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 (C) The Internet Society (2006).
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.
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].
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
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-
6. IANA Considerations
The IANA has assigned the Encryption Algorithm Names "arcfour128" and
"arcfour256" in accordance with [RFC4250].
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