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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group M. Friedl 3 Internet-Draft D. Miller 4 Expires: May 27, 2007 OpenSSH 5 November 23, 2006 7 Delayed compression the SSH Transport Layer Protocol 8 draft-miller-secsh-compression-delayed-00.txt 10 Status of this Memo 12 By submitting this Internet-Draft, each author represents that any 13 applicable patent or other IPR claims of which he or she is aware 14 have been or will be disclosed, and any of which he or she becomes 15 aware will be disclosed, in accordance with Section 6 of BCP 79. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that 19 other groups may also distribute working documents as Internet- 20 Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt. 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 This Internet-Draft will expire on May 27, 2007. 35 Copyright Notice 37 Copyright (C) The Internet Society (2006). 39 Abstract 41 This memo describes a new compression method for the SSH protocol. 42 This new method uses the same zlib compression algorithm as the 43 existing method described in the core SSH drafts but delays the start 44 of compression until after user authentication has completed. This 45 eliminates the risk of a bug in compression libraries resulting in a 46 pre-authentication compromise of an SSH server. 48 Table of Contents 50 1. Requirements notation . . . . . . . . . . . . . . . . . . . . . 3 51 2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 52 3. Delayed Compression . . . . . . . . . . . . . . . . . . . . . . 3 53 4. Security Considerations . . . . . . . . . . . . . . . . . . . . 3 54 5. Normative References . . . . . . . . . . . . . . . . . . . . . 4 55 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 5 56 Intellectual Property and Copyright Statements . . . . . . . . . . 6 58 1. Requirements notation 60 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 61 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 62 document are to be interpreted as described in [RFC2119]. 64 2. Overview 66 SSH [RFC4251] is a popular protocol for secure remote login and data 67 transfer on the Internet. Included in the protocol is the ability to 68 compress data between the client and the server, offering throughput 69 improvements and traffic savings when used on compressible data. 71 The "zlib" compression in method defined in [RFC4253] for the SSH 72 protocol uses ZLIB (LZ77) compression (as described in [RFC1950] and 73 [RFC1951]). This method activates compression after the completion 74 of the key exchange algorithm, upon the receipt of the 75 SSH_MSG_NEWKEYS message. This step occurs before user authentication 76 [RFC4252] completes, so a bug in the underlying compression code 77 could be exposed to unauthenticated users. 79 This new compression method "zlib@openssh.com" addresses this risk by 80 delaying the start of compression until the completion of the user 81 authentication exchange. 83 3. Delayed Compression 85 The "zlib@openssh.com" method operates identically to the "zlib" 86 method described in [RFC4252] except that packet compression does not 87 start until the server sends a SSH_MSG_USERAUTH_SUCCESS packet, 88 replacing the "zlib" method's start of compression when the server 89 sends SSH_MSG_NEWKEYS. 91 Specifically, this means that a server selecting this method MUST 92 compress the first packet after SSH_MSG_USERAUTH_SUCCESS and all 93 subsequent packets. The client MUST compress all packets it sends 94 after the time that it receives SSH_MSG_USERAUTH_SUCCESS. 96 An implementation of this method should be careful to ensure that 97 compression is enabled by subsequent re-keying exchanges after 98 authentication. 100 4. Security Considerations 102 Delaying compression until after authentication may reveal slightly 103 more exact information about the length of authentication data. It 104 is doubtful that this would assist an attacker; the compressed length 105 of an authentication secret is of similar interest to an attacker as 106 its uncompressed length. Implementations should be taking steps to 107 obscure the exact length of authentication secrets anyway. 109 The security considerations in [RFC4251] also apply to this 110 compression method. 112 5. Normative References 114 [RFC1950] Deutsch, L. and J-L. Gailly, "ZLIB Compressed Data Format 115 Specification version 3.3", RFC 1950, May 1996. 117 [RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification 118 version 1.3", RFC 1951, May 1996. 120 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 121 Requirement Levels", BCP 14, RFC 2119, March 1997. 123 [RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) 124 Protocol Architecture", RFC 4251, January 2006. 126 [RFC4252] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) 127 Authentication Protocol", RFC 4252, January 2006. 129 [RFC4253] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) 130 Transport Layer Protocol", RFC 4253, January 2006. 132 Authors' Addresses 134 Markus Friedl 135 OpenSSH 137 Email: markus@openssh.com 139 Damien Miller 140 OpenSSH 142 Email: djm@openssh.com 144 Intellectual Property Statement 146 The IETF takes no position regarding the validity or scope of any 147 Intellectual Property Rights or other rights that might be claimed to 148 pertain to the implementation or use of the technology described in 149 this document or the extent to which any license under such rights 150 might or might not be available; nor does it represent that it has 151 made any independent effort to identify any such rights. Information 152 on the procedures with respect to rights in RFC documents can be 153 found in BCP 78 and BCP 79. 155 Copies of IPR disclosures made to the IETF Secretariat and any 156 assurances of licenses to be made available, or the result of an 157 attempt made to obtain a general license or permission for the use of 158 such proprietary rights by implementers or users of this 159 specification can be obtained from the IETF on-line IPR repository at 160 http://www.ietf.org/ipr. 162 The IETF invites any interested party to bring to its attention any 163 copyrights, patents or patent applications, or other proprietary 164 rights that may cover technology that may be required to implement 165 this standard. Please address the information to the IETF at 166 ietf-ipr@ietf.org. 168 Disclaimer of Validity 170 This document and the information contained herein are provided on an 171 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 172 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET 173 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 174 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 175 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 176 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 178 Copyright Statement 180 Copyright (C) The Internet Society (2006). This document is subject 181 to the rights, licenses and restrictions contained in BCP 78, and 182 except as set forth therein, the authors retain all their rights. 184 Acknowledgment 186 Funding for the RFC Editor function is currently provided by the 187 Internet Society.