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Versions: 00 01 02 03 RFC 4345

Network Working Group                                          B. Harris
Internet-Draft                                                March 2005
Expires: September 2, 2005

      Improved Arcfour Modes for the SSH Transport Layer Protocol
                   draft-harris-ssh-arcfour-fixes-01

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 3 of RFC 3667.  By submitting this Internet-Draft, each
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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document specifies a method of using the Arcfour cipher in the
   SSH protocol which mitigates the weakness of its key-scheduling
   algorithm.

1.  Introduction

   Secure Shell (SSH) [I-D.ietf-secsh-architecture] is a secure


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   remote-login protocol.  It allows for the use of an extensible
   variety of symmetric cipher algorithms to provide confidentiality for
   data in transit.  One of the algorithms specified in the base
   protocol is "arcfour", which specifies the use of Arcfour (also known
   as RC4), a fast stream cipher.  As [I-D.ietf-secsh-transport] says,
   though, "Arcfour (and RC4) has problems with weak keys, and should be
   used with caution."  These problems are described in more detail in
   [MANTIN], along with a recommendation to discard the first 1536 bytes
   of keystream so as to ensure that the cipher's internal state is
   thoroughly mixed.  This document specifies new cipher algorithms for
   SSH which follow this recommendation.

2.  Conventions Used in this Document

   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].

3.  Algorithm Definitions

   The "arcfour128-draft-00@putty.projects.tartarus.org" algorithm is
   the RC4 cipher as described in [SCHNEIER], using a 128-bit key.  The
   first 1536 bytes of keystream generated by the cipher MUST be
   discarded, with the first byte of the first encrypted packet being
   encrypted using the 1537th byte of keystream.

   The "arcfour256-draft-00@putty.projects.tartarus.org" algorithm is
   the same, but using a 256-bit key.

4.  Security Considerations

   The security considerations in [I-D.ietf-secsh-architecture] apply.

   The discarded bytes of keystream MUST be kept secret and MUST NOT be
   transmitted over the network.  The contents of these bytes could
   reveal information about the key.

   Given about 2^30.6 bytes of Arcfour keystream, it is possible to
   distinguish it from a random byte sequence [FMcG].  These 2^30.6
   bytes need not all be from the same Arcfour stream, with the
   consequence that if the same plaintext (for instance a password) is
   encrypted 2^33.6 times with different Arcfour keys, it might be
   possible to deduce its content.  It is thus RECOMMENDED that Arcfour
   (either in the form described here or that described in
   [I-D.ietf-secsh-architecture]) not be used for high-volume
   password-authenticated connections.



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5.  IANA Considerations

   This document has no actions for IANA.

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.

   [I-D.ietf-secsh-architecture]
              Lonvick, C., "SSH Protocol Architecture",
              Internet-Draft draft-ietf-secsh-architecture-22, March
              2005.

   [I-D.ietf-secsh-transport]
              Lonvick, C., "SSH Transport Layer Protocol",
              Internet-Draft draft-ietf-secsh-transport-24, March 2005.

   [SCHNEIER]
              Schneier, B., "Applied Cryptography Second Edition:
              protocols algorithms and source in code in C", John Wiley
              and Sons, New York, NY, 1996.

6.2  Informative References

   [FMcG]    Fluhrer, S. and D. McGrew, "Statistical Analysis of the
             Alleged RC4 Keystream Generator", The Procedings of the
             Fast Software Encryption Workshop 2000,
             <http://www.mindspring.com/~dmcgrew/rc4-03.pdf>.

   [MANTIN]  Mantin, I., "Analysis of the Stream Cipher RC4", M.Sc.
             Thesis, Weizmann Institute of Science, 2001,
             <http://www.wisdom.weizmann.ac.il/~itsik/RC4/Papers/Mantin1
             .zip>.

Author's Address

   Ben Harris
   37 Milton Road
   CAMBRIDGE  CB4 1XA
   GB

   Email: bjh21@bjh21.me.uk



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