Network Working Group                                          B. Harris
Internet-Draft                                                 June 2005
Expires: December 3, 2005


   Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer
                                Protocol
                   draft-harris-ssh-arcfour-fixes-03

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   Copyright (C) The Internet Society (2005).

Abstract

   This document specifies methods of using the Arcfour cipher in the
   Secure Shell (SSH) protocol which mitigate the weakness of the
   cipher's key-scheduling algorithm.

.  Introduction

   Secure Shell (SSH) [I-D.ietf-secsh-architecture] is a secure remote-
   login protocol.  It allows for the use of an extensible variety of



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   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
   [MANTIN01], 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.

.  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].

.  Applicability Statement

   Implementations of Arcfour are typically slightly faster and much
   smaller than those of any other encryption algorithm currently
   defined for SSH.  This must be balanced, though, against the known
   security problems with Arcfour described in Section 5.  In most
   cases, where speed and code size aren't critical issues, the
   algorithms specified by [I-D.ietf-secsh-newmodes] should be used
   instead.

.  Algorithm Definitions

   The "arcfour128" 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" algorithm is the same, but using a 256-bit key.

.  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.

   There are two classes of attack on Arcfour described in [MIRINOV].
   Strong distinguishers distinguish an Arcfour keystream from
   randomness given the start of the stream, and are defended against by



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   the algorithm defined in this document.  Weak distinguishers can
   operate on any part of the keystream, and the best ones, described in
   [FMcG] and [MANTIN05], can use data from multiple different
   keystreams.  A consequence of this is that encrypting the same data
   (for instance, a password) sufficiently many times in separate
   Arcfour keystreams can be sufficient to leak information about it to
   an adversary.  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.

.  IANA Considerations

   IANA is requested to assign the Encryption Algorithm Names
   "arcfour128" and "arcfour256" in accordance with [I-D.ietf-secsh-
   assignednumbers].

.  References

7.1  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]
              Ylonen, T. and C. Lonvick, "SSH Protocol Architecture",
              draft-ietf-secsh-architecture-22 (work in progress),
              March 2005.

   [I-D.ietf-secsh-transport]
              Lonvick, C., "SSH Transport Layer Protocol",
              draft-ietf-secsh-transport-24 (work in progress),
              March 2005.

   [I-D.ietf-secsh-assignednumbers]
              Lehtinen, S. and C. Lonvick, "SSH Protocol Assigned
              Numbers", draft-ietf-secsh-assignednumbers-12 (work in
              progress), March 2005.

   [I-D.ietf-secsh-newmodes]
              Bellare, M., "SSH Transport Layer Encryption Modes",
              draft-ietf-secsh-newmodes-04 (work in progress),
              April 2005.

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



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7.2  Informative References

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

   [MANTIN01]
              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>.

   [MIRINOV]  Mirinov, I., "(Not So) Random Shuffles of RC4",
              CRYPTO 2002, 2002.

   [MANTIN05]
              Mantin, I., "Predicting and Distinguishing Attacks on RC4
              Keystream Generator", EUROCRYPT 2005, 2005.


Author's Address

   Ben Harris
   37 Milton Road
   CAMBRIDGE  CB4 1XA
   GB

   Email: bjh21@bjh21.me.uk

Trademark notice

   "RC4" and "SSH" are registered trademarks in the United States.



















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