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Versions: 00 01 02

Internet Engineering Task Force                                 A. Popov
Internet-Draft                                           Microsoft Corp.
Updates: 5246,4346,2246 (if approved)                     April 11, 2014
Intended status: Standards Track
Expires: October 13, 2014


                     Prohibiting RC4 Cipher Suites
                   draft-popov-tls-prohibiting-rc4-02

Abstract

   This document requires that Transport Layer Security (TLS) clients
   and servers never negotiate the use of RC4 cipher suites when they
   establish connections.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 13, 2014.

Copyright Notice

   Copyright (c) 2014 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
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   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Changes to TLS  . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   3
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   3
   Appendix A.  RC4 Cipher Suites  . . . . . . . . . . . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   4

1.  Introduction

   RC4 is a stream cipher described in [SCH], which is widely supported,
   and often preferred, by TLS servers.  However, RC4 has long been
   known to have a variety of cryptographic weaknesses, e.g. [PAU],
   [MAN], [FLU].  Recent cryptanalysis results [ALF] exploit biases in
   the RC4 keystream to recover repeatedly encrypted plaintexts.

   These recent results are on the verge of becoming practically
   exploitable; currently they require 2^26 sessions or 13x2^30
   encryptions.  As a result, RC4 can no longer be seen as providing a
   sufficient level of security for TLS sessions.

   This document requires that TLS ([RFC5246], [RFC4346], [RFC2246])
   clients and servers never negotiate the use of RC4 cipher suites.

1.1.  Requirements Language

   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 [RFC2119].

2.  Changes to TLS

   Because of the deficiencies noted in Section 1:

   o  TLS clients MUST NOT include RC4 cipher suites in the ClientHello
      message.

   o  TLS servers MUST NOT select an RC4 cipher suite when a TLS client
      sends such a cipher suite in the ClientHello message.






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   o  If the TLS client only offers RC4 cipher suites, the TLS server
      MUST terminate the handshake.  The TLS server MAY send the
      insufficient_security fatal alert in this case.

   Appendix A lists the RC4 cipher suites defined for TLS.

3.  Acknowledgements

   This document was inspired by discussions with Magnus Nystrom, Eric
   Rescorla, Joseph Salowey, Yaron Sheffer, Nagendra Modadugu and others
   on the TLS mailing list.

4.  IANA Considerations

   This memo includes no request to IANA.

5.  Security Considerations

   This document helps maintain the security guarantees of the TLS
   protocol by prohibiting the use of the RC4-based cipher suites
   (listed in Appendix A), which do not provide a sufficiently high
   level of security.

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

6.2.  Informative References

   [ALF]      AlFardan, N., Bernstein, D., Paterson, K., Poettering, B.,
              and J. Schuldt, "On the security of RC4 in TLS and WPA.
              USENIX Security Symposium.", 2013, <https://www.usenix.org
              /conference/usenixsecurity13/security-rc4-tls>.






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   [FLU]      Fluhrer, S., Mantin, I., and A. Shamir, "Weaknesses in the
              Key Scheduling Algorithm of RC4. Selected Areas in
              Cryptography, pp. 1-24", 2001.

   [MAN]      Mantin, I. and A. Shamir, "A Practical Attack on Broadcast
              RC4. FSE, pp. 152-164.", 2001.

   [PAU]      Paul, G. and S. Maitra, "Permutation after RC4 Key
              Scheduling Reveals the Secret Key. In Proceedings of the
              14th Workshop on Selected Areas in Cryptography (SAC), pp.
              360-377, vol. 4876, LNCS, Springer.", 2007.

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

Appendix A.  RC4 Cipher Suites

   The following cipher suites defined for TLS use RC4:

   o  TLS_RSA_WITH_RC4_128_MD5

   o  TLS_RSA_WITH_RC4_128_SHA

   o  TLS_DH_anon_WITH_RC4_128_MD5

   o  TLS_RSA_EXPORT_WITH_RC4_40_MD5

   o  TLS_DH_anon_EXPORT_WITH_RC4_40_MD5

Author's Address

   Andrei Popov
   Microsoft Corp.
   One Microsoft Way
   Redmond, WA  98052
   USA

   Email: andreipo@microsoft.com













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