Network Working Group                                           P. Chown
Request for Comments: 3268                            Skygate Technology
Category: Standards Track                                      June 2002


  Advanced Encryption Standard (AES) Ciphersuites for Transport Layer
                             Security (TLS)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document proposes several new ciphersuites.  At present, the
   symmetric ciphers supported by Transport Layer Security (TLS) are
   RC2, RC4, International Data Encryption Algorithm (IDEA), Data
   Encryption Standard (DES), and triple DES.  The protocol would be
   enhanced by the addition of Advanced Encryption Standard (AES)
   ciphersuites.

Overview

   At present, the symmetric ciphers supported by TLS are RC2, RC4,
   IDEA, DES, and triple DES.  The protocol would be enhanced by the
   addition of AES [AES] ciphersuites, for the following reasons:

   1. RC2, RC4, and IDEA are all subject to intellectual property
      claims.  RSA Security Inc. has trademark rights in the names RC2
      and RC4, and claims that the RC4 algorithm itself is a trade
      secret.  Ascom Systec Ltd. owns a patent on the IDEA algorithm.

   2. Triple DES is much less efficient than more modern ciphers.

   3. Now that the AES process is completed there will be commercial
      pressure to use the selected cipher.  The AES is efficient and has
      withstood extensive cryptanalytic efforts.  The AES is therefore a
      desirable choice.

   4. Currently the DHE ciphersuites only allow triple DES (along with
      some "export" variants which do not use a satisfactory key
      length).  At the same time the DHE ciphersuites are the only ones
      to offer forward secrecy.

   This document proposes several new ciphersuites, with the aim of
   overcoming these problems.

Cipher Usage

   The new ciphersuites proposed here are very similar to the following,
   defined in [TLS]:

   TLS_RSA_WITH_3DES_EDE_CBC_SHA
   TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA
   TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA
   TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA
   TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA
   TLS_DH_anon_WITH_3DES_EDE_CBC_SHA

   All the ciphersuites described here use the AES in cipher block
   chaining (CBC) mode.  Furthermore, they use SHA-1 [SHA-1] in an HMAC
   construction as described in section 5 of [TLS].  (Although the TLS
   ciphersuite names include the text "SHA", this actually refers to the
   modified SHA-1 version of the algorithm.)

   The ciphersuites differ in the type of certificate and key exchange
   method.  The ciphersuites defined here use the following options for
   this part of the protocol:

   CipherSuite                        Certificate type (if applicable)
                                      and key exchange algorithm

   TLS_RSA_WITH_AES_128_CBC_SHA       RSA
   TLS_DH_DSS_WITH_AES_128_CBC_SHA    DH_DSS
   TLS_DH_RSA_WITH_AES_128_CBC_SHA    DH_RSA
   TLS_DHE_DSS_WITH_AES_128_CBC_SHA   DHE_DSS
   TLS_DHE_RSA_WITH_AES_128_CBC_SHA   DHE_RSA
   TLS_DH_anon_WITH_AES_128_CBC_SHA   DH_anon

   TLS_RSA_WITH_AES_256_CBC_SHA       RSA
   TLS_DH_DSS_WITH_AES_256_CBC_SHA    DH_DSS
   TLS_DH_RSA_WITH_AES_256_CBC_SHA    DH_RSA
   TLS_DHE_DSS_WITH_AES_256_CBC_SHA   DHE_DSS
   TLS_DHE_RSA_WITH_AES_256_CBC_SHA   DHE_RSA
   TLS_DH_anon_WITH_AES_256_CBC_SHA   DH_anon

   For the meanings of the terms RSA, DH_DSS, DH_RSA, DHE_DSS, DHE_RSA
   and DH_anon, please refer to sections 7.4.2 and 7.4.3 of [TLS].

   The AES supports key lengths of 128, 192 and 256 bits.  However, this
   document only defines ciphersuites for 128- and 256-bit keys.  This
   is to avoid unnecessary proliferation of ciphersuites.  Rijndael
   actually allows for 192- and 256-bit block sizes as well as the 128-
   bit blocks mandated by the AES process.  The ciphersuites defined
   here all use 128-bit blocks.

   The new ciphersuites will have the following definitions:

   CipherSuite TLS_RSA_WITH_AES_128_CBC_SHA      = { 0x00, 0x2F };
   CipherSuite TLS_DH_DSS_WITH_AES_128_CBC_SHA   = { 0x00, 0x30 };
   CipherSuite TLS_DH_RSA_WITH_AES_128_CBC_SHA   = { 0x00, 0x31 };
   CipherSuite TLS_DHE_DSS_WITH_AES_128_CBC_SHA  = { 0x00, 0x32 };
   CipherSuite TLS_DHE_RSA_WITH_AES_128_CBC_SHA  = { 0x00, 0x33 };
   CipherSuite TLS_DH_anon_WITH_AES_128_CBC_SHA  = { 0x00, 0x34 };

   CipherSuite TLS_RSA_WITH_AES_256_CBC_SHA      = { 0x00, 0x35 };
   CipherSuite TLS_DH_DSS_WITH_AES_256_CBC_SHA   = { 0x00, 0x36 };
   CipherSuite TLS_DH_RSA_WITH_AES_256_CBC_SHA   = { 0x00, 0x37 };
   CipherSuite TLS_DHE_DSS_WITH_AES_256_CBC_SHA  = { 0x00, 0x38 };
   CipherSuite TLS_DHE_RSA_WITH_AES_256_CBC_SHA  = { 0x00, 0x39 };
   CipherSuite TLS_DH_anon_WITH_AES_256_CBC_SHA  = { 0x00, 0x3A };

Security Considerations

   It is not believed that the new ciphersuites are ever less secure
   than the corresponding older ones.  The AES is believed to be secure,
   and it has withstood extensive cryptanalytic attack.

   The ephemeral Diffie-Hellman ciphersuites provide forward secrecy
   without any known reduction in security in other areas.  To obtain
   the maximum benefit from these ciphersuites:

   1. The ephemeral keys should only be used once.  With the TLS
      protocol as currently defined there is no significant efficiency
      gain from reusing ephemeral keys.

   2. Ephemeral keys should be destroyed securely when they are no
      longer required.

   3. The random number generator used to create ephemeral keys must not
      reveal past output even when its internal state is compromised.

   [TLS] describes the anonymous Diffie-Hellman (ADH) ciphersuites as
   deprecated.  The ADH ciphersuites defined here are not deprecated.
   However, when they are used, particular care must be taken:

   1. ADH provides confidentiality but not authentication.  This means
      that (if authentication is required) the communicating parties
      must authenticate to each other by some means other than TLS.

   2. ADH is vulnerable to man-in-the-middle attacks, as a consequence
      of the lack of authentication.  The parties must have a way of
      determining whether they are participating in the same TLS
      connection.  If they are not, they can deduce that they are under
      attack, and presumably abort the connection.

      For example, if the parties share a secret, it is possible to
      compute a MAC of the TLS Finished message.  An attacker would have
      to negotiate two different TLS connections; one with each
      communicating party.  The Finished messages would be different in
      each case, because they depend on the parties' public keys (among
      other things).  For this reason, the MACs computed by each party
      would be different.

      It is important to note that authentication techniques which do
      not use the Finished message do not usually provide protection
      from this attack.  For example, the client could authenticate to
      the server with a password, but it would still be vulnerable to
      man-in-the-middle attacks.

      Recent research has identified a chosen plaintext attack which
      applies to all ciphersuites defined in [TLS] which use CBC mode.
      This weakness does not affect the common use of TLS on the World
      Wide Web, but may affect the use of TLS in other applications.
      When TLS is used in an application where this attack is possible,
      attackers can determine the truth or otherwise of a hypothesis
      that particular plaintext data was sent earlier in the session.
      No key material is compromised.

      It is likely that the CBC construction will be changed in a future
      revision of the TLS protocol.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use other technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the

   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.

   During the development of the AES, NIST published the following
   statement on intellectual property:

      SPECIAL NOTE - Intellectual Property

      NIST reminds all interested parties that the adoption of AES is
      being conducted as an open standards-setting activity.
      Specifically, NIST has requested that all interested parties
      identify to NIST any patents or inventions that may be required
      for the use of AES.  NIST hereby gives public notice that it may
      seek redress under the antitrust laws of the United States against
      any party in the future who might seek to exercise patent rights
      against any user of AES that have not been disclosed to NIST in
      response to this request for information.

Acknowledgements

   I would like to thank the ietf-tls mailing list contributors who have
   made helpful suggestions for this document.

References

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

   [AES]    National Institute of Standards and Technology,
            "Specification for the Advanced Encryption Standard (AES)"
            FIPS 197.  November 26, 2001.

   [SHA-1]  FIPS PUB 180-1, "Secure Hash Standard," National Institute
            of Standards and Technology, U.S. Department of Commerce,
            April 17, 1995.

Author's Address

   Pete Chown
   Skygate Technology Ltd
   8 Lombard Road
   London
   SW19 3TZ
   United Kingdom

   Phone: +44 20 8542 7856
   EMail: pc@skygate.co.uk

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