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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 RFC 5430

Network Working Group                                          M. Salter
Internet-Draft                                  National Security Agency
Intended status:  Informational                              E. Rescorla
Expires:  April 5, 2009                                Network Resonance
                                                              R. Housley
                                                          Vigil Security
                                                         October 2, 2008


                     Suite B Cipher Suites for TLS
                    draft-rescorla-tls-suiteb-07.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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   This Internet-Draft will expire on April 5, 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
   TLS which is conformant with Suite B.








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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used In This Document  . . . . . . . . . . . . . .  3
   3.  Suite B Requirements . . . . . . . . . . . . . . . . . . . . .  3
   4.  Suite B Compliance Requirements  . . . . . . . . . . . . . . .  4
     4.1.  Security Levels  . . . . . . . . . . . . . . . . . . . . .  6
     4.2.  Acceptable Curves  . . . . . . . . . . . . . . . . . . . .  7
     4.3.  Certificates . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  9
   Intellectual Property and Copyright Statements . . . . . . . . . . 11


































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1.  Introduction

   National Security Agency posted a Fact Sheet on Suite B Cryptography,
   and at the time of this 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
       cryptographic algorithms should be used to satisfy a particular
       requirement.

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

   This document is a profile of TLS 1.2 [RFC5246] and of the cipher
   suites defined in [RFC5289], but does not itself define any new
   cipher suites.


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, that the encryption algorithm be AES [AES], and
   that the function used for key derivation and data integrity be SHA
   [SHS].  It defines two security levels, of 128 and 192 bits.

   In particular, Suite B includes:






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       Encryption:         Advanced Encryption Standard (AES) [AES] -
                           FIPS 197 (with keys 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)

       Hashing:            Secure Hash Algorithm - FIPS 180-2 [SHS]
                           (using SHA-256 and SHA-384)

   The 128-bit security level corresponds to an elliptic curve size of
   256 bits, AES-128, and SHA-256.  The 192-bit security level
   corresponds to an elliptic curve size of 384 bits, AES-256, and SHA-
   384.

   Note:  Some people refer to the two security levels based on the AES
   key size that is employed.  At the 128-bit security level, an AES key
   length of 128 bits is used.  However, at the 192-bit security level,
   an AES key length of 256 bits is used.


4.  Suite B Compliance Requirements

   TLS version 1.1 [RFC4346] and earlier does not support Galois Counter
   Mode (GCM) cipher suites [RFC5289].  However, TLS version 1.2
   [RFC5246] and later does support GCM.  For Suite B TLS compliance,
   GCM cipher suites are REQUIRED to be used whenever possible.  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 be considered Suite B compliant, the following cipher
   suite rules apply:

   o  A Suite B compliant TLS version 1.1 or earlier client MUST offer
      one cipher suite for each supported security level.  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 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,



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      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.
   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 (of any version) 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.

   For a server that is configured to support the 128-bit security level
   to be considered Suite B compliant, the following cipher suite rules
   apply:

   o  A Suite B compliant TLS version 1.1 or earlier server MUST accept
      the TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA cipher suite if it is
      offered.  If the server is not offered this cipher suite and
      interoperability with clients that are not compliant with Suite B
      is desired, then the server MAY accept another offered cipher
      suite that is considered acceptable by the server administrator.
   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.  If this cipher suite is not offered, then the server
      MUST accept the TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 cipher
      suite if it is offered.  If neither of these cipher suites is
      offered, then the server MUST accept the
      TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA cipher suite if it is



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      offered.  If the server is not offered any of those three cipher
      suites and interoperability with clients that are not compliant
      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 that is configured to support the 192-bit security level
   to be considered Suite B compliant, the following cipher suite rules
   apply:

   o  A Suite B compliant TLS version 1.1 or earlier server MUST accept
      the TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA cipher suite if it is
      offered.  If the server is not offered this cipher suite and
      interoperability with clients that are not compliant with Suite B
      is desired, then the server MAY accept another offered cipher
      suite that is considered acceptable by the server administrator.
   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.  If this cipher suite is not offered, then the server
      MUST accept the TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 cipher
      suite if it is offered.  If neither of these cipher suites is
      offered, then the server MUST accept the
      TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA cipher suite if it is
      offered.  If the server is not offered any of those three cipher
      suites and interoperability with clients that are not compliant
      with Suite B 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 uncompressed(0) form be supported.
   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 which 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 which is willing to do either Suite B-compliant or
   non-Suite B-compliant modes to interoperate with a server which 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.  For certificates used at the
   128-bit security level, the subject public key MUST use the P-256
   curve, and the digital signature MUST be calculated using the P-256
   curve and the SHA-256 hash algorithm.  For certificates used at the
   192-bit security level, the subject public key MUST use the P-384



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   curve, and the digital signature MUST be calculated using the P-384
   curve and the SHA-384 hash algorithm.

   While the key exchange algorithm used in TLS_ECDHE_ECDSA-collection
   of cipher suites require the server's certificate to be signed with a
   particular signature scheme, this specification does not impose
   restrictions on 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.


5.  Security Considerations

   Most of the security considerations for this document are described
   in TLS 1.2 [RFC5246], Elliptic Curve Cryptography (ECC) Cipher Suites
   for TLS [RFC4492], AES-GCM Cipher Suites for TLS [RFC5288], and TLS
   ECC Cipher Suites with SHA-256/384 and AES-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.  IANA Considerations

   This document defines no actions for IANA.


7.  Acknowledgements

   This work was supported by the US Department of Defense.


8.  References




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8.1.  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.

8.2.  Informative References

   [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.












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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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