Network Working Group                                     P. Eronen, Ed.
Request for Comments: 5469                                         Nokia
Category: Informational                                    February 2009


    DES and IDEA Cipher Suites for Transport Layer Security (TLS)

Status of This Memo

  This memo provides information for the Internet community.  It does
  not specify an Internet standard of any kind.  Distribution of this
  memo is unlimited.

Copyright Notice

  Copyright (c) 2009 IETF Trust and the persons identified as the
  document authors.  All rights reserved.

  This document is subject to BCP 78 and the IETF Trust's Legal
  Provisions Relating to IETF Documents (http://trustee.ietf.org/
  license-info) in effect on the date of publication of this document.
  Please review these documents carefully, as they describe your rights
  and restrictions with respect to this document.

Abstract

  Transport Layer Security (TLS) versions 1.0 (RFC 2246) and 1.1 (RFC
  4346) include 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 version 1.2 (RFC
  5246).  This document specifies these cipher suites for completeness
  and discusses reasons why their use is no longer recommended.

1.  Introduction

  TLS versions 1.0 [TLS10] and 1.1 [TLS11] include 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 version 1.2 [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].



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RFC 5469           DES and IDEA Cipher Suites for TLS      February 2009


2.  DES Cipher Suites

  DES (Data Encryption Standard) is a block cipher that was originally
  approved as a 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 Cipher
  Block Chaining (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 (Message Authentication Code) algorithm
  (SHA) are defined in the base TLS specification.

3.  IDEA Cipher Suite

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

  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 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 that performs an exhaustive key search in less
  than nine days on average, and costs less than 10,000 USD to build.





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RFC 5469           DES and IDEA Cipher Suites for TLS      February 2009


  Given this, 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 Suite

  IDEA has a 128-bit key, and thus is not vulnerable to an exhaustive
  key search.  However, the IDEA cipher suite for TLS has not seen
  widespread use: most implementations either do not support it, do not
  enable it by default, or do not negotiate it 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 suite
  SHOULD NOT be implemented by TLS libraries and SHOULD be removed from
  existing implementations.

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 has updated the references of these cipher suites to point to
  this document:

  Value         Description                             Reference
  -----------   --------------------------------------  ---------
  0x00,0x07     TLS_RSA_WITH_IDEA_CBC_SHA               [RFC5469]
  0x00,0x09     TLS_RSA_WITH_DES_CBC_SHA                [RFC5469]
  0x00,0x0C     TLS_DH_DSS_WITH_DES_CBC_SHA             [RFC5469]
  0x00,0x0F     TLS_DH_RSA_WITH_DES_CBC_SHA             [RFC5469]
  0x00,0x12     TLS_DHE_DSS_WITH_DES_CBC_SHA            [RFC5469]
  0x00,0x15     TLS_DHE_RSA_WITH_DES_CBC_SHA            [RFC5469]
  0x00,0x1A     TLS_DH_anon_WITH_DES_CBC_SHA            [RFC5469]

  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, Tom
  Petch, Martin Rex, and Len Sassaman for their contributions to
  preparing this document.




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RFC 5469           DES and IDEA Cipher Suites for TLS      February 2009


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.

  [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", RFC 5246, August 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: [email protected]



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