Suite B Profile for Transport Layer Security (TLS)
RFC 5430

Network Working Group                                          M. Salter
Request for Comments: 5430                      National Security Agency
Category: Informational                                      E. Rescorla
                                                       Network Resonance
                                                              R. Housley
                                                          Vigil Security
                                                              March 2009

           Suite B Profile for Transport Layer Security (TLS)

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

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RFC 5430                    Suite B for TLS                   March 2009

Abstract

   The United States government has published guidelines for "NSA Suite
   B Cryptography", which defines cryptographic algorithm policy for
   national security applications.  This document defines a profile of
   Transport Layer Security (TLS) version 1.2 that is fully conformant
   with Suite B.  This document also defines a transitional profile for
   use with TLS version 1.0 and TLS version 1.1 which employs Suite B
   algorithms to the greatest extent possible.

Table of Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................3
   3. Suite B Requirements ............................................3
   4. Suite B Compliance and Interoperability Requirements ............4
      4.1. Security Levels ............................................7
      4.2. Acceptable Curves ..........................................8
      4.3. Certificates ...............................................8
      4.4. signature_algorithms Extension .............................9
      4.5. CertificateRequest Message .................................9
      4.6. CertificateVerify Message .................................10
      4.7. ServerKeyExchange Message Signature .......................10
   5. Security Considerations ........................................10
   6. Acknowledgements ...............................................10
   7. References .....................................................11
      7.1. Normative References ......................................11
      7.2. Informative References ....................................11

1.  Introduction

   The United States government has posted the Fact Sheet on National
   Security Agency (NSA) Suite B Cryptography [NSA], and at the time of
   writing, it states:

       To complement the existing policy for the use of the Advanced
       Encryption Standard (AES) to protect national security systems
       and information as specified in The National Policy on the use of
       the Advanced Encryption Standard (AES) to Protect National
       Security Systems and National Security Information (CNSSP-15),
       the National Security Agency (NSA) announced Suite B Cryptography
       at the 2005 RSA Conference.  In addition to the AES, Suite B
       includes cryptographic algorithms for hashing, digital
       signatures, and key exchange.

       Suite B only specifies the cryptographic algorithms to be
       used. Many other factors need to be addressed in determining
       whether a particular device implementing a particular set of

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RFC 5430                    Suite B for TLS                   March 2009

       cryptographic algorithms should be used to satisfy a particular
       requirement.

   Among those factors are "requirements for interoperability both
   domestically and internationally".

   This document does not define any new cipher suites; instead, it
   defines two profiles:

   o  A Suite B compliant profile for use with TLS version 1.2 [RFC5246]
      and the cipher suites defined in [RFC5289].  This profile uses
      only Suite B algorithms.

   o  A transitional profile for use with TLS version 1.0 [RFC2246] or
      TLS version 1.1 [RFC4346] and the cipher suites defined in
      [RFC4492].  This profile uses the Suite B cryptographic algorithms
      to the greatest extent possible and provides backward
      compatibility.  While the transitional profile is not Suite B
      compliant, it provides a transition path towards the Suite B
      compliant profile.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Suite B Requirements

   The Fact Sheet on Suite B Cryptography requires that key
   establishment and authentication algorithms be based on Elliptic
   Curve Cryptography, and that the encryption algorithm be AES [AES].
   Suite B defines two security levels, of 128 and 192 bits.

   In particular, Suite B includes:

       Encryption:         Advanced Encryption Standard (AES) [AES] --
                           FIPS 197 (with key sizes of 128 and 256 bits)

       Digital Signature:  Elliptic Curve Digital Signature Algorithm
                           (ECDSA) [DSS] - FIPS 186-2 (using the
                           curves with 256- and 384-bit prime moduli)

       Key Exchange:       Elliptic Curve Diffie-Hellman (ECDH) - NIST
                           Special Publication 800-56A [PWKE] (using the
                           curves with 256- and 384-bit prime moduli)

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   The 128-bit security level corresponds to an elliptic curve size of
   256 bits and AES-128; it also makes use of SHA-256 [SHS].  The 192-
   bit security level corresponds to an elliptic curve size of 384 bits
   and AES-256; it also makes use of SHA-384 [SHS].

   Note: Some people refer to the two security levels based on the AES
   key size that is employed instead of the overall security provided by
   the combination of Suite B algorithms.  At the 128-bit security
   level, an AES key size of 128 bits is used, which does not lead to
   any confusion.  However, at the 192-bit security level, an AES key
   size of 256 bits is used, which sometimes leads to an expectation of
   more security than is offered by the combination of Suite B
   algorithms.

   To accommodate backward compatibility, a Suite B compliant client or
   server can be configured to accept a cipher suite that is not part of
   Suite B. However, whenever a Suite B compliant client and a Suite B
   compliant server establish a TLS version 1.2 session, only Suite B
   algorithms are employed.

4.  Suite B Compliance and Interoperability Requirements

   TLS version 1.1 [RFC4346] and earlier do not support Galois Counter
   Mode (GCM) cipher suites [RFC5289].  However, TLS version 1.2
   [RFC5246] and later do support GCM.  For Suite B TLS compliance, GCM
   cipher suites are REQUIRED to be used whenever both the client and
   the server support the necessary cipher suites.  Also, for Suite B
   TLS compliance, Cipher Block Chaining (CBC) cipher suites are
   employed when GCM cipher suites cannot be employed.

   For a client to implement the Suite B compliant profile, it MUST
   implement TLS version 1.2 or later, and the following cipher suite
   rules apply:

   o  A Suite B compliant TLS version 1.2 or later client MUST offer at
      least two cipher suites for each supported security level.  For
      the 128-bit security level,
      TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and
      TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 MUST be offered in this
      order in the ClientHello message.  For the 192-bit security level,
      TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 and
      TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 MUST be offered in this
      order in the ClientHello message.  One of these cipher suites MUST
      be the first (most preferred) cipher suite in the ClientHello
      message.

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   o  A Suite B compliant TLS version 1.2 or later client that offers
      backward compatibility with TLS version 1.1 or earlier servers MAY
      offer an additional cipher suite for each supported security
      level.  If these cipher suites are offered, they MUST appear after
      the ones discussed above.  For the 128-bit security level,
      TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA MAY be offered in the
      ClientHello message.  For the 192-bit security level,
      TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA MAY be offered in the
      ClientHello message.

   o  A Suite B compliant TLS version 1.2 or later client that offers
      interoperability with non-Suite B compliant servers MAY offer
      additional cipher suites.  If any additional cipher suites are
      offered, they MUST appear after the ones discussed above in the
      ClientHello message.

   For a client to implement the Suite B transitional profile, it MUST
   implement TLS version 1.1 or earlier and the following cipher suite
   rules apply:

   o  A Suite B transitional TLS version 1.1 or earlier client MUST
      offer the cipher suite for the 128-bit security level, the cipher
      suite for the 192-bit security level, or both.  For the 128-bit
      security level, TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA MUST be
      offered in the ClientHello message.  For the 192-bit security
      level, TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA MUST be offered in the
      ClientHello message.  One of these cipher suites MUST be the first
      (most preferred) cipher suite in the ClientHello message.

   o  A Suite B transitional TLS version 1.1 or earlier client that
      offers interoperability with non-Suite B compliant servers MAY
      offer additional cipher suites.  If any additional cipher suites
      are offered, they MUST appear after the ones discussed above in
      the ClientHello message.

   A Suite B compliant TLS server MUST be configured to support the 128-
   bit security level, the 192-bit security level, or both security
   levels.  The cipher suite rules for each of these security levels is
   described below.  If a Suite B compliant TLS server is configured to
   support both security levels, then the configuration MUST prefer one
   security level over the other.  In practice, this means that the
   cipher suite rules associated with the cipher suites listed in
   Section 4.1 for the preferred security level are processed before the
   cipher suite rules for the less preferred security level.

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   For a server to implement the Suite B conformant profile at the 128-
   bit security level, the following cipher suite rules apply:

   o  A Suite B compliant TLS version 1.2 or later server MUST accept
      the TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 cipher suite if it is
      offered.

   o  If the preceding cipher suite is not offered, then a Suite B
      compliant TLS version 1.2 or later server MUST accept the
      TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 cipher suite if it is
      offered.

   o  If neither of the preceding two cipher suites is offered, then a
      Suite B compliant TLS version 1.2 or later server that offers
      backward compatibility with TLS version 1.1 or earlier clients MAY
      accept the TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA cipher suite if it
      is offered.

   o  If the server is not offered any of the preceding three cipher
      suites and interoperability with clients that are not compliant or
      interoperable with Suite B is desired, then the server MAY accept
      another offered cipher suite that is considered acceptable by the
      server administrator.

   For a server to implement the Suite B transitional profile at the
   128-bit security level, the following cipher suite rules apply:

   o  A Suite B transitional TLS version 1.1 or earlier server MUST
      accept the TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA cipher suite if it
      is offered.

   o  If the server is not offered the preceding cipher suite and
      interoperability with clients that are not Suite B transitional is
      desired, then the server MAY accept another offered cipher suite
      that is considered acceptable by the server administrator.

   For a server to implement the Suite B conformant profile at the 192-
   bit security level, the following cipher suite rules apply:

   o  A Suite B compliant TLS version 1.2 or later server MUST accept
      the TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suite if it is
      offered.

   o  If the preceding cipher suite is not offered, then a Suite B
      compliant TLS version 1.2 or later server MUST accept the
      TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 cipher suite if it is
      offered.

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   o  If neither of the preceding two cipher suites is offered, then a
      Suite B compliant TLS version 1.2 or later server that offers
      backward compatibility with TLS version 1.1 or earlier clients MAY
      accept the TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA cipher suite if it
      is offered.

   o  If the server is not offered any of the preceding three cipher
      suites and interoperability with clients that are not compliant or
      interoperable with Suite B is desired, then the server MAY accept
      another offered cipher suite that is considered acceptable by the
      server administrator.

   For a server to implement the Suite B transitional profile at the
   192-bit security level, the following cipher suite rules apply:

   o  A Suite B transitional TLS version 1.1 or earlier server MUST
      accept the TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA cipher suite if it
      is offered.

   o  If the server is not offered the preceding cipher suite and
      interoperability with clients that are not Suite B transitional is
      desired, then the server MAY accept another offered cipher suite
      that is considered acceptable by the server administrator.

   Note that these rules explicitly permit the use of CBC cipher suites
   in TLS version 1.2 connections in order to permit operation between
   Suite B compliant and non-Suite B compliant implementations.  For
   instance, a Suite B compliant TLS version 1.2 client might offer TLS
   version 1.2 with both GCM and CBC cipher suites when communicating
   with a non-Suite B TLS version 1.2 server, which then selected the
   CBC cipher suites.  This connection would nevertheless meet the
   requirements of this specification.  However, any two Suite B
   compliant implementations will negotiate a GCM cipher suite when
   doing TLS version 1.2.

4.1.  Security Levels

   As described in Section 1, Suite B specifies two security levels:
   128-bit and 192-bit.  The following table lists the cipher suites for
   each security level.  Within each security level, the cipher suites
   are listed in their preferred order for selection by a TLS version
   1.2 implementation.

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       +-----------------------------------------+----------------+
       | Cipher Suite                            | Security Level |
       +-----------------------------------------+----------------+
       | TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 | 128            |
       | TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 | 128            |
       | TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA    | 128            |
       | TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 | 192            |
       | TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 | 192            |
       | TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA    | 192            |
       +-----------------------------------------+----------------+

4.2.  Acceptable Curves

   RFC 4492 defines a variety of elliptic curves.  For cipher suites
   defined in this specification, only secp256r1(23) or secp384r1(24)
   may be used.  These are the same curves that appear in FIPS 186-2
   [DSS] as P-256 and P-384, respectively.  For cipher suites at the
   128-bit security level, secp256r1 MUST be used.  For cipher suites at
   the 192-bit security level, secp384r1 MUST be used.  RFC 4492
   requires that the uncompressed(0) form be supported.  The
   ansiX962_compressed_prime(1) point formats MAY also be supported.

   Clients desiring to negotiate only a Suite B compliant connection
   MUST generate a "Supported Elliptic Curves Extension" containing only
   the allowed curves.  These curves MUST match the cipher suite
   security levels being offered.  Clients that are willing to do both
   Suite B compliant and non-Suite B compliant connections MAY omit the
   extension or send the extension but offer other curves as well as the
   appropriate Suite B ones.

   Servers desiring to negotiate a Suite B compliant connection SHOULD
   check for the presence of the extension, but MUST NOT negotiate
   inappropriate curves even if they are offered by the client.  This
   allows a client that is willing to do either Suite B compliant or
   non-Suite B compliant modes to interoperate with a server that will
   only do Suite B compliant modes.  If the client does not advertise an
   acceptable curve, the server MUST generate a fatal
   "handshake_failure" alert and terminate the connection.  Clients MUST
   check the chosen curve to make sure it is acceptable.

4.3.  Certificates

   Server and client certificates used to establish a Suite B compliant
   connection MUST be signed with ECDSA.  Digital signatures MUST be
   calculated using either the P-256 curve along with the SHA-256 hash
   algorithm or calculated using the P-384 curve along with the SHA-384
   hash algorithm.  For certificates used at the 128-bit security level,
   the subject public key MUST use the P-256 curve and be signed with

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   either the P-384 curve or the P-256 curve.  For certificates used at
   the 192-bit security level, the subject public key MUST use the P-384
   curve and be signed with the P-384 curve.

   In TLS version 1.0 and TLS version 1.1, the key exchange algorithm
   used in the TLS_ECDHE_ECDSA-collection of cipher suites requires the
   server's certificate to be signed with a particular signature scheme.
   TLS version 1.2 offers more flexibility.  This specification does not
   impose any additional restrictions on the server certificate
   signature or the signature schemes used elsewhere in the
   certification path.  (Often such restrictions will be useful, and it
   is expected that this will be taken into account in practices of
   certification authorities.  However, such restrictions are not
   strictly required, even if it is beyond the capabilities of a client
   to completely validate a given certification path, the client may be
   able to validate the server's certificate by relying on a trusted
   certification authority whose certificate appears as one of the
   intermediate certificates in the certification path.)

   Likewise, this specification does not impose restrictions on
   signature schemes used in the certification path for the client's
   certificate when mutual authentication is employed.

4.4.  signature_algorithms Extension

   The signature_algorithms extension is defined in Section 7.4.1.4.1 of
   TLS version 1.2 [RFC5246].  A Suite B compliant TLS version 1.2 or
   later client MUST include the signature_algorithms extension.  For
   the 128-bit security level, SHA-256 with ECDSA MUST be offered in the
   signature_algorithms extension.  For the 192-bit security level, SHA-
   384 with ECDSA MUST be offered in the signature_algorithms extension.
   Other offerings MAY be included to indicate the signature algorithms
   that are acceptable in cipher suites that are offered for
   interoperability with servers that are not compliant with Suite B and
   to indicate the signature algorithms that are acceptable for
   certification path validation.

4.5.  CertificateRequest Message

   A Suite B compliant TLS version 1.2 or later server MUST include SHA-
   256 with ECDSA and/or SHA-384 with ECDSA in the
   supported_signature_algorithms field of the CertificateRequest
   message.  For the 128-bit security level, SHA-256 with ECDSA MUST
   appear in the supported_signature_algorithms field.  For the 192-bit
   security level, SHA-384 with ECDSA MUST appear in the
   supported_signature_algorithms field.

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4.6.  CertificateVerify Message

   A Suite B compliant TLS version 1.2 or later client MUST use SHA-256
   with ECDSA or SHA-384 with ECDSA for the signature in the
   CertificateVerify message.  For the 128-bit security level, SHA-256
   with ECDSA MUST be used.  For the 192-bit security level, SHA-384
   with ECDSA MUST be used.

4.7.  ServerKeyExchange Message Signature

   In the TLS_ECDHE_ECDSA-collection of cipher suites, the server sends
   its ephemeral ECDH public key and a specification of the
   corresponding curve in the ServerKeyExchange message.  These
   parameters MUST be signed with ECDSA using the private key
   corresponding to the public key in the server's certificate.

   A TLS version 1.1 or earlier server MUST sign the ServerKeyExchange
   message using SHA-1 with ECDSA.

   A Suite B compliant TLS version 1.2 or later server MUST sign the
   ServerKeyExchange message using either SHA-256 with ECDSA or SHA-384
   with ECDSA.  For the 128-bit security level, SHA-256 with ECDSA MUST
   be used.  For the 192-bit security level, SHA-384 with ECDSA MUST be
   used.

5.  Security Considerations

   Most of the security considerations for this document are described
   in "The Transport Layer Security (TLS) Protocol Version 1.2"
   [RFC5246], "Elliptic Curve Cryptography (ECC) Cipher Suites for
   Transport Layer Security (TLS)" [RFC4492], "AES Galois Counter Mode
   (GCM) Cipher Suites for TLS" [RFC5288], and "TLS Elliptic Curve
   Cipher Suites with SHA-256/384 and AES Galois Counter Mode (GCM)"
   [RFC5289].  Readers should consult those documents.

   In order to meet the goal of a consistent security level for the
   entire cipher suite, in Suite B mode TLS implementations MUST ONLY
   use the curves defined in Section 4.2.  Otherwise, it is possible to
   have a set of symmetric algorithms with much weaker or stronger
   security properties than the asymmetric (ECC) algorithms.

6.  Acknowledgements

   Thanks to Pasi Eronen, Steve Hanna, and Paul Hoffman for their
   review, comments, and insightful suggestions.

   This work was supported by the US Department of Defense.

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7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
              Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
              for Transport Layer Security (TLS)", RFC 4492, May 2006.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5289]  Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-
              256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
              August 2008.

   [AES]      National Institute of Standards and Technology,
              "Specification for the Advanced Encryption Standard
              (AES)", FIPS 197, November 2001.

   [DSS]      National Institute of Standards and Technology, "Digital
              Signature Standard", FIPS 186-2, January 2000.

   [PWKE]     National Institute of Standards and Technology,
              "Recommendation for Pair-Wise Key Establishment Schemes
              Using Discrete Logarithm Cryptography (Revised)", NIST
              Special Publication 800-56A, March 2007.

   [SHS]      National Institute of Standards and Technology, "Secure
              Hash Standard", FIPS 180-2, August 2002.

7.2.  Informative References

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC5288]  Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
              Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
              August 2008.

   [NSA]      National Security Agency, "Fact Sheet NSA Suite B
              Cryptography",
              <http://www.nsa.gov/ia/Industry/crypto_suite_b.cfm>.

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Authors' Addresses

   Margaret Salter
   National Security Agency
   9800 Savage Rd.
   Fort Meade  20755-6709
   USA

   EMail: msalter@restarea.ncsc.mil

   Eric Rescorla
   Network Resonance
   2064 Edgewood Drive
   Palo Alto  94303
   USA

   EMail: ekr@rtfm.com

   Russ Housley
   Vigil Security
   918 Spring Knoll Drive
   Herndon  21070
   USA

   EMail: housley@vigilsec.com

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