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.





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  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





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  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.






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  [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



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  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.





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Author's Address

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

  Phone: +44 20 8542 7856
  EMail: [email protected]








































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Full Copyright Statement

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

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain it
  or assist in its implementation may be prepared, copied, published
  and distributed, in whole or in part, without restriction of any
  kind, provided that the above copyright notice and this paragraph are
  included on all such copies and derivative works.  However, this
  document itself may not be modified in any way, such as by removing
  the copyright notice or references to the Internet Society or other
  Internet organizations, except as needed for the purpose of
  developing Internet standards in which case the procedures for
  copyrights defined in the Internet Standards process must be
  followed, or as required to translate it into languages other than
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  The limited permissions granted above are perpetual and will not be
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  This document and the information contained herein is provided on an
  "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
  TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
  BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
  HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
  MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

  Funding for the RFC Editor function is currently provided by the
  Internet Society.



















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