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 have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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. Friedl & Miller Expires May 27, 2007 [Page 1] Internet-Draft SSH zlib@openssh.com compression method November 2006 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 Friedl & Miller Expires May 27, 2007 [Page 2] Internet-Draft SSH zlib@openssh.com compression method November 2006 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 Friedl & Miller Expires May 27, 2007 [Page 3] 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. Friedl & Miller Expires May 27, 2007 [Page 4] 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 Friedl & Miller Expires May 27, 2007 [Page 5] Internet-Draft SSH zlib@openssh.com compression method November 2006 Intellectual Property Statement 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. 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Disclaimer of Validity 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. 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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Friedl & Miller Expires May 27, 2007 [Page 6]