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rfc:rfc4345

Network Working Group B. Harris Request for Comments: 4345 January 2006 Category: Standards Track

                    Improved Arcfour Modes for
          the Secure Shell (SSH) Transport Layer Protocol

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

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

1. Introduction

 Secure Shell (SSH) [RFC4251] is a secure 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 [RFC4253] 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 that 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].

Harris Standards Track [Page 1] RFC 4345 Improved Arcfour Modes for SSH January 2006

3. 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 are not critical issues, the
 algorithms specified by [RFC4344] should be used instead.

4. 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, and the first byte of the
 first encrypted packet MUST be encrypted using the 1537th byte of
 keystream.
 The "arcfour256" algorithm is the same, but uses a 256-bit key.

5. Security Considerations

 The security considerations in [RFC4251] 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 [MIRONOV].
 Strong distinguishers distinguish an Arcfour keystream from
 randomness at the start of the stream and are defended against by 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 [RFC4251]) not be used for high-volume password-
 authenticated connections.

6. IANA Considerations

 The IANA has assigned the Encryption Algorithm Names "arcfour128" and
 "arcfour256" in accordance with [RFC4250].

Harris Standards Track [Page 2] RFC 4345 Improved Arcfour Modes for SSH January 2006

7. References

7.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC4250]  Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
            Protocol Assigned Numbers", RFC 4250, January 2006.
 [RFC4251]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
            Protocol Architecture", RFC 4251, January 2006.
 [RFC4253]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
            Transport Layer Protocol", RFC 4253, January 2006
 [RFC4344]  Bellare, M., Kohno, T., and C. Namprempre, "The Secure
            Shell (SSH) Transport Layer Encryption Modes", RFC 4344,
            January 2006.
 [SCHNEIER] Schneier, B., "Applied Cryptography Second Edition:
            protocols algorithms and source in code in C", John Wiley
            and Sons, New York, NY, 1996.

7.2. Informative References

 [FMcG]     Fluhrer, S. and D. McGrew, "Statistical Analysis of the
            Alleged RC4 Keystream Generator", Fast Software
            Encryption:  7th International Workshop, FSE 2000, April
            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>.
 [MIRONOV]  Mironov, I., "(Not So) Random Shuffles of RC4", Advances
            in Cryptology -- CRYPTO 2002: 22nd Annual International
            Cryptology Conference, August 2002,
            <http://eprint.iacr.org/2002/067.pdf>.
 [MANTIN05] Mantin, I., "Predicting and Distinguishing Attacks on RC4
            Keystream Generator", Advances in Cryptology -- EUROCRYPT
            2005: 24th Annual International Conference on the Theory
            and Applications of Cryptographic Techniques, May 2005.

Harris Standards Track [Page 3] RFC 4345 Improved Arcfour Modes for SSH January 2006

Author's Address

 Ben Harris
 2a Eachard Road
 CAMBRIDGE
 CB3 0HY
 UNITED KINGDOM
 EMail: bjh21@bjh21.me.uk

Trademark Notice

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

Harris Standards Track [Page 4] RFC 4345 Improved Arcfour Modes for SSH January 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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 The IETF invites any interested party to bring to its attention any
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Acknowledgement

 Funding for the RFC Editor function is provided by the IETF
 Administrative Support Activity (IASA).

Harris Standards Track [Page 5]

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