Network Working Group M. Friedl
Internet-Draft D. Miller
Expires: May 27, 2007 OpenSSH
November 23, 2006
Delayed compression the SSH Transport Layer Protocoldraft-miller-secsh-compression-delayed-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This memo describes a new compression method for the SSH protocol.
This new method uses the same zlib compression algorithm as the
existing method described in the core SSH drafts but delays the start
of compression until after user authentication has completed. This
eliminates the risk of a bug in compression libraries resulting in a
pre-authentication compromise of an SSH server.
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Internet-Draft SSH zlib@openssh.com compression method November 20061. Requirements notation
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].
2. Overview
SSH [RFC4251] is a popular protocol for secure remote login and data
transfer on the Internet. Included in the protocol is the ability to
compress data between the client and the server, offering throughput
improvements and traffic savings when used on compressible data.
The "zlib" compression in method defined in [RFC4253] for the SSH
protocol uses ZLIB (LZ77) compression (as described in [RFC1950] and
[RFC1951]). This method activates compression after the completion
of the key exchange algorithm, upon the receipt of the
SSH_MSG_NEWKEYS message. This step occurs before user authentication
[RFC4252] completes, so a bug in the underlying compression code
could be exposed to unauthenticated users.
This new compression method "zlib@openssh.com" addresses this risk by
delaying the start of compression until the completion of the user
authentication exchange.
3. Delayed Compression
The "zlib@openssh.com" method operates identically to the "zlib"
method described in [RFC4252] except that packet compression does not
start until the server sends a SSH_MSG_USERAUTH_SUCCESS packet,
replacing the "zlib" method's start of compression when the server
sends SSH_MSG_NEWKEYS.
Specifically, this means that a server selecting this method MUST
compress the first packet after SSH_MSG_USERAUTH_SUCCESS and all
subsequent packets. The client MUST compress all packets it sends
after the time that it receives SSH_MSG_USERAUTH_SUCCESS.
An implementation of this method should be careful to ensure that
compression is enabled by subsequent re-keying exchanges after
authentication.
4. Security Considerations
Delaying compression until after authentication may reveal slightly
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Internet-Draft SSH zlib@openssh.com compression method November 2006
more exact information about the length of authentication data. It
is doubtful that this would assist an attacker; the compressed length
of an authentication secret is of similar interest to an attacker as
its uncompressed length. Implementations should be taking steps to
obscure the exact length of authentication secrets anyway.
The security considerations in [RFC4251] also apply to this
compression method.
5. Normative References
[RFC1950] Deutsch, L. and J-L. Gailly, "ZLIB Compressed Data Format
Specification version 3.3", RFC 1950, May 1996.
[RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, May 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[RFC4252] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Authentication Protocol", RFC 4252, January 2006.
[RFC4253] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, January 2006.
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Internet-Draft SSH zlib@openssh.com compression method November 2006
Authors' Addresses
Markus Friedl
OpenSSH
Email: markus@openssh.com
Damien Miller
OpenSSH
Email: djm@openssh.com
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Internet-Draft SSH zlib@openssh.com compression method November 2006
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