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Network Working Group                                         Pete Chown
INTERNET DRAFT                                        Skygate Technology
<draft-ietf-tls-ciphersuite-05.txt>                       14 August 2001

                        AES Ciphersuites for TLS

Status of this Memo

     This document is an Internet-Draft and is in full conformance with
     all provisions of Section 10 of RFC2026.

     Internet-Drafts are working documents of the Internet Engineering
     Task Force (IETF), its areas, and its working groups. Note that
     other groups may also distribute working documents as Internet-
     Drafts.

     Internet-Drafts are draft documents valid for a maximum of six
     months and may be updated, replaced, or obsoleted by other docu¡
     ments at any time. It is inappropriate to use Internet-Drafts as
     reference material or to cite them other than as ``work in
     progress.''

     The list of current Internet-Drafts can be accessed at
     <http://www.ietf.org/ietf/1id-abstracts.txt>

     The list of Internet-Draft Shadow Directories can be accessed at
     <http://www.ietf.org/shadow.html>

     Distribution of this document is unlimited. Please send comments to
     the author (pc@skygate.co.uk) or to the Transport Layer Security
     Working Group's discussion list (ietf-tls@lists.certicom.com).

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 the AES process is completed there will be commercial pres¡
         sure to use the selected cipher.  The AES is efficient and has
         withstood extensive cryptanalytic efforts.  The AES is



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         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 follow¡
     ing, 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:




















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     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.  Rijn¡
     dael 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



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     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 pro¡
         tocol 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 compro¡
         mised.

     [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 conse¡
         quence 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.

Copyright




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     Copyright (C) The Internet Society 2001. 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, pub¡
     lished and distributed, in whole or in part, without restriction of
     any kind, provided that the above copyright notice and this para¡
     graph are included on all such copies and derivative works. How¡
     ever, 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 proce¡
     dures for copyrights defined in the Internet Standards process must
     be followed, or as required to translate it into languages other
     than English.

     The limited permissions granted above are perpetual and will not be
     revoked by the Internet Society or its successors or assignees.

     This document and the information contained herein is provided on
     an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGI¡
     NEERING 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 WAR¡
     RANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

     The IETF takes no position regarding the validity or scope of any
     intellectual property or other rights that might be claimed to per¡
     tain 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
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     prietary 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.




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     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 inven¡
          tions 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.

     One of the authors of Rijndael signed the following disclaimer when
     submitting the algorithm to NIST for consideration in the AES pro¡
     cess:

          I, Joan Daemen, do hereby declare that to the best of my
          knowledge the practice of the algorithm, reference imple¡
          mentation, and mathematically optimized implementations,
          I have submitted, known as Rijndael may be covered by the
          following U.S. and/or foreign patents:

          none

          I do hereby declare that I am aware of no patent applica¡
          tions which may cover the practice of my submitted algo¡
          rithm, reference implementation or mathematically opti¡
          mized implementations.

          I do hereby understand that my submitted algorithm may
          not be selected for inclusion in the Advanced Encryption
          Standard.  I also understand and agree that after the
          close of the submission period, my submission may not be
          withdrawn from public consideration for inclusion in the
          Federal Information Processing Standard (FIPS) for
          Advanced Encryption Standard (AES).  I further understand
          that I will not receive financial compensation from the
          government for my submission.  I certify that, to the
          best of my knowledge, I have fully disclosed all patents
          and patent applications relating to my algorithm.  I also
          understand that the U.S. Government may, during the
          course of the lifetime of the AES or during the FIPS pub¡
          lic review process, modify the algorithm's specifications
          (e.g., to protect against a newly discovered



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          vulnerability).  Should my submission be selected for
          inclusion in the AES, I hereby agree not to place any
          restrictions on the use of the algorithm intending it to
          be available on a worldwide, non-exclusive, royalty-free
          basis.

          I do hereby agree to provide the statements for any
          patent or patent application identified to cover practice
          of my algorithm, reference implementation or mathemati¡
          cally optimized implementations and the right to use such
          implementations for the purposes of the AES evaluation
          process.

          I understand that NIST will announce the selected algo¡
          rithm(s) and proceed to publish the draft FIPS for public
          comment.  If my algorithm (or the derived algorithm) is
          not selected for inclusion in the FIPS (including those
          not selected for second round of public evaluation), I
          understand that all rights, including use rights of the
          reference and mathematically optimized implementations,
          revert back to the submitter (and other owner[s] as
          appropriate).  Additionally, should the U.S. Government
          not select my algorithm for inclusion in the AES after a
          period of four years from the close of the submission
          date for candidate algorithms, all rights revert to the
          submitter (and other owner[s] as appropriate).

          [signed]


          Title: Cryptographer
          Dated: 10-6-98
          Place: Brussels

     The following disclaimer was signed at the start of the second
     "round" of the AES process:

          Dear Mr Foti [of NIST],

          Hereby we confirm that the original patent and patent
          application information, as provided to NIST with our
          original submission in June 1998, has not changed.  To
          the best of our knowledge, there are no patents or patent
          applications covering the practice of the algorithm, ref¡
          erence implementation or the mathematically optimized
          implementations.





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          [signed]
          Joan Daemen, Vincent Rijmen

Acknowledgements

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

References

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

     [AES] J. Daemen, V. Rijmen, "The Rijndael Block Cipher"
     http://csrc.nist.gov/encryption/aes/rijndael/Rijndael.pdf 3rd
     September 1999.

     [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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