CCM Cipher Suites for Transport Layer
Security (TLS), Internet Engineering Task Force, Request for Comments 6655, July
2012, http://www.ietf.org/rfc/rfc6655.txt[RFC6818] Yee, P., Updates to the Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile, Internet Engineering Task Force,
Request for Comments 6818, January 2013, http://www.ietf.org/rfc/rfc6818.txt[RFC6960] Santesson, S., et al., X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP, Internet Engineering Task Force, Request forComments 6960, June 2013, http://www.ietf.org/rfc/rfc6960.txt[RFC6961] Pettersen, Y., The Transport Layer Security (TLS) Multiple Certificate Status
Request Extension, Internet Engineering Task Force, Request for Comments 6961,
June 2013, http://www.ietf.org/rfc/rfc6961.txt[RFC6962] Laurie, B., et al., Certificate Transparency, Internet Engineering Task Force,
Request for Comments 6962, June 2013, http://www.ietf.org/rfc/rfc6962.txt
58

Guidelines for TLS Implementations[SOVER] Sovereign Key Cryptography for Internet Domains, Electronic Frontier
Foundation, https://git.eff.org/?p=sovereign-keys.git;a=blob_plain;f=sovereign-keydesign.txt;hb=master[SP800-32] NIST Special Publication 800-32, Introduction to Public Key Technology
and the Federal PKI Infrastructure, February 2001,
http://csrc.nist.gov/publications/nistpubs/800-32/sp800-32.pdf[SP800-53] NIST Special Publication 800-53, Security and Privacy Controls for Federal
Information Systems and Organizations, April 2013,
http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-53r4.pdf[SP800-56A] NIST Special Publication 800-56A, Recommendation for Pair-Wise Key
Establishment Schemes Using Discrete Logarithm Cryptography, May 2013,
http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-56Ar2.pdf[SP800-56B] NIST Special Publication 800-56B, Recommendation for Pair-Wise Key
Establishment Schemes Using Integer Factorization Cryptography, August 2009,http://csrc.nist.gov/publications/nistpubs/800-56B/sp800-56B.pdf[SP800-57p1] NIST Special Publication 800-57 Part 1, Recommendation for Key
Management – Part 1: General (Revision 3), July 2012,
http://csrc.nist.gov/publications/nistpubs/800-57/sp800-57_part1_rev3_general.pdf[SP800-63] NIST Special Publication 800-63-2, Electronic Authentication Guide, August
2013, http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-63-2.pdf[SP800-67] NIST Special Publication 800-67 Revision 1, Recommendation for the Triple
Data Encryption Algorithm (TDEA) Block Cipher, January 2012,
http://csrc.nist.gov/publications/nistpubs/800-67-Rev1/SP-800-67-Rev1.pdf[SP800-90A] NIST Special Publication 800-90A Revision 1, Recommendation for
Random Number Generation Using Deterministic Random Bit Generators, January
2012, http://csrc.nist.gov/publications/nistpubs/800-90A/SP800-90A.pdf[SP800-107] NIST Special Publication 800-107 Revision 1, Recommendation forApplications Using Approved Hash Algorithms, August 2012,
http://csrc.nist.gov/publications/nistpubs/800-107-rev1/sp800-107-rev1.pdf[SP800-131A] NIST Special Publication 800-131A, Transitions: Recommendation forTransitioning the Use of Cryptographic Algorithms and Key Lengths, January 2011,
http://csrc.nist.gov/publications/nistpubs/800-131A/sp800-131A.pdf[SP800-135] NIST Special Publication 800-135 Revision 1, Recommendation for
Existing Application-Specific Key Derivation Functions, December 2011,
http://csrc.nist.gov/publications/nistpubs/800-135-rev1/sp800-135-rev1.pdf[Schneier96] Schneier, B., Applied Cryptography: Protocols, Algorithms, and Source
Code in C, 2nd ed., (John Wiley & Sons, Inc. 1996).

59