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matrix invertmatrix matrix concatmatrix - matrix invertmatrix put -%%EndSetup -%%Page: (0,1) 1 -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 0.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -/showpage{}def/copypage{}def/erasepage{}def -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 1 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 701 M -(Network Working Group T. Ylonen) s -5 690 M -(Internet-Draft SSH Communications Security Corp) s -5 679 M -(Expires: March 31, 2004 D. Moffat, Ed.) s -5 668 M -( Sun Microsystems, Inc) s -5 657 M -( Oct 2003) s -5 624 M -( SSH Protocol Architecture) s -5 613 M -( draft-ietf-secsh-architecture-15.txt) s -5 591 M -(Status of this Memo) s -5 569 M -( This document is an Internet-Draft and is in full conformance with) s -5 558 M -( all provisions of Section 10 of RFC2026.) s -5 536 M -( Internet-Drafts are working documents of the Internet Engineering) s -5 525 M -( Task Force \(IETF\), its areas, and its working groups. Note that other) s -5 514 M -( groups may also distribute working documents as Internet-Drafts.) s -5 492 M -( Internet-Drafts are draft documents valid for a maximum of six months) s -5 481 M -( and may be updated, replaced, or obsoleted by other documents at any) s -5 470 M -( time. It is inappropriate to use Internet-Drafts as reference) s -5 459 M -( material or to cite them other than as "work in progress.") s -5 437 M -( The list of current Internet-Drafts can be accessed at http://) s -5 426 M -( www.ietf.org/ietf/1id-abstracts.txt.) s -5 404 M -( The list of Internet-Draft Shadow Directories can be accessed at) s -5 393 M -( http://www.ietf.org/shadow.html.) s -5 371 M -( This Internet-Draft will expire on March 31, 2004.) s -5 349 M -(Copyright Notice) s -5 327 M -( Copyright \(C\) The Internet Society \(2003\). All Rights Reserved.) s -5 305 M -(Abstract) s -5 283 M -( SSH is a protocol for secure remote login and other secure network) s -5 272 M -( services over an insecure network. This document describes the) s -5 261 M -( architecture of the SSH protocol, as well as the notation and) s -5 250 M -( terminology used in SSH protocol documents. It also discusses the SSH) s -5 239 M -( algorithm naming system that allows local extensions. The SSH) s -5 228 M -( protocol consists of three major components: The Transport Layer) s -5 217 M -( Protocol provides server authentication, confidentiality, and) s -5 206 M -( integrity with perfect forward secrecy. The User Authentication) s -5 195 M -( Protocol authenticates the client to the server. The Connection) s -5 184 M -( Protocol multiplexes the encrypted tunnel into several logical) s -5 173 M -( channels. Details of these protocols are described in separate) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 1]) s -_R -S -PStoPSsaved restore -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 421.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 2 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( documents.) s -5 668 M -(Table of Contents) s -5 646 M -( 1. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 3) s -5 635 M -( 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3) s -5 624 M -( 3. Specification of Requirements . . . . . . . . . . . . . . . 3) s -5 613 M -( 4. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 3) s -5 602 M -( 4.1 Host Keys . . . . . . . . . . . . . . . . . . . . . . . . . 4) s -5 591 M -( 4.2 Extensibility . . . . . . . . . . . . . . . . . . . . . . . 5) s -5 580 M -( 4.3 Policy Issues . . . . . . . . . . . . . . . . . . . . . . . 5) s -5 569 M -( 4.4 Security Properties . . . . . . . . . . . . . . . . . . . . 6) s -5 558 M -( 4.5 Packet Size and Overhead . . . . . . . . . . . . . . . . . . 6) s -5 547 M -( 4.6 Localization and Character Set Support . . . . . . . . . . . 7) s -5 536 M -( 5. Data Type Representations Used in the SSH Protocols . . . . 8) s -5 525 M -( 6. Algorithm Naming . . . . . . . . . . . . . . . . . . . . . . 10) s -5 514 M -( 7. Message Numbers . . . . . . . . . . . . . . . . . . . . . . 11) s -5 503 M -( 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . 11) s -5 492 M -( 9. Security Considerations . . . . . . . . . . . . . . . . . . 12) s -5 481 M -( 9.1 Pseudo-Random Number Generation . . . . . . . . . . . . . . 12) s -5 470 M -( 9.2 Transport . . . . . . . . . . . . . . . . . . . . . . . . . 13) s -5 459 M -( 9.2.1 Confidentiality . . . . . . . . . . . . . . . . . . . . . . 13) s -5 448 M -( 9.2.2 Data Integrity . . . . . . . . . . . . . . . . . . . . . . . 16) s -5 437 M -( 9.2.3 Replay . . . . . . . . . . . . . . . . . . . . . . . . . . . 16) s -5 426 M -( 9.2.4 Man-in-the-middle . . . . . . . . . . . . . . . . . . . . . 17) s -5 415 M -( 9.2.5 Denial-of-service . . . . . . . . . . . . . . . . . . . . . 19) s -5 404 M -( 9.2.6 Covert Channels . . . . . . . . . . . . . . . . . . . . . . 19) s -5 393 M -( 9.2.7 Forward Secrecy . . . . . . . . . . . . . . . . . . . . . . 20) s -5 382 M -( 9.3 Authentication Protocol . . . . . . . . . . . . . . . . . . 20) s -5 371 M -( 9.3.1 Weak Transport . . . . . . . . . . . . . . . . . . . . . . . 21) s -5 360 M -( 9.3.2 Debug messages . . . . . . . . . . . . . . . . . . . . . . . 21) s -5 349 M -( 9.3.3 Local security policy . . . . . . . . . . . . . . . . . . . 21) s -5 338 M -( 9.3.4 Public key authentication . . . . . . . . . . . . . . . . . 22) s -5 327 M -( 9.3.5 Password authentication . . . . . . . . . . . . . . . . . . 22) s -5 316 M -( 9.3.6 Host based authentication . . . . . . . . . . . . . . . . . 23) s -5 305 M -( 9.4 Connection protocol . . . . . . . . . . . . . . . . . . . . 23) s -5 294 M -( 9.4.1 End point security . . . . . . . . . . . . . . . . . . . . . 23) s -5 283 M -( 9.4.2 Proxy forwarding . . . . . . . . . . . . . . . . . . . . . . 23) s -5 272 M -( 9.4.3 X11 forwarding . . . . . . . . . . . . . . . . . . . . . . . 24) s -5 261 M -( Normative References . . . . . . . . . . . . . . . . . . . . 24) s -5 250 M -( Informative References . . . . . . . . . . . . . . . . . . . 25) s -5 239 M -( Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 27) s -5 228 M -( Intellectual Property and Copyright Statements . . . . . . . 28) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 2]) s -_R -S -PStoPSsaved restore -%%Page: (2,3) 2 -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 0.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -/showpage{}def/copypage{}def/erasepage{}def -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 3 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -(1. Contributors) s -5 668 M -( The major original contributors of this document were: Tatu Ylonen,) s -5 657 M -( Tero Kivinen, Timo J. Rinne, Sami Lehtinen \(all of SSH Communications) s -5 646 M -( Security Corp\), and Markku-Juhani O. Saarinen \(University of) s -5 635 M -( Jyvaskyla\)) s -5 613 M -( The document editor is: [email protected]. Comments on this) s -5 602 M -( internet draft should be sent to the IETF SECSH working group,) s -5 591 M -( details at: http://ietf.org/html.charters/secsh-charter.html) s -5 569 M -(2. Introduction) s -5 547 M -( SSH is a protocol for secure remote login and other secure network) s -5 536 M -( services over an insecure network. It consists of three major) s -5 525 M -( components:) s -5 514 M -( o The Transport Layer Protocol [SSH-TRANS] provides server) s -5 503 M -( authentication, confidentiality, and integrity. It may optionally) s -5 492 M -( also provide compression. The transport layer will typically be) s -5 481 M -( run over a TCP/IP connection, but might also be used on top of any) s -5 470 M -( other reliable data stream.) s -5 459 M -( o The User Authentication Protocol [SSH-USERAUTH] authenticates the) s -5 448 M -( client-side user to the server. It runs over the transport layer) s -5 437 M -( protocol.) s -5 426 M -( o The Connection Protocol [SSH-CONNECT] multiplexes the encrypted) s -5 415 M -( tunnel into several logical channels. It runs over the user) s -5 404 M -( authentication protocol.) s -5 382 M -( The client sends a service request once a secure transport layer) s -5 371 M -( connection has been established. A second service request is sent) s -5 360 M -( after user authentication is complete. This allows new protocols to) s -5 349 M -( be defined and coexist with the protocols listed above.) s -5 327 M -( The connection protocol provides channels that can be used for a wide) s -5 316 M -( range of purposes. Standard methods are provided for setting up) s -5 305 M -( secure interactive shell sessions and for forwarding \("tunneling"\)) s -5 294 M -( arbitrary TCP/IP ports and X11 connections.) s -5 272 M -(3. Specification of Requirements) s -5 250 M -( All documents related to the SSH protocols shall use the keywords) s -5 239 M -( "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",) s -5 228 M -( "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" to describe) s -5 217 M -( requirements. They are to be interpreted as described in [RFC2119].) s -5 195 M -(4. Architecture) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 3]) s -_R -S -PStoPSsaved restore -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 421.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 4 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -(4.1 Host Keys) s -5 668 M -( Each server host SHOULD have a host key. Hosts MAY have multiple) s -5 657 M -( host keys using multiple different algorithms. Multiple hosts MAY) s -5 646 M -( share the same host key. If a host has keys at all, it MUST have at) s -5 635 M -( least one key using each REQUIRED public key algorithm \(DSS) s -5 624 M -( [FIPS-186]\).) s -5 602 M -( The server host key is used during key exchange to verify that the) s -5 591 M -( client is really talking to the correct server. For this to be) s -5 580 M -( possible, the client must have a priori knowledge of the server's) s -5 569 M -( public host key.) s -5 547 M -( Two different trust models can be used:) s -5 536 M -( o The client has a local database that associates each host name \(as) s -5 525 M -( typed by the user\) with the corresponding public host key. This) s -5 514 M -( method requires no centrally administered infrastructure, and no) s -5 503 M -( third-party coordination. The downside is that the database of) s -5 492 M -( name-to-key associations may become burdensome to maintain.) s -5 481 M -( o The host name-to-key association is certified by some trusted) s -5 470 M -( certification authority. The client only knows the CA root key,) s -5 459 M -( and can verify the validity of all host keys certified by accepted) s -5 448 M -( CAs.) s -5 426 M -( The second alternative eases the maintenance problem, since) s -5 415 M -( ideally only a single CA key needs to be securely stored on the) s -5 404 M -( client. On the other hand, each host key must be appropriately) s -5 393 M -( certified by a central authority before authorization is possible.) s -5 382 M -( Also, a lot of trust is placed on the central infrastructure.) s -5 360 M -( The protocol provides the option that the server name - host key) s -5 349 M -( association is not checked when connecting to the host for the first) s -5 338 M -( time. This allows communication without prior communication of host) s -5 327 M -( keys or certification. The connection still provides protection) s -5 316 M -( against passive listening; however, it becomes vulnerable to active) s -5 305 M -( man-in-the-middle attacks. Implementations SHOULD NOT normally allow) s -5 294 M -( such connections by default, as they pose a potential security) s -5 283 M -( problem. However, as there is no widely deployed key infrastructure) s -5 272 M -( available on the Internet yet, this option makes the protocol much) s -5 261 M -( more usable during the transition time until such an infrastructure) s -5 250 M -( emerges, while still providing a much higher level of security than) s -5 239 M -( that offered by older solutions \(e.g. telnet [RFC-854] and rlogin) s -5 228 M -( [RFC-1282]\).) s -5 206 M -( Implementations SHOULD try to make the best effort to check host) s -5 195 M -( keys. An example of a possible strategy is to only accept a host key) s -5 184 M -( without checking the first time a host is connected, save the key in) s -5 173 M -( a local database, and compare against that key on all future) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 4]) s -_R -S -PStoPSsaved restore -%%Page: (4,5) 3 -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 0.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -/showpage{}def/copypage{}def/erasepage{}def -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 5 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( connections to that host.) s -5 668 M -( Implementations MAY provide additional methods for verifying the) s -5 657 M -( correctness of host keys, e.g. a hexadecimal fingerprint derived from) s -5 646 M -( the SHA-1 hash of the public key. Such fingerprints can easily be) s -5 635 M -( verified by using telephone or other external communication channels.) s -5 613 M -( All implementations SHOULD provide an option to not accept host keys) s -5 602 M -( that cannot be verified.) s -5 580 M -( We believe that ease of use is critical to end-user acceptance of) s -5 569 M -( security solutions, and no improvement in security is gained if the) s -5 558 M -( new solutions are not used. Thus, providing the option not to check) s -5 547 M -( the server host key is believed to improve the overall security of) s -5 536 M -( the Internet, even though it reduces the security of the protocol in) s -5 525 M -( configurations where it is allowed.) s -5 503 M -(4.2 Extensibility) s -5 481 M -( We believe that the protocol will evolve over time, and some) s -5 470 M -( organizations will want to use their own encryption, authentication) s -5 459 M -( and/or key exchange methods. Central registration of all extensions) s -5 448 M -( is cumbersome, especially for experimental or classified features.) s -5 437 M -( On the other hand, having no central registration leads to conflicts) s -5 426 M -( in method identifiers, making interoperability difficult.) s -5 404 M -( We have chosen to identify algorithms, methods, formats, and) s -5 393 M -( extension protocols with textual names that are of a specific format.) s -5 382 M -( DNS names are used to create local namespaces where experimental or) s -5 371 M -( classified extensions can be defined without fear of conflicts with) s -5 360 M -( other implementations.) s -5 338 M -( One design goal has been to keep the base protocol as simple as) s -5 327 M -( possible, and to require as few algorithms as possible. However, all) s -5 316 M -( implementations MUST support a minimal set of algorithms to ensure) s -5 305 M -( interoperability \(this does not imply that the local policy on all) s -5 294 M -( hosts would necessary allow these algorithms\). The mandatory) s -5 283 M -( algorithms are specified in the relevant protocol documents.) s -5 261 M -( Additional algorithms, methods, formats, and extension protocols can) s -5 250 M -( be defined in separate drafts. See Section Algorithm Naming \(Section) s -5 239 M -( 6\) for more information.) s -5 217 M -(4.3 Policy Issues) s -5 195 M -( The protocol allows full negotiation of encryption, integrity, key) s -5 184 M -( exchange, compression, and public key algorithms and formats.) s -5 173 M -( Encryption, integrity, public key, and compression algorithms can be) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 5]) s -_R -S -PStoPSsaved restore -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 421.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 6 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( different for each direction.) s -5 668 M -( The following policy issues SHOULD be addressed in the configuration) s -5 657 M -( mechanisms of each implementation:) s -5 646 M -( o Encryption, integrity, and compression algorithms, separately for) s -5 635 M -( each direction. The policy MUST specify which is the preferred) s -5 624 M -( algorithm \(e.g. the first algorithm listed in each category\).) s -5 613 M -( o Public key algorithms and key exchange method to be used for host) s -5 602 M -( authentication. The existence of trusted host keys for different) s -5 591 M -( public key algorithms also affects this choice.) s -5 580 M -( o The authentication methods that are to be required by the server) s -5 569 M -( for each user. The server's policy MAY require multiple) s -5 558 M -( authentication for some or all users. The required algorithms MAY) s -5 547 M -( depend on the location where the user is trying to log in from.) s -5 536 M -( o The operations that the user is allowed to perform using the) s -5 525 M -( connection protocol. Some issues are related to security; for) s -5 514 M -( example, the policy SHOULD NOT allow the server to start sessions) s -5 503 M -( or run commands on the client machine, and MUST NOT allow) s -5 492 M -( connections to the authentication agent unless forwarding such) s -5 481 M -( connections has been requested. Other issues, such as which TCP/) s -5 470 M -( IP ports can be forwarded and by whom, are clearly issues of local) s -5 459 M -( policy. Many of these issues may involve traversing or bypassing) s -5 448 M -( firewalls, and are interrelated with the local security policy.) s -5 426 M -(4.4 Security Properties) s -5 404 M -( The primary goal of the SSH protocol is improved security on the) s -5 393 M -( Internet. It attempts to do this in a way that is easy to deploy,) s -5 382 M -( even at the cost of absolute security.) s -5 371 M -( o All encryption, integrity, and public key algorithms used are) s -5 360 M -( well-known, well-established algorithms.) s -5 349 M -( o All algorithms are used with cryptographically sound key sizes) s -5 338 M -( that are believed to provide protection against even the strongest) s -5 327 M -( cryptanalytic attacks for decades.) s -5 316 M -( o All algorithms are negotiated, and in case some algorithm is) s -5 305 M -( broken, it is easy to switch to some other algorithm without) s -5 294 M -( modifying the base protocol.) s -5 272 M -( Specific concessions were made to make wide-spread fast deployment) s -5 261 M -( easier. The particular case where this comes up is verifying that) s -5 250 M -( the server host key really belongs to the desired host; the protocol) s -5 239 M -( allows the verification to be left out \(but this is NOT RECOMMENDED\).) s -5 228 M -( This is believed to significantly improve usability in the short) s -5 217 M -( term, until widespread Internet public key infrastructures emerge.) s -5 195 M -(4.5 Packet Size and Overhead) s -5 173 M -( Some readers will worry about the increase in packet size due to new) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 6]) s -_R -S -PStoPSsaved restore -%%Page: (6,7) 4 -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 0.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -/showpage{}def/copypage{}def/erasepage{}def -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 7 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( headers, padding, and MAC. The minimum packet size is in the order) s -5 679 M -( of 28 bytes \(depending on negotiated algorithms\). The increase is) s -5 668 M -( negligible for large packets, but very significant for one-byte) s -5 657 M -( packets \(telnet-type sessions\). There are, however, several factors) s -5 646 M -( that make this a non-issue in almost all cases:) s -5 635 M -( o The minimum size of a TCP/IP header is 32 bytes. Thus, the) s -5 624 M -( increase is actually from 33 to 51 bytes \(roughly\).) s -5 613 M -( o The minimum size of the data field of an Ethernet packet is 46) s -5 602 M -( bytes [RFC-894]. Thus, the increase is no more than 5 bytes. When) s -5 591 M -( Ethernet headers are considered, the increase is less than 10) s -5 580 M -( percent.) s -5 569 M -( o The total fraction of telnet-type data in the Internet is) s -5 558 M -( negligible, even with increased packet sizes.) s -5 536 M -( The only environment where the packet size increase is likely to have) s -5 525 M -( a significant effect is PPP [RFC-1134] over slow modem lines \(PPP) s -5 514 M -( compresses the TCP/IP headers, emphasizing the increase in packet) s -5 503 M -( size\). However, with modern modems, the time needed to transfer is in) s -5 492 M -( the order of 2 milliseconds, which is a lot faster than people can) s -5 481 M -( type.) s -5 459 M -( There are also issues related to the maximum packet size. To) s -5 448 M -( minimize delays in screen updates, one does not want excessively) s -5 437 M -( large packets for interactive sessions. The maximum packet size is) s -5 426 M -( negotiated separately for each channel.) s -5 404 M -(4.6 Localization and Character Set Support) s -5 382 M -( For the most part, the SSH protocols do not directly pass text that) s -5 371 M -( would be displayed to the user. However, there are some places where) s -5 360 M -( such data might be passed. When applicable, the character set for the) s -5 349 M -( data MUST be explicitly specified. In most places, ISO 10646 with) s -5 338 M -( UTF-8 encoding is used [RFC-2279]. When applicable, a field is also) s -5 327 M -( provided for a language tag [RFC-3066].) s -5 305 M -( One big issue is the character set of the interactive session. There) s -5 294 M -( is no clear solution, as different applications may display data in) s -5 283 M -( different formats. Different types of terminal emulation may also be) s -5 272 M -( employed in the client, and the character set to be used is) s -5 261 M -( effectively determined by the terminal emulation. Thus, no place is) s -5 250 M -( provided for directly specifying the character set or encoding for) s -5 239 M -( terminal session data. However, the terminal emulation type \(e.g.) s -5 228 M -( "vt100"\) is transmitted to the remote site, and it implicitly) s -5 217 M -( specifies the character set and encoding. Applications typically use) s -5 206 M -( the terminal type to determine what character set they use, or the) s -5 195 M -( character set is determined using some external means. The terminal) s -5 184 M -( emulation may also allow configuring the default character set. In) s -5 173 M -( any case, the character set for the terminal session is considered) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 7]) s -_R -S -PStoPSsaved restore -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 421.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 8 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( primarily a client local issue.) s -5 668 M -( Internal names used to identify algorithms or protocols are normally) s -5 657 M -( never displayed to users, and must be in US-ASCII.) s -5 635 M -( The client and server user names are inherently constrained by what) s -5 624 M -( the server is prepared to accept. They might, however, occasionally) s -5 613 M -( be displayed in logs, reports, etc. They MUST be encoded using ISO) s -5 602 M -( 10646 UTF-8, but other encodings may be required in some cases. It) s -5 591 M -( is up to the server to decide how to map user names to accepted user) s -5 580 M -( names. Straight bit-wise binary comparison is RECOMMENDED.) s -5 558 M -( For localization purposes, the protocol attempts to minimize the) s -5 547 M -( number of textual messages transmitted. When present, such messages) s -5 536 M -( typically relate to errors, debugging information, or some externally) s -5 525 M -( configured data. For data that is normally displayed, it SHOULD be) s -5 514 M -( possible to fetch a localized message instead of the transmitted) s -5 503 M -( message by using a numerical code. The remaining messages SHOULD be) s -5 492 M -( configurable.) s -5 470 M -(5. Data Type Representations Used in the SSH Protocols) s -5 459 M -( byte) s -5 437 M -( A byte represents an arbitrary 8-bit value \(octet\) [RFC-1700].) s -5 426 M -( Fixed length data is sometimes represented as an array of bytes,) s -5 415 M -( written byte[n], where n is the number of bytes in the array.) s -5 393 M -( boolean) s -5 371 M -( A boolean value is stored as a single byte. The value 0) s -5 360 M -( represents FALSE, and the value 1 represents TRUE. All non-zero) s -5 349 M -( values MUST be interpreted as TRUE; however, applications MUST NOT) s -5 338 M -( store values other than 0 and 1.) s -5 316 M -( uint32) s -5 294 M -( Represents a 32-bit unsigned integer. Stored as four bytes in the) s -5 283 M -( order of decreasing significance \(network byte order\). For) s -5 272 M -( example, the value 699921578 \(0x29b7f4aa\) is stored as 29 b7 f4) s -5 261 M -( aa.) s -5 239 M -( uint64) s -5 217 M -( Represents a 64-bit unsigned integer. Stored as eight bytes in) s -5 206 M -( the order of decreasing significance \(network byte order\).) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 8]) s -_R -S -PStoPSsaved restore -%%Page: (8,9) 5 -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 0.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -/showpage{}def/copypage{}def/erasepage{}def -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 9 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( string) s -5 668 M -( Arbitrary length binary string. Strings are allowed to contain) s -5 657 M -( arbitrary binary data, including null characters and 8-bit) s -5 646 M -( characters. They are stored as a uint32 containing its length) s -5 635 M -( \(number of bytes that follow\) and zero \(= empty string\) or more) s -5 624 M -( bytes that are the value of the string. Terminating null) s -5 613 M -( characters are not used.) s -5 591 M -( Strings are also used to store text. In that case, US-ASCII is) s -5 580 M -( used for internal names, and ISO-10646 UTF-8 for text that might) s -5 569 M -( be displayed to the user. The terminating null character SHOULD) s -5 558 M -( NOT normally be stored in the string.) s -5 536 M -( For example, the US-ASCII string "testing" is represented as 00 00) s -5 525 M -( 00 07 t e s t i n g. The UTF8 mapping does not alter the encoding) s -5 514 M -( of US-ASCII characters.) s -5 492 M -( mpint) s -5 470 M -( Represents multiple precision integers in two's complement format,) s -5 459 M -( stored as a string, 8 bits per byte, MSB first. Negative numbers) s -5 448 M -( have the value 1 as the most significant bit of the first byte of) s -5 437 M -( the data partition. If the most significant bit would be set for a) s -5 426 M -( positive number, the number MUST be preceded by a zero byte.) s -5 415 M -( Unnecessary leading bytes with the value 0 or 255 MUST NOT be) s -5 404 M -( included. The value zero MUST be stored as a string with zero) s -5 393 M -( bytes of data.) s -5 371 M -( By convention, a number that is used in modular computations in) s -5 360 M -( Z_n SHOULD be represented in the range 0 <= x < n.) s -5 338 M -( Examples:) s -5 327 M -( value \(hex\) representation \(hex\)) s -5 316 M -( ---------------------------------------------------------------) s -5 305 M -( 0 00 00 00 00) s -5 294 M -( 9a378f9b2e332a7 00 00 00 08 09 a3 78 f9 b2 e3 32 a7) s -5 283 M -( 80 00 00 00 02 00 80) s -5 272 M -( -1234 00 00 00 02 ed cc) s -5 261 M -( -deadbeef 00 00 00 05 ff 21 52 41 11) s -5 217 M -( name-list) s -5 195 M -( A string containing a comma separated list of names. A name list) s -5 184 M -( is represented as a uint32 containing its length \(number of bytes) s -5 173 M -( that follow\) followed by a comma-separated list of zero or more) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 9]) s -_R -S -PStoPSsaved restore -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 421.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 10 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( names. A name MUST be non-zero length, and it MUST NOT contain a) s -5 679 M -( comma \(','\). Context may impose additional restrictions on the) s -5 668 M -( names; for example, the names in a list may have to be valid) s -5 657 M -( algorithm identifier \(see Algorithm Naming below\), or [RFC-3066]) s -5 646 M -( language tags. The order of the names in a list may or may not be) s -5 635 M -( significant, also depending on the context where the list is is) s -5 624 M -( used. Terminating NUL characters are not used, neither for the) s -5 613 M -( individual names, nor for the list as a whole.) s -5 591 M -( Examples:) s -5 580 M -( value representation \(hex\)) s -5 569 M -( ---------------------------------------) s -5 558 M -( \(\), the empty list 00 00 00 00) s -5 547 M -( \("zlib"\) 00 00 00 04 7a 6c 69 62) s -5 536 M -( \("zlib", "none"\) 00 00 00 09 7a 6c 69 62 2c 6e 6f 6e 65) s -5 481 M -(6. Algorithm Naming) s -5 459 M -( The SSH protocols refer to particular hash, encryption, integrity,) s -5 448 M -( compression, and key exchange algorithms or protocols by names.) s -5 437 M -( There are some standard algorithms that all implementations MUST) s -5 426 M -( support. There are also algorithms that are defined in the protocol) s -5 415 M -( specification but are OPTIONAL. Furthermore, it is expected that) s -5 404 M -( some organizations will want to use their own algorithms.) s -5 382 M -( In this protocol, all algorithm identifiers MUST be printable) s -5 371 M -( US-ASCII non-empty strings no longer than 64 characters. Names MUST) s -5 360 M -( be case-sensitive.) s -5 338 M -( There are two formats for algorithm names:) s -5 327 M -( o Names that do not contain an at-sign \(@\) are reserved to be) s -5 316 M -( assigned by IETF consensus \(RFCs\). Examples include `3des-cbc',) s -5 305 M -( `sha-1', `hmac-sha1', and `zlib' \(the quotes are not part of the) s -5 294 M -( name\). Names of this format MUST NOT be used without first) s -5 283 M -( registering them. Registered names MUST NOT contain an at-sign) s -5 272 M -( \(@\) or a comma \(,\).) s -5 261 M -( o Anyone can define additional algorithms by using names in the) s -5 250 M -( format name@domainname, e.g. "[email protected]". The) s -5 239 M -( format of the part preceding the at sign is not specified; it MUST) s -5 228 M -( consist of US-ASCII characters except at-sign and comma. The part) s -5 217 M -( following the at-sign MUST be a valid fully qualified internet) s -5 206 M -( domain name [RFC-1034] controlled by the person or organization) s -5 195 M -( defining the name. It is up to each domain how it manages its) s -5 184 M -( local namespace.) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 10]) s -_R -S -PStoPSsaved restore -%%Page: (10,11) 6 -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 0.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -/showpage{}def/copypage{}def/erasepage{}def -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 11 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -(7. Message Numbers) s -5 668 M -( SSH packets have message numbers in the range 1 to 255. These numbers) s -5 657 M -( have been allocated as follows:) s -5 624 M -( Transport layer protocol:) s -5 602 M -( 1 to 19 Transport layer generic \(e.g. disconnect, ignore, debug,) s -5 591 M -( etc.\)) s -5 580 M -( 20 to 29 Algorithm negotiation) s -5 569 M -( 30 to 49 Key exchange method specific \(numbers can be reused for) s -5 558 M -( different authentication methods\)) s -5 536 M -( User authentication protocol:) s -5 514 M -( 50 to 59 User authentication generic) s -5 503 M -( 60 to 79 User authentication method specific \(numbers can be) s -5 492 M -( reused for different authentication methods\)) s -5 470 M -( Connection protocol:) s -5 448 M -( 80 to 89 Connection protocol generic) s -5 437 M -( 90 to 127 Channel related messages) s -5 415 M -( Reserved for client protocols:) s -5 393 M -( 128 to 191 Reserved) s -5 371 M -( Local extensions:) s -5 349 M -( 192 to 255 Local extensions) s -5 305 M -(8. IANA Considerations) s -5 283 M -( The initial state of the IANA registry is detailed in [SSH-NUMBERS].) s -5 261 M -( Allocation of the following types of names in the SSH protocols is) s -5 250 M -( assigned by IETF consensus:) s -5 239 M -( o SSH encryption algorithm names,) s -5 228 M -( o SSH MAC algorithm names,) s -5 217 M -( o SSH public key algorithm names \(public key algorithm also implies) s -5 206 M -( encoding and signature/encryption capability\),) s -5 195 M -( o SSH key exchange method names, and) s -5 184 M -( o SSH protocol \(service\) names.) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 11]) s -_R -S -PStoPSsaved restore -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 421.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 12 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( These names MUST be printable US-ASCII strings, and MUST NOT contain) s -5 679 M -( the characters at-sign \('@'\), comma \(','\), or whitespace or control) s -5 668 M -( characters \(ASCII codes 32 or less\). Names are case-sensitive, and) s -5 657 M -( MUST NOT be longer than 64 characters.) s -5 635 M -( Names with the at-sign \('@'\) in them are allocated by the owner of) s -5 624 M -( DNS name after the at-sign \(hierarchical allocation in [RFC-2343]\),) s -5 613 M -( otherwise the same restrictions as above.) s -5 591 M -( Each category of names listed above has a separate namespace.) s -5 580 M -( However, using the same name in multiple categories SHOULD be avoided) s -5 569 M -( to minimize confusion.) s -5 547 M -( Message numbers \(see Section Message Numbers \(Section 7\)\) in the) s -5 536 M -( range of 0..191 are allocated via IETF consensus; message numbers in) s -5 525 M -( the 192..255 range \(the "Local extensions" set\) are reserved for) s -5 514 M -( private use.) s -5 492 M -(9. Security Considerations) s -5 470 M -( In order to make the entire body of Security Considerations more) s -5 459 M -( accessible, Security Considerations for the transport,) s -5 448 M -( authentication, and connection documents have been gathered here.) s -5 426 M -( The transport protocol [1] provides a confidential channel over an) s -5 415 M -( insecure network. It performs server host authentication, key) s -5 404 M -( exchange, encryption, and integrity protection. It also derives a) s -5 393 M -( unique session id that may be used by higher-level protocols.) s -5 371 M -( The authentication protocol [2] provides a suite of mechanisms which) s -5 360 M -( can be used to authenticate the client user to the server.) s -5 349 M -( Individual mechanisms specified in the in authentication protocol use) s -5 338 M -( the session id provided by the transport protocol and/or depend on) s -5 327 M -( the security and integrity guarantees of the transport protocol.) s -5 305 M -( The connection protocol [3] specifies a mechanism to multiplex) s -5 294 M -( multiple streams [channels] of data over the confidential and) s -5 283 M -( authenticated transport. It also specifies channels for accessing an) s -5 272 M -( interactive shell, for 'proxy-forwarding' various external protocols) s -5 261 M -( over the secure transport \(including arbitrary TCP/IP protocols\), and) s -5 250 M -( for accessing secure 'subsystems' on the server host.) s -5 228 M -(9.1 Pseudo-Random Number Generation) s -5 206 M -( This protocol binds each session key to the session by including) s -5 195 M -( random, session specific data in the hash used to produce session) s -5 184 M -( keys. Special care should be taken to ensure that all of the random) s -5 173 M -( numbers are of good quality. If the random data here \(e.g., DH) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 12]) s -_R -S -PStoPSsaved restore -%%Page: (12,13) 7 -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 0.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -/showpage{}def/copypage{}def/erasepage{}def -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 13 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( parameters\) are pseudo-random then the pseudo-random number generator) s -5 679 M -( should be cryptographically secure \(i.e., its next output not easily) s -5 668 M -( guessed even when knowing all previous outputs\) and, furthermore,) s -5 657 M -( proper entropy needs to be added to the pseudo-random number) s -5 646 M -( generator. RFC 1750 [1750] offers suggestions for sources of random) s -5 635 M -( numbers and entropy. Implementors should note the importance of) s -5 624 M -( entropy and the well-meant, anecdotal warning about the difficulty in) s -5 613 M -( properly implementing pseudo-random number generating functions.) s -5 591 M -( The amount of entropy available to a given client or server may) s -5 580 M -( sometimes be less than what is required. In this case one must) s -5 569 M -( either resort to pseudo-random number generation regardless of) s -5 558 M -( insufficient entropy or refuse to run the protocol. The latter is) s -5 547 M -( preferable.) s -5 525 M -(9.2 Transport) s -5 503 M -(9.2.1 Confidentiality) s -5 481 M -( It is beyond the scope of this document and the Secure Shell Working) s -5 470 M -( Group to analyze or recommend specific ciphers other than the ones) s -5 459 M -( which have been established and accepted within the industry. At the) s -5 448 M -( time of this writing, ciphers commonly in use include 3DES, ARCFOUR,) s -5 437 M -( twofish, serpent and blowfish. AES has been accepted by The) s -5 426 M -( published as a US Federal Information Processing Standards [FIPS-197]) s -5 415 M -( and the cryptographic community as being acceptable for this purpose) s -5 404 M -( as well has accepted AES. As always, implementors and users should) s -5 393 M -( check current literature to ensure that no recent vulnerabilities) s -5 382 M -( have been found in ciphers used within products. Implementors should) s -5 371 M -( also check to see which ciphers are considered to be relatively) s -5 360 M -( stronger than others and should recommend their use to users over) s -5 349 M -( relatively weaker ciphers. It would be considered good form for an) s -5 338 M -( implementation to politely and unobtrusively notify a user that a) s -5 327 M -( stronger cipher is available and should be used when a weaker one is) s -5 316 M -( actively chosen.) s -5 294 M -( The "none" cipher is provided for debugging and SHOULD NOT be used) s -5 283 M -( except for that purpose. It's cryptographic properties are) s -5 272 M -( sufficiently described in RFC 2410, which will show that its use does) s -5 261 M -( not meet the intent of this protocol.) s -5 239 M -( The relative merits of these and other ciphers may also be found in) s -5 228 M -( current literature. Two references that may provide information on) s -5 217 M -( the subject are [SCHNEIER] and [KAUFMAN,PERLMAN,SPECINER]. Both of) s -5 206 M -( these describe the CBC mode of operation of certain ciphers and the) s -5 195 M -( weakness of this scheme. Essentially, this mode is theoretically) s -5 184 M -( vulnerable to chosen cipher-text attacks because of the high) s -5 173 M -( predictability of the start of packet sequence. However, this attack) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 13]) s -_R -S -PStoPSsaved restore -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 421.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 14 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( is still deemed difficult and not considered fully practicable) s -5 679 M -( especially if relatively longer block sizes are used.) s -5 657 M -( Additionally, another CBC mode attack may be mitigated through the) s -5 646 M -( insertion of packets containing SSH_MSG_IGNORE. Without this) s -5 635 M -( technique, a specific attack may be successful. For this attack) s -5 624 M -( \(commonly known as the Rogaway attack) s -5 613 M -( [ROGAWAY],[DAI],[BELLARE,KOHNO,NAMPREMPRE]\) to work, the attacker) s -5 602 M -( would need to know the IV of the next block that is going to be) s -5 591 M -( encrypted. In CBC mode that is the output of the encryption of the) s -5 580 M -( previous block. If the attacker does not have any way to see the) s -5 569 M -( packet yet \(i.e it is in the internal buffers of the ssh) s -5 558 M -( implementation or even in the kernel\) then this attack will not work.) s -5 547 M -( If the last packet has been sent out to the network \(i.e the attacker) s -5 536 M -( has access to it\) then he can use the attack.) s -5 514 M -( In the optimal case an implementor would need to add an extra packet) s -5 503 M -( only if the packet has been sent out onto the network and there are) s -5 492 M -( no other packets waiting for transmission. Implementors may wish to) s -5 481 M -( check to see if there are any unsent packets awaiting transmission,) s -5 470 M -( but unfortunately it is not normally easy to obtain this information) s -5 459 M -( from the kernel or buffers. If there are not, then a packet) s -5 448 M -( containing SSH_MSG_IGNORE SHOULD be sent. If a new packet is added) s -5 437 M -( to the stream every time the attacker knows the IV that is supposed) s -5 426 M -( to be used for the next packet, then the attacker will not be able to) s -5 415 M -( guess the correct IV, thus the attack will never be successfull.) s -5 393 M -( As an example, consider the following case:) s -5 360 M -( Client Server) s -5 349 M -( ------ ------) s -5 338 M -( TCP\(seq=x, len=500\) ->) s -5 327 M -( contains Record 1) s -5 305 M -( [500 ms passes, no ACK]) s -5 283 M -( TCP\(seq=x, len=1000\) ->) s -5 272 M -( contains Records 1,2) s -5 250 M -( ACK) s -5 217 M -( 1. The Nagle algorithm + TCP retransmits mean that the two records) s -5 206 M -( get coalesced into a single TCP segment) s -5 195 M -( 2. Record 2 is *not* at the beginning of the TCP segment and never) s -5 184 M -( will be, since it gets ACKed.) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 14]) s -_R -S -PStoPSsaved restore -%%Page: (14,15) 8 -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 0.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -/showpage{}def/copypage{}def/erasepage{}def -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 15 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( 3. Yet, the attack is possible because Record 1 has already been) s -5 679 M -( seen.) s -5 657 M -( As this example indicates, it's totally unsafe to use the existence) s -5 646 M -( of unflushed data in the TCP buffers proper as a guide to whether you) s -5 635 M -( need an empty packet, since when you do the second write\(\), the) s -5 624 M -( buffers will contain the un-ACKed Record 1.) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 15]) s -_R -S -PStoPSsaved restore -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 421.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 16 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( On the other hand, it's perfectly safe to have the following) s -5 679 M -( situation:) s -5 646 M -( Client Server) s -5 635 M -( ------ ------) s -5 624 M -( TCP\(seq=x, len=500\) ->) s -5 613 M -( contains SSH_MSG_IGNORE) s -5 591 M -( TCP\(seq=y, len=500\) ->) s -5 580 M -( contains Data) s -5 558 M -( Provided that the IV for second SSH Record is fixed after the data for) s -5 547 M -( the Data packet is determined -i.e. you do:) s -5 536 M -( read from user) s -5 525 M -( encrypt null packet) s -5 514 M -( encrypt data packet) s -5 481 M -(9.2.2 Data Integrity) s -5 459 M -( This protocol does allow the Data Integrity mechanism to be disabled.) s -5 448 M -( Implementors SHOULD be wary of exposing this feature for any purpose) s -5 437 M -( other than debugging. Users and administrators SHOULD be explicitly) s -5 426 M -( warned anytime the "none" MAC is enabled.) s -5 404 M -( So long as the "none" MAC is not used, this protocol provides data) s -5 393 M -( integrity.) s -5 371 M -( Because MACs use a 32 bit sequence number, they might start to leak) s -5 360 M -( information after 2**32 packets have been sent. However, following) s -5 349 M -( the rekeying recommendations should prevent this attack. The) s -5 338 M -( transport protocol [1] recommends rekeying after one gigabyte of) s -5 327 M -( data, and the smallest possible packet is 16 bytes. Therefore,) s -5 316 M -( rekeying SHOULD happen after 2**28 packets at the very most.) s -5 294 M -(9.2.3 Replay) s -5 272 M -( The use of a MAC other than 'none' provides integrity and) s -5 261 M -( authentication. In addition, the transport protocol provides a) s -5 250 M -( unique session identifier \(bound in part to pseudo-random data that) s -5 239 M -( is part of the algorithm and key exchange process\) that can be used) s -5 228 M -( by higher level protocols to bind data to a given session and prevent) s -5 217 M -( replay of data from prior sessions. For example, the authentication) s -5 206 M -( protocol uses this to prevent replay of signatures from previous) s -5 195 M -( sessions. Because public key authentication exchanges are) s -5 184 M -( cryptographically bound to the session \(i.e., to the initial key) s -5 173 M -( exchange\) they cannot be successfully replayed in other sessions.) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 16]) s -_R -S -PStoPSsaved restore -%%Page: (16,17) 9 -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 0.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -/showpage{}def/copypage{}def/erasepage{}def -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 17 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( Note that the session ID can be made public without harming the) s -5 679 M -( security of the protocol.) s -5 657 M -( If two session happen to have the same session ID [hash of key) s -5 646 M -( exchanges] then packets from one can be replayed against the other.) s -5 635 M -( It must be stressed that the chances of such an occurrence are,) s -5 624 M -( needless to say, minimal when using modern cryptographic methods.) s -5 613 M -( This is all the more so true when specifying larger hash function) s -5 602 M -( outputs and DH parameters.) s -5 580 M -( Replay detection using monotonically increasing sequence numbers as) s -5 569 M -( input to the MAC, or HMAC in some cases, is described in [RFC2085] />) s -5 558 M -( [RFC2246], [RFC2743], [RFC1964], [RFC2025], and [RFC1510]. The) s -5 547 M -( underlying construct is discussed in [RFC2104]. Essentially a) s -5 536 M -( different sequence number in each packet ensures that at least this) s -5 525 M -( one input to the MAC function will be unique and will provide a) s -5 514 M -( nonrecurring MAC output that is not predictable to an attacker. If) s -5 503 M -( the session stays active long enough, however, this sequence number) s -5 492 M -( will wrap. This event may provide an attacker an opportunity to) s -5 481 M -( replay a previously recorded packet with an identical sequence number) s -5 470 M -( but only if the peers have not rekeyed since the transmission of the) s -5 459 M -( first packet with that sequence number. If the peers have rekeyed,) s -5 448 M -( then the replay will be detected as the MAC check will fail. For) s -5 437 M -( this reason, it must be emphasized that peers MUST rekey before a) s -5 426 M -( wrap of the sequence numbers. Naturally, if an attacker does attempt) s -5 415 M -( to replay a captured packet before the peers have rekeyed, then the) s -5 404 M -( receiver of the duplicate packet will not be able to validate the MAC) s -5 393 M -( and it will be discarded. The reason that the MAC will fail is) s -5 382 M -( because the receiver will formulate a MAC based upon the packet) s -5 371 M -( contents, the shared secret, and the expected sequence number. Since) s -5 360 M -( the replayed packet will not be using that expected sequence number) s -5 349 M -( \(the sequence number of the replayed packet will have already been) s -5 338 M -( passed by the receiver\) then the calculated MAC will not match the) s -5 327 M -( MAC received with the packet.) s -5 305 M -(9.2.4 Man-in-the-middle) s -5 283 M -( This protocol makes no assumptions nor provisions for an) s -5 272 M -( infrastructure or means for distributing the public keys of hosts. It) s -5 261 M -( is expected that this protocol will sometimes be used without first) s -5 250 M -( verifying the association between the server host key and the server) s -5 239 M -( host name. Such usage is vulnerable to man-in-the-middle attacks.) s -5 228 M -( This section describes this and encourages administrators and users) s -5 217 M -( to understand the importance of verifying this association before any) s -5 206 M -( session is initiated.) s -5 184 M -( There are three cases of man-in-the-middle attacks to consider. The) s -5 173 M -( first is where an attacker places a device between the client and the) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 17]) s -_R -S -PStoPSsaved restore -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 421.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 18 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( server before the session is initiated. In this case, the attack) s -5 679 M -( device is trying to mimic the legitimate server and will offer its) s -5 668 M -( public key to the client when the client initiates a session. If it) s -5 657 M -( were to offer the public key of the server, then it would not be able) s -5 646 M -( to decrypt or sign the transmissions between the legitimate server) s -5 635 M -( and the client unless it also had access to the private-key of the) s -5 624 M -( host. The attack device will also, simultaneously to this, initiate) s -5 613 M -( a session to the legitimate server masquerading itself as the client.) s -5 602 M -( If the public key of the server had been securely distributed to the) s -5 591 M -( client prior to that session initiation, the key offered to the) s -5 580 M -( client by the attack device will not match the key stored on the) s -5 569 M -( client. In that case, the user SHOULD be given a warning that the) s -5 558 M -( offered host key does not match the host key cached on the client.) s -5 547 M -( As described in Section 3.1 of [ARCH], the user may be free to accept) s -5 536 M -( the new key and continue the session. It is RECOMMENDED that the) s -5 525 M -( warning provide sufficient information to the user of the client) s -5 514 M -( device so they may make an informed decision. If the user chooses to) s -5 503 M -( continue the session with the stored public-key of the server \(not) s -5 492 M -( the public-key offered at the start of the session\), then the session) s -5 481 M -( specific data between the attacker and server will be different) s -5 470 M -( between the client-to-attacker session and the attacker-to-server) s -5 459 M -( sessions due to the randomness discussed above. From this, the) s -5 448 M -( attacker will not be able to make this attack work since the attacker) s -5 437 M -( will not be able to correctly sign packets containing this session) s -5 426 M -( specific data from the server since he does not have the private key) s -5 415 M -( of that server.) s -5 393 M -( The second case that should be considered is similar to the first) s -5 382 M -( case in that it also happens at the time of connection but this case) s -5 371 M -( points out the need for the secure distribution of server public) s -5 360 M -( keys. If the server public keys are not securely distributed then) s -5 349 M -( the client cannot know if it is talking to the intended server. An) s -5 338 M -( attacker may use social engineering techniques to pass off server) s -5 327 M -( keys to unsuspecting users and may then place a man-in-the-middle) s -5 316 M -( attack device between the legitimate server and the clients. If this) s -5 305 M -( is allowed to happen then the clients will form client-to-attacker) s -5 294 M -( sessions and the attacker will form attacker-to-server sessions and) s -5 283 M -( will be able to monitor and manipulate all of the traffic between the) s -5 272 M -( clients and the legitimate servers. Server administrators are) s -5 261 M -( encouraged to make host key fingerprints available for checking by) s -5 250 M -( some means whose security does not rely on the integrity of the) s -5 239 M -( actual host keys. Possible mechanisms are discussed in Section 3.1) s -5 228 M -( of [SSH-ARCH] and may also include secured Web pages, physical pieces) s -5 217 M -( of paper, etc. Implementors SHOULD provide recommendations on how) s -5 206 M -( best to do this with their implementation. Because the protocol is) s -5 195 M -( extensible, future extensions to the protocol may provide better) s -5 184 M -( mechanisms for dealing with the need to know the server's host key) s -5 173 M -( before connecting. For example, making the host key fingerprint) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 18]) s -_R -S -PStoPSsaved restore -%%Page: (18,19) 10 -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 0.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -/showpage{}def/copypage{}def/erasepage{}def -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 19 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( available through a secure DNS lookup, or using kerberos over gssapi) s -5 679 M -( during key exchange to authenticate the server are possibilities.) s -5 657 M -( In the third man-in-the-middle case, attackers may attempt to) s -5 646 M -( manipulate packets in transit between peers after the session has) s -5 635 M -( been established. As described in the Replay part of this section, a) s -5 624 M -( successful attack of this nature is very improbable. As in the) s -5 613 M -( Replay section, this reasoning does assume that the MAC is secure and) s -5 602 M -( that it is infeasible to construct inputs to a MAC algorithm to give) s -5 591 M -( a known output. This is discussed in much greater detail in Section) s -5 580 M -( 6 of RFC 2104. If the MAC algorithm has a vulnerability or is weak) s -5 569 M -( enough, then the attacker may be able to specify certain inputs to) s -5 558 M -( yield a known MAC. With that they may be able to alter the contents) s -5 547 M -( of a packet in transit. Alternatively the attacker may be able to) s -5 536 M -( exploit the algorithm vulnerability or weakness to find the shared) s -5 525 M -( secret by reviewing the MACs from captured packets. In either of) s -5 514 M -( those cases, an attacker could construct a packet or packets that) s -5 503 M -( could be inserted into an SSH stream. To prevent that, implementors) s -5 492 M -( are encouraged to utilize commonly accepted MAC algorithms and) s -5 481 M -( administrators are encouraged to watch current literature and) s -5 470 M -( discussions of cryptography to ensure that they are not using a MAC) s -5 459 M -( algorithm that has a recently found vulnerability or weakness.) s -5 437 M -( In summary, the use of this protocol without a reliable association) s -5 426 M -( of the binding between a host and its host keys is inherently) s -5 415 M -( insecure and is NOT RECOMMENDED. It may however be necessary in) s -5 404 M -( non-security critical environments, and will still provide protection) s -5 393 M -( against passive attacks. Implementors of protocols and applications) s -5 382 M -( running on top of this protocol should keep this possibility in mind.) s -5 360 M -(9.2.5 Denial-of-service) s -5 338 M -( This protocol is designed to be used over a reliable transport. If) s -5 327 M -( transmission errors or message manipulation occur, the connection is) s -5 316 M -( closed. The connection SHOULD be re-established if this occurs.) s -5 305 M -( Denial of service attacks of this type \("wire cutter"\) are almost) s -5 294 M -( impossible to avoid.) s -5 272 M -( In addition, this protocol is vulnerable to Denial of Service attacks) s -5 261 M -( because an attacker can force the server to go through the CPU and) s -5 250 M -( memory intensive tasks of connection setup and key exchange without) s -5 239 M -( authenticating. Implementors SHOULD provide features that make this) s -5 228 M -( more difficult. For example, only allowing connections from a subset) s -5 217 M -( of IPs known to have valid users.) s -5 195 M -(9.2.6 Covert Channels) s -5 173 M -( The protocol was not designed to eliminate covert channels. For) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 19]) s -_R -S -PStoPSsaved restore -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 421.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 20 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( example, the padding, SSH_MSG_IGNORE messages, and several other) s -5 679 M -( places in the protocol can be used to pass covert information, and) s -5 668 M -( the recipient has no reliable way to verify whether such information) s -5 657 M -( is being sent.) s -5 635 M -(9.2.7 Forward Secrecy) s -5 613 M -( It should be noted that the Diffie-Hellman key exchanges may provide) s -5 602 M -( perfect forward secrecy \(PFS\). PFS is essentially defined as the) s -5 591 M -( cryptographic property of a key-establishment protocol in which the) s -5 580 M -( compromise of a session key or long-term private key after a given) s -5 569 M -( session does not cause the compromise of any earlier session. [ANSI) s -5 558 M -( T1.523-2001] SSHv2 sessions resulting from a key exchange using) s -5 547 M -( diffie-hellman-group1-sha1 are secure even if private keying/) s -5 536 M -( authentication material is later revealed, but not if the session) s -5 525 M -( keys are revealed. So, given this definition of PFS, SSHv2 does have) s -5 514 M -( PFS. It is hoped that all other key exchange mechanisms proposed and) s -5 503 M -( used in the future will also provide PFS. This property is not) s -5 492 M -( commuted to any of the applications or protocols using SSH as a) s -5 481 M -( transport however. The transport layer of SSH provides) s -5 470 M -( confidentiality for password authentication and other methods that) s -5 459 M -( rely on secret data.) s -5 437 M -( Of course, if the DH private parameters for the client and server are) s -5 426 M -( revealed then the session key is revealed, but these items can be) s -5 415 M -( thrown away after the key exchange completes. It's worth pointing) s -5 404 M -( out that these items should not be allowed to end up on swap space) s -5 393 M -( and that they should be erased from memory as soon as the key) s -5 382 M -( exchange completes.) s -5 360 M -(9.3 Authentication Protocol) s -5 338 M -( The purpose of this protocol is to perform client user) s -5 327 M -( authentication. It assumes that this run over a secure transport) s -5 316 M -( layer protocol, which has already authenticated the server machine,) s -5 305 M -( established an encrypted communications channel, and computed a) s -5 294 M -( unique session identifier for this session.) s -5 272 M -( Several authentication methods with different security) s -5 261 M -( characteristics are allowed. It is up to the server's local policy) s -5 250 M -( to decide which methods \(or combinations of methods\) it is willing to) s -5 239 M -( accept for each user. Authentication is no stronger than the weakest) s -5 228 M -( combination allowed.) s -5 206 M -( The server may go into a "sleep" period after repeated unsuccessful) s -5 195 M -( authentication attempts to make key search more difficult for) s -5 184 M -( attackers. Care should be taken so that this doesn't become a) s -5 173 M -( self-denial of service vector.) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 20]) s -_R -S -PStoPSsaved restore -%%Page: (20,21) 11 -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 0.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -/showpage{}def/copypage{}def/erasepage{}def -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 21 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -(9.3.1 Weak Transport) s -5 668 M -( If the transport layer does not provide confidentiality,) s -5 657 M -( authentication methods that rely on secret data SHOULD be disabled.) s -5 646 M -( If it does not provide strong integrity protection, requests to) s -5 635 M -( change authentication data \(e.g. a password change\) SHOULD be) s -5 624 M -( disabled to prevent an attacker from modifying the ciphertext) s -5 613 M -( without being noticed, or rendering the new authentication data) s -5 602 M -( unusable \(denial of service\).) s -5 580 M -( The assumption as stated above that the Authentication Protocol only) s -5 569 M -( run over a secure transport that has previously authenticated the) s -5 558 M -( server is very important to note. People deploying SSH are reminded) s -5 547 M -( of the consequences of man-in-the-middle attacks if the client does) s -5 536 M -( not have a very strong a priori association of the server with the) s -5 525 M -( host key of that server. Specifically for the case of the) s -5 514 M -( Authentication Protocol the client may form a session to a) s -5 503 M -( man-in-the-middle attack device and divulge user credentials such as) s -5 492 M -( their username and password. Even in the cases of authentication) s -5 481 M -( where no user credentials are divulged, an attacker may still gain) s -5 470 M -( information they shouldn't have by capturing key-strokes in much the) s -5 459 M -( same way that a honeypot works.) s -5 437 M -(9.3.2 Debug messages) s -5 415 M -( Special care should be taken when designing debug messages. These) s -5 404 M -( messages may reveal surprising amounts of information about the host) s -5 393 M -( if not properly designed. Debug messages can be disabled \(during) s -5 382 M -( user authentication phase\) if high security is required.) s -5 371 M -( Administrators of host machines should make all attempts to) s -5 360 M -( compartmentalize all event notification messages and protect them) s -5 349 M -( from unwarranted observation. Developers should be aware of the) s -5 338 M -( sensitive nature of some of the normal event messages and debug) s -5 327 M -( messages and may want to provide guidance to administrators on ways) s -5 316 M -( to keep this information away from unauthorized people. Developers) s -5 305 M -( should consider minimizing the amount of sensitive information) s -5 294 M -( obtainable by users during the authentication phase in accordance) s -5 283 M -( with the local policies. For this reason, it is RECOMMENDED that) s -5 272 M -( debug messages be initially disabled at the time of deployment and) s -5 261 M -( require an active decision by an administrator to allow them to be) s -5 250 M -( enabled. It is also RECOMMENDED that a message expressing this) s -5 239 M -( concern be presented to the administrator of a system when the action) s -5 228 M -( is taken to enable debugging messages.) s -5 206 M -(9.3.3 Local security policy) s -5 184 M -( Implementer MUST ensure that the credentials provided validate the) s -5 173 M -( professed user and also MUST ensure that the local policy of the) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 21]) s -_R -S -PStoPSsaved restore -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 421.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 22 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( server permits the user the access requested. In particular, because) s -5 679 M -( of the flexible nature of the SSH connection protocol, it may not be) s -5 668 M -( possible to determine the local security policy, if any, that should) s -5 657 M -( apply at the time of authentication because the kind of service being) s -5 646 M -( requested is not clear at that instant. For example, local policy) s -5 635 M -( might allow a user to access files on the server, but not start an) s -5 624 M -( interactive shell. However, during the authentication protocol, it is) s -5 613 M -( not known whether the user will be accessing files or attempting to) s -5 602 M -( use an interactive shell, or even both. In any event, where local) s -5 591 M -( security policy for the server host exists, it MUST be applied and) s -5 580 M -( enforced correctly.) s -5 558 M -( Implementors are encouraged to provide a default local policy and) s -5 547 M -( make its parameters known to administrators and users. At the) s -5 536 M -( discretion of the implementors, this default policy may be along the) s -5 525 M -( lines of 'anything goes' where there are no restrictions placed upon) s -5 514 M -( users, or it may be along the lines of 'excessively restrictive' in) s -5 503 M -( which case the administrators will have to actively make changes to) s -5 492 M -( this policy to meet their needs. Alternatively, it may be some) s -5 481 M -( attempt at providing something practical and immediately useful to) s -5 470 M -( the administrators of the system so they don't have to put in much) s -5 459 M -( effort to get SSH working. Whatever choice is made MUST be applied) s -5 448 M -( and enforced as required above.) s -5 426 M -(9.3.4 Public key authentication) s -5 404 M -( The use of public-key authentication assumes that the client host has) s -5 393 M -( not been compromised. It also assumes that the private-key of the) s -5 382 M -( server host has not been compromised.) s -5 360 M -( This risk can be mitigated by the use of passphrases on private keys;) s -5 349 M -( however, this is not an enforceable policy. The use of smartcards,) s -5 338 M -( or other technology to make passphrases an enforceable policy is) s -5 327 M -( suggested.) s -5 305 M -( The server could require both password and public-key authentication,) s -5 294 M -( however, this requires the client to expose its password to the) s -5 283 M -( server \(see section on password authentication below.\)) s -5 261 M -(9.3.5 Password authentication) s -5 239 M -( The password mechanism as specified in the authentication protocol) s -5 228 M -( assumes that the server has not been compromised. If the server has) s -5 217 M -( been compromised, using password authentication will reveal a valid) s -5 206 M -( username / password combination to the attacker, which may lead to) s -5 195 M -( further compromises.) s -5 173 M -( This vulnerability can be mitigated by using an alternative form of) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 22]) s -_R -S -PStoPSsaved restore -%%Page: (22,23) 12 -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 0.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -/showpage{}def/copypage{}def/erasepage{}def -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 23 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( authentication. For example, public-key authentication makes no) s -5 679 M -( assumptions about security on the server.) s -5 657 M -(9.3.6 Host based authentication) s -5 635 M -( Host based authentication assumes that the client has not been) s -5 624 M -( compromised. There are no mitigating strategies, other than to use) s -5 613 M -( host based authentication in combination with another authentication) s -5 602 M -( method.) s -5 580 M -(9.4 Connection protocol) s -5 558 M -(9.4.1 End point security) s -5 536 M -( End point security is assumed by the connection protocol. If the) s -5 525 M -( server has been compromised, any terminal sessions, port forwarding,) s -5 514 M -( or systems accessed on the host are compromised. There are no) s -5 503 M -( mitigating factors for this.) s -5 481 M -( If the client end point has been compromised, and the server fails to) s -5 470 M -( stop the attacker at the authentication protocol, all services) s -5 459 M -( exposed \(either as subsystems or through forwarding\) will be) s -5 448 M -( vulnerable to attack. Implementors SHOULD provide mechanisms for) s -5 437 M -( administrators to control which services are exposed to limit the) s -5 426 M -( vulnerability of other services.) s -5 404 M -( These controls might include controlling which machines and ports can) s -5 393 M -( be target in 'port-forwarding' operations, which users are allowed to) s -5 382 M -( use interactive shell facilities, or which users are allowed to use) s -5 371 M -( exposed subsystems.) s -5 349 M -(9.4.2 Proxy forwarding) s -5 327 M -( The SSH connection protocol allows for proxy forwarding of other) s -5 316 M -( protocols such as SNMP, POP3, and HTTP. This may be a concern for) s -5 305 M -( network administrators who wish to control the access of certain) s -5 294 M -( applications by users located outside of their physical location.) s -5 283 M -( Essentially, the forwarding of these protocols may violate site) s -5 272 M -( specific security policies as they may be undetectably tunneled) s -5 261 M -( through a firewall. Implementors SHOULD provide an administrative) s -5 250 M -( mechanism to control the proxy forwarding functionality so that site) s -5 239 M -( specific security policies may be upheld.) s -5 217 M -( In addition, a reverse proxy forwarding functionality is available,) s -5 206 M -( which again can be used to bypass firewall controls.) s -5 184 M -( As indicated above, end-point security is assumed during proxy) s -5 173 M -( forwarding operations. Failure of end-point security will compromise) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 23]) s -_R -S -PStoPSsaved restore -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 421.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 24 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( all data passed over proxy forwarding.) s -5 668 M -(9.4.3 X11 forwarding) s -5 646 M -( Another form of proxy forwarding provided by the ssh connection) s -5 635 M -( protocol is the forwarding of the X11 protocol. If end-point) s -5 624 M -( security has been compromised, X11 forwarding may allow attacks) s -5 613 M -( against the X11 server. Users and administrators should, as a matter) s -5 602 M -( of course, use appropriate X11 security mechanisms to prevent) s -5 591 M -( unauthorized use of the X11 server. Implementors, administrators and) s -5 580 M -( users who wish to further explore the security mechanisms of X11 are) s -5 569 M -( invited to read [SCHEIFLER] and analyze previously reported problems) s -5 558 M -( with the interactions between SSH forwarding and X11 in CERT) s -5 547 M -( vulnerabilities VU#363181 and VU#118892 [CERT].) s -5 525 M -( X11 display forwarding with SSH, by itself, is not sufficient to) s -5 514 M -( correct well known problems with X11 security [VENEMA]. However, X11) s -5 503 M -( display forwarding in SSHv2 \(or other, secure protocols\), combined) s -5 492 M -( with actual and pseudo-displays which accept connections only over) s -5 481 M -( local IPC mechanisms authorized by permissions or ACLs, does correct) s -5 470 M -( many X11 security problems as long as the "none" MAC is not used. It) s -5 459 M -( is RECOMMENDED that X11 display implementations default to allowing) s -5 448 M -( display opens only over local IPC. It is RECOMMENDED that SSHv2) s -5 437 M -( server implementations that support X11 forwarding default to) s -5 426 M -( allowing display opens only over local IPC. On single-user systems) s -5 415 M -( it might be reasonable to default to allowing local display opens) s -5 404 M -( over TCP/IP.) s -5 382 M -( Implementors of the X11 forwarding protocol SHOULD implement the) s -5 371 M -( magic cookie access checking spoofing mechanism as described in) s -5 360 M -( [ssh-connect] as an additional mechanism to prevent unauthorized use) s -5 349 M -( of the proxy.) s -5 327 M -(Normative References) s -5 305 M -( [SSH-ARCH]) s -5 294 M -( Ylonen, T., "SSH Protocol Architecture", I-D) s -5 283 M -( draft-ietf-architecture-15.txt, Oct 2003.) s -5 261 M -( [SSH-TRANS]) s -5 250 M -( Ylonen, T., "SSH Transport Layer Protocol", I-D) s -5 239 M -( draft-ietf-transport-17.txt, Oct 2003.) s -5 217 M -( [SSH-USERAUTH]) s -5 206 M -( Ylonen, T., "SSH Authentication Protocol", I-D) s -5 195 M -( draft-ietf-userauth-18.txt, Oct 2003.) s -5 173 M -( [SSH-CONNECT]) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 24]) s -_R -S -PStoPSsaved restore -%%Page: (24,25) 13 -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 0.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -/showpage{}def/copypage{}def/erasepage{}def -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 25 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( Ylonen, T., "SSH Connection Protocol", I-D) s -5 679 M -( draft-ietf-connect-18.txt, Oct 2003.) s -5 657 M -( [SSH-NUMBERS]) s -5 646 M -( Lehtinen, S. and D. Moffat, "SSH Protocol Assigned) s -5 635 M -( Numbers", I-D draft-ietf-secsh-assignednumbers-05.txt, Oct) s -5 624 M -( 2003.) s -5 602 M -( [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate) s -5 591 M -( Requirement Levels", BCP 14, RFC 2119, March 1997.) s -5 569 M -(Informative References) s -5 547 M -( [FIPS-186]) s -5 536 M -( Federal Information Processing Standards Publication,) s -5 525 M -( "FIPS PUB 186, Digital Signature Standard", May 1994.) s -5 503 M -( [FIPS-197]) s -5 492 M -( National Institue of Standards and Technology, "FIPS 197,) s -5 481 M -( Specification for the Advanced Encryption Standard",) s -5 470 M -( November 2001.) s -5 448 M -( [ANSI T1.523-2001]) s -5 437 M -( American National Standards Insitute, Inc., "Telecom) s -5 426 M -( Glossary 2000", February 2001.) s -5 404 M -( [SCHEIFLER]) s -5 393 M -( Scheifler, R., "X Window System : The Complete Reference) s -5 382 M -( to Xlib, X Protocol, Icccm, Xlfd, 3rd edition.", Digital) s -5 371 M -( Press ISBN 1555580882, Feburary 1992.) s -5 349 M -( [RFC0854] Postel, J. and J. Reynolds, "Telnet Protocol) s -5 338 M -( Specification", STD 8, RFC 854, May 1983.) s -5 316 M -( [RFC0894] Hornig, C., "Standard for the transmission of IP datagrams) s -5 305 M -( over Ethernet networks", STD 41, RFC 894, April 1984.) s -5 283 M -( [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",) s -5 272 M -( STD 13, RFC 1034, November 1987.) s -5 250 M -( [RFC1134] Perkins, D., "Point-to-Point Protocol: A proposal for) s -5 239 M -( multi-protocol transmission of datagrams over) s -5 228 M -( Point-to-Point links", RFC 1134, November 1989.) s -5 206 M -( [RFC1282] Kantor, B., "BSD Rlogin", RFC 1282, December 1991.) s -5 184 M -( [RFC1510] Kohl, J. and B. Neuman, "The Kerberos Network) s -5 173 M -( Authentication Service \(V5\)", RFC 1510, September 1993.) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 25]) s -_R -S -PStoPSsaved restore -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 421.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 26 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( [RFC1700] Reynolds, J. and J. Postel, "Assigned Numbers", RFC 1700,) s -5 679 M -( October 1994.) s -5 657 M -( [RFC1750] Eastlake, D., Crocker, S. and J. Schiller, "Randomness) s -5 646 M -( Recommendations for Security", RFC 1750, December 1994.) s -5 624 M -( [RFC3066] Alvestrand, H., "Tags for the Identification of) s -5 613 M -( Languages", BCP 47, RFC 3066, January 2001.) s -5 591 M -( [RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC) s -5 580 M -( 1964, June 1996.) s -5 558 M -( [RFC2025] Adams, C., "The Simple Public-Key GSS-API Mechanism) s -5 547 M -( \(SPKM\)", RFC 2025, October 1996.) s -5 525 M -( [RFC2085] Oehler, M. and R. Glenn, "HMAC-MD5 IP Authentication with) s -5 514 M -( Replay Prevention", RFC 2085, February 1997.) s -5 492 M -( [RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:) s -5 481 M -( Keyed-Hashing for Message Authentication", RFC 2104,) s -5 470 M -( February 1997.) s -5 448 M -( [RFC2246] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A.) s -5 437 M -( and P. Kocher, "The TLS Protocol Version 1.0", RFC 2246,) s -5 426 M -( January 1999.) s -5 404 M -( [RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO) s -5 393 M -( 10646", RFC 2279, January 1998.) s -5 371 M -( [RFC2410] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and) s -5 360 M -( Its Use With IPsec", RFC 2410, November 1998.) s -5 338 M -( [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an) s -5 327 M -( IANA Considerations Section in RFCs", BCP 26, RFC 2434,) s -5 316 M -( October 1998.) s -5 294 M -( [RFC2743] Linn, J., "Generic Security Service Application Program) s -5 283 M -( Interface Version 2, Update 1", RFC 2743, January 2000.) s -5 261 M -( [SCHNEIER]) s -5 250 M -( Schneier, B., "Applied Cryptography Second Edition:) s -5 239 M -( protocols algorithms and source in code in C", 1996.) s -5 217 M -( [KAUFMAN,PERLMAN,SPECINER]) s -5 206 M -( Kaufman, C., Perlman, R. and M. Speciner, "Network) s -5 195 M -( Security: PRIVATE Communication in a PUBLIC World", 1995.) s -5 173 M -( [CERT] CERT Coordination Center, The., "http://www.cert.org/nav/) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 26]) s -_R -S -PStoPSsaved restore -%%Page: (26,27) 14 -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 0.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -/showpage{}def/copypage{}def/erasepage{}def -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 27 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( index_red.html".) s -5 668 M -( [VENEMA] Venema, W., "Murphy's Law and Computer Security",) s -5 657 M -( Proceedings of 6th USENIX Security Symposium, San Jose CA) s -5 646 M -( http://www.usenix.org/publications/library/proceedings/) s -5 635 M -( sec96/venema.html, July 1996.) s -5 613 M -( [ROGAWAY] Rogaway, P., "Problems with Proposed IP Cryptography",) s -5 602 M -( Unpublished paper http://www.cs.ucdavis.edu/~rogaway/) s -5 591 M -( papers/draft-rogaway-ipsec-comments-00.txt, 1996.) s -5 569 M -( [DAI] Dai, W., "An attack against SSH2 protocol", Email to the) s -5 558 M -( SECSH Working Group [email protected] ftp://) s -5 547 M -( ftp.ietf.org/ietf-mail-archive/secsh/2002-02.mail, Feb) s -5 536 M -( 2002.) s -5 514 M -( [BELLARE,KOHNO,NAMPREMPRE]) s -5 503 M -( Bellaire, M., Kohno, T. and C. Namprempre, "Authenticated) s -5 492 M -( Encryption in SSH: Fixing the SSH Binary Packet Protocol",) s -5 481 M -( , Sept 2002.) s -5 448 M -(Authors' Addresses) s -5 426 M -( Tatu Ylonen) s -5 415 M -( SSH Communications Security Corp) s -5 404 M -( Fredrikinkatu 42) s -5 393 M -( HELSINKI FIN-00100) s -5 382 M -( Finland) s -5 360 M -( EMail: [email protected]) s -5 327 M -( Darren J. Moffat \(editor\)) s -5 316 M -( Sun Microsystems, Inc) s -5 305 M -( 17 Network Circle) s -5 294 M -( Menlo Park CA 94025) s -5 283 M -( USA) s -5 261 M -( EMail: [email protected]) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 27]) s -_R -S -PStoPSsaved restore -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 421.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 28 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -(Intellectual Property Statement) s -5 668 M -( The IETF takes no position regarding the validity or scope of any) s -5 657 M -( intellectual property or other rights that might be claimed to) s -5 646 M -( pertain to the implementation or use of the technology described in) s -5 635 M -( this document or the extent to which any license under such rights) s -5 624 M -( might or might not be available; neither does it represent that it) s -5 613 M -( has made any effort to identify any such rights. Information on the) s -5 602 M -( IETF's procedures with respect to rights in standards-track and) s -5 591 M -( standards-related documentation can be found in BCP-11. Copies of) s -5 580 M -( claims of rights made available for publication and any assurances of) s -5 569 M -( licenses to be made available, or the result of an attempt made to) s -5 558 M -( obtain a general license or permission for the use of such) s -5 547 M -( proprietary rights by implementors or users of this specification can) s -5 536 M -( be obtained from the IETF Secretariat.) s -5 514 M -( The IETF invites any interested party to bring to its attention any) s -5 503 M -( copyrights, patents or patent applications, or other proprietary) s -5 492 M -( rights which may cover technology that may be required to practice) s -5 481 M -( this standard. Please address the information to the IETF Executive) s -5 470 M -( Director.) s -5 448 M -( The IETF has been notified of intellectual property rights claimed in) s -5 437 M -( regard to some or all of the specification contained in this) s -5 426 M -( document. For more information consult the online list of claimed) s -5 415 M -( rights.) s -5 382 M -(Full Copyright Statement) s -5 360 M -( Copyright \(C\) The Internet Society \(2003\). All Rights Reserved.) s -5 338 M -( This document and translations of it may be copied and furnished to) s -5 327 M -( others, and derivative works that comment on or otherwise explain it) s -5 316 M -( or assist in its implementation may be prepared, copied, published) s -5 305 M -( and distributed, in whole or in part, without restriction of any) s -5 294 M -( kind, provided that the above copyright notice and this paragraph are) s -5 283 M -( included on all such copies and derivative works. However, this) s -5 272 M -( document itself may not be modified in any way, such as by removing) s -5 261 M -( the copyright notice or references to the Internet Society or other) s -5 250 M -( Internet organizations, except as needed for the purpose of) s -5 239 M -( developing Internet standards in which case the procedures for) s -5 228 M -( copyrights defined in the Internet Standards process must be) s -5 217 M -( followed, or as required to translate it into languages other than) s -5 206 M -( English.) s -5 184 M -( The limited permissions granted above are perpetual and will not be) s -5 173 M -( revoked by the Internet Society or its successors or assignees.) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 28]) s -_R -S -PStoPSsaved restore -%%Page: (28,29) 15 -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 0.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -/showpage{}def/copypage{}def/erasepage{}def -PStoPSxform concat -%%BeginPageSetup -_S -75 0 translate -/pagenum 29 def -/fname () def -/fdir () def -/ftail () def -/user_header_p false def -%%EndPageSetup -5 723 M -(Internet-Draft SSH Protocol Architecture Oct 2003) s -5 690 M -( This document and the information contained herein is provided on an) s -5 679 M -( "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING) s -5 668 M -( TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING) s -5 657 M -( BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION) s -5 646 M -( HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF) s -5 635 M -( MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.) s -5 602 M -(Acknowledgment) s -5 580 M -( Funding for the RFC Editor function is currently provided by the) s -5 569 M -( Internet Society.) s -5 129 M -(Ylonen & Moffat Expires March 31, 2004 [Page 29]) s -_R -S -PStoPSsaved restore -userdict/PStoPSsaved save put -PStoPSmatrix setmatrix -595.000000 421.271378 translate -90 rotate -0.706651 dup scale -userdict/PStoPSmatrix matrix currentmatrix put -userdict/PStoPSclip{0 0 moveto - 595.000000 0 rlineto 0 842.000000 rlineto -595.000000 0 rlineto - closepath}put initclip -PStoPSxform concat -showpage -PStoPSsaved restore -%%Trailer -%%Pages: 29 -%%DocumentNeededResources: font Courier-Bold Courier -%%EOF |