


TLS Working Group                                         P. Eronen, Ed.
Internet-Draft                                                     Nokia
Intended status: Informational                         February 14, 2008
Expires: August 17, 2008


     DES and IDEA Cipher Suites for Transport Layer Security (TLS)
                     draft-ietf-tls-des-idea-00.txt

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   Copyright (C) The IETF Trust (2008).

Abstract

   TLS specification versions 1.0 (RFC 2246) and 1.1 (RFC 4346) included
   cipher suites based on DES (Data Encryption Standard) and IDEA
   (International Data Encryption Algorithm) algorithms.  DES (when used
   in single-DES mode) and IDEA are no longer recommended for general
   use in TLS, and have been removed from TLS 1.2 main specification
   (RFC NNNN).  This document specifies these cipher suites for
   completeness, and discusses reasons why their use is no longer
   recommended.



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1.  Introduction

   TLS specification versions 1.0 [TLS10] and 1.1 [TLS11] included
   cipher suites based on DES (Data Encryption Standard) and IDEA
   (International Data Encryption Algorithm) algorithms.  DES (when used
   in single-DES mode) and IDEA are no longer recommended for general
   use in TLS, and have been removed from TLS 1.2 main specification
   [TLS12].

   This document specifies these cipher suites for completeness, and
   discusses reasons why their use is no longer recommended.

   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 [REQ].


2.  DES Cipher Suites

   DES (Data Encryption Standard) is a block cipher which was originally
   approved as US federal standard in 1976, and is specified in [DES].

   For TLS key generation purposes, DES is treated as having a 64-bit
   key, but it still provides only 56 bits of protection, as 8 of the 64
   bits are not used by the algorithm.  DES uses a 64-bit block size.

   The following cipher suites have been defined for using DES in CBC
   mode in TLS:

      CipherSuite TLS_RSA_WITH_DES_CBC_SHA            = { 0x00,0x09 };
      CipherSuite TLS_DH_DSS_WITH_DES_CBC_SHA         = { 0x00,0x0C };
      CipherSuite TLS_DH_RSA_WITH_DES_CBC_SHA         = { 0x00,0x0F };
      CipherSuite TLS_DHE_DSS_WITH_DES_CBC_SHA        = { 0x00,0x12 };
      CipherSuite TLS_DHE_RSA_WITH_DES_CBC_SHA        = { 0x00,0x15 };
      CipherSuite TLS_DH_anon_WITH_DES_CBC_SHA        = { 0x00,0x1A };

   The key exchange algorithms (RSA, DH_DSS, DH_RSA, DHE_DSS, DHE_RSA,
   and DH_anon) and the MAC algorithm (SHA) are defined in the base TLS
   specification.


3.  IDEA Cipher Suites

   IDEA (International Data Encryption Algorithm) is block cipher
   designed by Xuejia Lai and James Massey [IDEA] [SCH].  IDEA uses a
   128-bit key and operates on 64-bit blocks.





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   The following cipher suite has been defined for using IDEA in CBC
   mode in TLS:

      CipherSuite TLS_RSA_WITH_IDEA_CBC_SHA           = { 0x00,0x07 };

   The key exchange algorithm (RSA) and the MAC algorithm (SHA) are
   defined in the base TLS specification.


4.  Security Considerations

4.1.  DES Cipher Suites

   DES has an effective key strength of 56 bits, which has been been
   known to be vulnerable to practical brute force attacks for over 20
   years [DH].  A relatively recent 2006 paper by Kumar et al.  [COPA]
   describes a system which performs exhaustive key search in less than
   nine days on average, and costs less than 10,000 USD to build.

   Given these, the single-DES cipher suites SHOULD NOT be implemented
   by TLS libraries.  If a TLS library implements these cipher suites,
   it SHOULD NOT enable them by default.  Experience has also shown that
   rarely used code is a source of security and interoperability
   problems, so existing implementations SHOULD consider removing these
   cipher suites.

4.2.  IDEA Cipher Suites

   IDEA has a 128-bit key, and thus is not vulnerable to exhaustive key
   search.  However, IDEA cipher suites for TLS have not seen widespread
   use: most implementations either do not support them, do not enable
   them by default, or do not negotiate them when other algorithms (such
   as AES, 3DES, or RC4) are available.

   Experience has shown that rarely used code is a source of security
   and interoperability problems; given this, the IDEA cipher suites
   SHOULD NOT be implemented by TLS libraries, and SHOULD be removed
   from existing implementations.

   Several reasons have been suggested to explain why the IDEA cipher
   suites have been rarely used.  These include the existence of IPR
   disclosures (which can be obtained from the IETF on-line IPR
   repository at http://www.ietf.org/ipr); poor performance in software
   on common CPU architectures; a 64-bit block size which is considered
   short by modern standards; the existence of weak keys; lack of
   government approval in many countries; and the availability of other
   algorithms which addressed at least some of these reasons.




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5.  IANA Considerations

   IANA has already allocated values for the cipher suites described in
   this document in the TLS Cipher Suite Registry, defined in [TLS11].
   IANA is requested to update (has updated) the references of these
   cipher suites to point to this document:

   Value         Description                             Reference
   -----------   --------------------------------------  ---------
   0x00,0x07     TLS_RSA_WITH_IDEA_CBC_SHA               [RFCnnnn]
   0x00,0x09     TLS_RSA_WITH_DES_CBC_SHA                [RFCnnnn]
   0x00,0x0C     TLS_DH_DSS_WITH_DES_CBC_SHA             [RFCnnnn]
   0x00,0x0F     TLS_DH_RSA_WITH_DES_CBC_SHA             [RFCnnnn]
   0x00,0x12     TLS_DHE_DSS_WITH_DES_CBC_SHA            [RFCnnnn]
   0x00,0x15     TLS_DHE_RSA_WITH_DES_CBC_SHA            [RFCnnnn]
   0x00,0x1A     TLS_DH_anon_WITH_DES_CBC_SHA            [RFCnnnn]

   This document does not create any new registries to be maintained by
   IANA, and does not require any new assignments from existing
   registries.


6.  Acknowledgments

   The editor would like to thank Steven Bellovin, Uri Blumenthal,
   Michael D'Errico, Paul Hoffman, Simon Josefsson, Bodo Moeller, Martin
   Rex, and Len Sassaman for their contributions to preparing this
   document.


7.  References

7.1.  Normative References

   [DES]    National Institute of Standards and Technology, "Data
            Encryption Standard (DES)", FIPS PUB 46-3, October 1999.

   [IDEA]   Lai, X., "On the Design and Security of Block Ciphers",
            ETH Series in Information Processing, v. 1, Konstanz:
            Hartung-Gorre Verlag, 1992.

   [REQ]    Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.

   [SCH]    Schneier, B., "Applied Cryptography: Protocols, Algorithms,
            and Source Code in C", 2nd ed., John Wiley & Sons, Inc.,
            1996.




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   [TLS10]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
            RFC 2246, January 1999.

   [TLS11]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [TLS12]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", draft-ietf-tls-rfc4346-bis-09
            (work in progress), February 2008.

7.2.  Informative References

   [COPA]   Kumar, S., Paar, C., Pelzl, J., Pfeiffer, G., and M.
            Schimmler, "Breaking Ciphers with COPACOBANA - A Cost-
            Optimized Parallel Code Breaker", Workshop on Cryptographic
            Hardware and Embedded Systems (CHES 2006), Yokohama, Japan,
            October 2006.

   [DH]     Diffie, W. and M. Hellman, "Exhaustive Cryptanalysis of the
            NBS Data Encryption Standard", IEEE Computer, volume 10,
            issue 6, June 1977.


Author's Address

   Pasi Eronen (editor)
   Nokia Research Center
   P.O. Box 407
   FIN-00045 Nokia Group
   Finland

   Email: pasi.eronen@nokia.com



















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