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+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.
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+Harris                      Standards Track                     [Page 4]
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+RFC 4345             Improved Arcfour Modes for SSH         January 2006
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+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.
+
+Intellectual Property
+
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+
+Acknowledgement
+
+   Funding for the RFC Editor function is provided by the IETF
+   Administrative Support Activity (IASA).
+
+
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+Harris                      Standards Track                     [Page 5]
+