Network Working Group B. Harris
Internet-Draft March 17, 2005
Expires: September 18, 2005
Improved Arcfour Modes for the SSH Transport Layer Protocol
draft-harris-ssh-arcfour-fixes-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document specifies a method of using the Arcfour cipher in the
SSH protocol which mitigates the weakness of its key-scheduling
algorithm.
1. Introduction
Secure Shell (SSH) [I-D.ietf-secsh-architecture] is a secure
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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 [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
[MANTIN], 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.
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].
3. Algorithm Definitions
The "arcfour128-draft-00@putty.projects.tartarus.org" 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-draft-00@putty.projects.tartarus.org" algorithm is
the same, but using a 256-bit key.
4. 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.
Given about 2^30.6 bytes of Arcfour keystream, it is possible to
distinguish it from a random byte sequence [FMcG]. Thus, it is
RECOMMENDED that implementations using these algorithms re-key at
least every 2^30 bytes.
5. IANA Considerations
This document has no actions for IANA.
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6. References
6.1 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]
Lonvick, C., "SSH Protocol Architecture",
Internet-Draft draft-ietf-secsh-architecture-22, March
2005.
[I-D.ietf-secsh-transport]
Lonvick, C., "SSH Transport Layer Protocol",
Internet-Draft draft-ietf-secsh-transport-24, March 2005.
[SCHNEIER]
Schneier, B., "Applied Cryptography Second Edition:
protocols algorithms and source in code in C", John Wiley
and Sons, New York, NY, 1996.
6.2 Informative References
[FMcG] Fluhrer, S. and D. McGrew, "Statistical Analysis of the
Alleged RC4 Keystream Generator", The Procedings of the
Fast Software Encryption Workshop 2000,
<http://www.mindspring.com/~dmcgrew/rc4-03.pdf>.
[MANTIN] 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>.
Author's Address
Ben Harris
37 Milton Road
CAMBRIDGE CB4 1XA
GB
Email: bjh21@bjh21.me.uk
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