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Network Working Group                                          M. Friedl
Internet-Draft                                                 D. Miller
Expires: May 27, 2007                                            OpenSSH
                                                       November 23, 2006


          Delayed compression the SSH Transport Layer Protocol
             draft-miller-secsh-compression-delayed-00.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on May 27, 2007.

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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Table of Contents

   1.  Requirements notation . . . . . . . . . . . . . . . . . . . . . 3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Delayed Compression . . . . . . . . . . . . . . . . . . . . . . 3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 3
   5.  Normative References  . . . . . . . . . . . . . . . . . . . . . 4
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 5
   Intellectual Property and Copyright Statements  . . . . . . . . . . 6










































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1.  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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   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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Authors' Addresses

   Markus Friedl
   OpenSSH

   Email: markus@openssh.com


   Damien Miller
   OpenSSH

   Email: djm@openssh.com







































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Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.




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