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RFC 4345
Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol.
B. Harris. January 2006.

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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, <>. [MANTIN01] Mantin, I., "Analysis of the Stream Cipher RC4", M.Sc. Thesis, Weizmann Institute of Science, 2001, <http://>. [MIRONOV] Mironov, I., "(Not So) Random Shuffles of RC4", Advances in Cryptology -- CRYPTO 2002: 22nd Annual International Cryptology Conference, August 2002, <>. [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: 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. 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 in BCP 78 and BCP 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 at 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 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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