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Network Working Group                                          B. HarrisRequest for Comments: 4345                                  January 2006Category: Standards TrackImproved Arcfour Modes forthe Secure Shell (SSH) Transport Layer ProtocolStatus 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 20063.  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 inSection 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 20067.  References7.1.  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 2006Author's Address   Ben Harris   2a Eachard Road   CAMBRIDGE   CB3 0HY   UNITED KINGDOM   EMail: bjh21@bjh21.me.ukTrademark Notice   "RC4" and "SSH" are registered trademarks in the United States.Harris                      Standards Track                     [Page 4]

RFC 4345             Improved Arcfour Modes for SSH         January 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 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   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Harris                      Standards Track                     [Page 5]