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Network Working Group                                          M. Salter
Internet-Draft                                  National Security Agency
Intended status:  Informational                              E. Rescorla
Expires:  March 9, 2009                                Network Resonance
                                                              R. Housley
                                                          Vigil Security
                                                       September 5, 2008


                     Suite B Cipher Suites for TLS
                    draft-rescorla-tls-suiteb-03.txt

Status of this Memo

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   This Internet-Draft will expire on March 9, 2009.

Abstract

   The United States Government has published guidelines for "NSA Suite
   B Cryptography," which defines cryptographic algorithm polcy 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 . . . . . . . . . . . . . . . . . . . . . . 5
     4.2.  Acceptable Curves . . . . . . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 7
   Intellectual Property and Copyright Statements  . . . . . . . . . . 9



































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1.  Introduction

   National Security Agency posted a Fact Sheet on Suite B Cryptography,
   and 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 of TLS 1.2 [I-D.ietf-tls-rfc4346-bis]
   and of the cipher suites defined in [I-D.ietf-tls-ecc-new-mac], 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)  -
                            FIPS 197 (with keys sizes of 128 and 256
                            bits)

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

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

        Hashing:            Secure Hash Algorithm - FIPS 180-2
                            (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

   Galois Counter Mode (GCM) cipher suites [I-D.ietf-tls-ecc-new-mac]
   are preferred for Suite B implementations.  However, TLS 1.1 (or
   earlier) does not support GCM.  Therefore, when TLS 1.1 (or earlier)
   implementations are used in Suite B mode, they support Cipher Block
   Chaining (CBC) mode.

   Therefore, in order for a connection to be Suite B compliant, the
   following rules apply:

   o  TLS 1.1 or earlier connections MUST use either
      TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA or
      TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA.  A compliant TLS 1.1 or
      earlier client MUST offer at least one of these cipher suites.  A
      compliant TLS 1.1 or earlier server MUST select one of them if
      offered.
   o  TLS 1.2 or greater connections connections MUST use one of
      TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256,
      TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384,
      TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256, or
      TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384.  A compliant TLS 1.2 or



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      greater client MUST offer one or both of the two GCM cipher
      suites.  If backward compatibility with TLS 1.1 or earlier is
      desired, it MAY offer one or both of the CBC cipher suites as
      well.  A compliant TLS 1.2 server MUST select one of these cipher
      suites if offered and MUST select a GCM cipher suite in TLS 1.2
      mode if one is offered.

   Note that these rules explicitly permit the use of CBC cipher suites
   in TLS 1.2 connections in order to permit operation between Suite B
   and non-Suite B implementations.  For instance, a Suite B compliant
   TLS 1.2 client might offer TLS 1.2 with both CBC and GCM cipher
   suites when communicating with a non-Suite B TLS 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 implementations will negotiate a GCM cipher suite
   when doing TLS 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 security levels
   for each cipher suite:

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



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   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.


5.  Security Considerations

   Most of the security considerations for this document are described
   in TLS 1.2 [I-D.ietf-tls-rfc4346-bis], RFC 4492 [RFC4492],
   [I-D.ietf-tls-rsa-aes-gcm], and [I-D.ietf-tls-ecc-new-mac].  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

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.



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   [I-D.ietf-tls-rfc4346-bis]
              Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", draft-ietf-tls-rfc4346-bis-10
              (work in progress), March 2008.

   [I-D.ietf-tls-ecc-new-mac]
              Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-
              256/384 and AES Galois Counter  Mode",
              draft-ietf-tls-ecc-new-mac-07 (work in progress),
              May 2008.

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

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

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

8.2.  Informative References

   [I-D.ietf-tls-rsa-aes-gcm]
              Salowey, J., Choudhury, A., and D. McGrew, "AES-GCM Cipher
              Suites for TLS", draft-ietf-tls-rsa-aes-gcm-03 (work in
              progress), April 2008.


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




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   Russ Housley
   Vigil Security
   918 Spring Knoll Drive
   Herndon  21070
   USA

   Email:  housley@vigilsec.com












































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