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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Network Working Group T. Ylonen 2 INTERNET-DRAFT T. Kivinen 3 draft-ietf-secsh-architecture-08.txt M. Saarinen 4 Expires: 2 September, 2001 T. Rinne 5 S. Lehtinen 6 SSH Communications Security 7 2 March, 2001 9 Secure Shell Remote Login Protocol Architecture 11 Status of This Memo 13 This document is an Internet-Draft and is in full conformance 14 with all provisions of Section 10 of RFC2026. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as 19 Internet-Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six 22 months and may be updated, replaced, or obsoleted by other 23 documents at any time. It is inappropriate to use Internet- 24 Drafts as reference material or to cite them other than as 25 "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 Abstract 35 The Secure Shell Remote Login Protocol is a suite of protocols for 36 secure remote logins and other secure network services over an insecure 37 network. This document describes the overall architecture of the Secure 38 Shell protocols, as well as the notation and terminology used in the 39 protocol documents. It also discusses the algorithm naming system that 40 allows local extensions. The Secure Shell protocol consists of three 41 major components: The Transport Layer Protocol provides server authenti- 42 cation, confidentiality, and integrity with perfect forward secrecy. The 43 User Authentication Protocol authenticates the client to the server. The 44 Connection Protocol multiplexes the encrypted tunnel into several logi- 45 cal channels. Details of these protocols are described in separate doc- 46 uments. 48 Table of Contents 50 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2 51 2. Specification of Requirements . . . . . . . . . . . . . . . . . 2 52 3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . 3 53 3.1. Host Keys . . . . . . . . . . . . . . . . . . . . . . . . . 3 54 3.2. Extensibility . . . . . . . . . . . . . . . . . . . . . . . 4 55 3.3. Policy Issues . . . . . . . . . . . . . . . . . . . . . . . 4 56 3.4. Security Properties . . . . . . . . . . . . . . . . . . . . 5 57 3.5. Packet Size and Overhead . . . . . . . . . . . . . . . . . . 5 58 3.6. Localization and Character Set Support . . . . . . . . . . . 6 59 4. Data Type Representations Used in the Secure Shell Protocols . . 7 60 5. Algorithm Naming . . . . . . . . . . . . . . . . . . . . . . . . 8 61 6. Message Numbers . . . . . . . . . . . . . . . . . . . . . . . . 8 62 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 9 63 8. Security Considerations . . . . . . . . . . . . . . . . . . . . 10 64 9. Trademark Issues . . . . . . . . . . . . . . . . . . . . . . . . 10 65 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 66 11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11 68 1. Introduction 70 The Secure Shell Remote Login Protocol is a protocol for secure remote 71 login and other secure network services over an insecure network. It 72 consists of three major components: 74 o The Transport Layer Protocol [SECSH-TRANS] provides server 75 authentication, confidentiality, and integrity. It may optionally 76 also provide compression. The transport layer will typically be run 77 over a TCP/IP connection, but might also be used on top of any other 78 reliable data stream. 80 o The User Authentication Protocol [SECSH-USERAUTH] authenticates the 81 client-side user to the server. It runs over the transport layer 82 protocol. 84 o The Connection Protocol [SECSH-CONN] multiplexes the encrypted tunnel 85 into several logical channels. It runs over the user authentication 86 protocol. 88 The client sends a service request once a secure transport layer 89 connection has been established. A second service request is sent after 90 user authentication is complete. This allows new protocols to be defined 91 and coexist with the protocols listed above. 93 The connection protocol provides channels that can be used for a wide 94 range of purposes. Standard methods are provided for setting up secure 95 interactive shell sessions and for forwarding ("tunneling") arbitrary 96 TCP/IP ports and X11 connections. 98 2. Specification of Requirements 99 All documents related to the Secure Shell protocols shall use the 100 keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", 101 "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" to describe 102 requirements. They are to be interpreted as described in [RFC-2119]. 104 3. Architecture 106 3.1. Host Keys 108 Each server host SHOULD have a host key. Hosts MAY have multiple host 109 keys using multiple different algorithms. Multiple hosts MAY share the 110 same host key. If a host has keys at all, it MUST have at least one key 111 using each REQUIRED public key algorithm (currently DSS [FIPS-186]). 113 The server host key is used during key exchange to verify that the 114 client is really talking to the correct server. For this to be possible, 115 the client must have a priori knowledge of the server's public host key. 117 Two different trust models can be used: 119 o The client has a local database that associates each host name (as 120 typed by the user) with the corresponding public host key. This 121 method requires no centrally administered infrastructure, and no 122 third-party coordination. The downside is that the database of name- 123 to-key associations may become burdensome to maintain. 125 o The host name-to-key association is certified by some trusted 126 certification authority. The client only knows the CA root key, and 127 can verify the validity of all host keys certified by accepted CAs. 129 The second alternative eases the maintenance problem, since ideally 130 only a single CA key needs to be securely stored on the client. On 131 the other hand, each host key must be appropriately certified by a 132 central authority before authorization is possible. Also, a lot of 133 trust is placed on the central infrastructure. 135 The protocol provides the option that the server name - host key 136 association is not checked when connecting to the host for the first 137 time. This allows communication without prior communication of host keys 138 or certification. The connection still provides protection against 139 passive listening; however, it becomes vulnerable to active man-in-the- 140 middle attacks. Implementations SHOULD NOT normally allow such 141 connections by default, as they pose a potential security problem. 142 However, as there is no widely deployed key infrastructure available on 143 the Internet yet, this option makes the protocol much more usable during 144 the transition time until such an infrastructure emerges, while still 145 providing a much higher level of security than that offered by older 146 solutions (e.g. telnet [RFC-854] and rlogin [RFC-1282]). 148 Implementations SHOULD try to make the best effort to check host keys. 149 An example of a possible strategy is to only accept a host key without 150 checking the first time a host is connected, save the key in a local 151 database, and compare against that key on all future connections to that 152 host. 154 Implementations MAY provide additional methods for verifying the 155 correctness of host keys, e.g. a hexadecimal fingerprint derived from 156 the SHA-1 hash of the public key. Such fingerprints can easily be 157 verified by using telephone or other external communication channels. 159 All implementations SHOULD provide an option to not accept host keys 160 that cannot be verified. 162 We believe that ease of use is critical to end-user acceptance of 163 security solutions, and no improvement in security is gained if the new 164 solutions are not used. Thus, providing the option not to check the 165 server host key is believed to improve the overall security of the 166 Internet, even though it reduces the security of the protocol in 167 configurations where it is allowed. 169 3.2. Extensibility 171 We believe that the protocol will evolve over time, and some 172 organizations will want to use their own encryption, authentication 173 and/or key exchange methods. Central registration of all extensions is 174 cumbersome, especially for experimental or classified features. On the 175 other hand, having no central registration leads to conflicts in method 176 identifiers, making interoperability difficult. 178 We have chosen to identify algorithms, methods, formats, and extension 179 protocols with textual names that are of a specific format. DNS names 180 are used to create local namespaces where experimental or classified 181 extensions can be defined without fear of conflicts with other 182 implementations. 184 One design goal has been to keep the base protocol as simple as 185 possible, and to require as few algorithms as possible. However, all 186 implementations MUST support a minimal set of algorithms to ensure 187 interoperability (this does not imply that the local policy on all hosts 188 would necessary allow these algorithms). The mandatory algorithms are 189 specified in the relevant protocol documents. 191 Additional algorithms, methods, formats, and extension protocols can be 192 defined in separate drafts. See Section ``Algorithm Naming'' for more 193 information. 195 3.3. Policy Issues 197 The protocol allows full negotiation of encryption, integrity, key 198 exchange, compression, and public key algorithms and formats. 199 Encryption, integrity, public key, and compression algorithms can be 200 different for each direction. 202 The following policy issues SHOULD be addressed in the configuration 203 mechanisms of each implementation: 205 o Encryption, integrity, and compression algorithms, separately for 206 each direction. The policy MUST specify which is the preferred 207 algorithm (e.g. the first algorithm listed in each category). 209 o Public key algorithms and key exchange method to be used for host 210 authentication. The existence of trusted host keys for different 211 public key algorithms also affects this choice. 213 o The authentication methods that are to be required by the server for 214 each user. The server's policy MAY require multiple authentication 215 for some or all users. The required algorithms MAY depend on the 216 location where the user is trying to log in from. 218 o The operations that the user is allowed to perform using the 219 connection protocol. Some issues are related to security; for 220 example, the policy SHOULD NOT allow the server to start sessions or 221 run commands on the client machine, and MUST NOT allow connections to 222 the authentication agent unless forwarding such connections has been 223 requested. Other issues, such as which TCP/IP ports can be forwarded 224 and by whom, are clearly issues of local policy. Many of these issues 225 may involve traversing or bypassing firewalls, and are interrelated 226 with the local security policy. 228 3.4. Security Properties 230 The primary goal of the Secure Shell protocol is improved security on 231 the Internet. It attempts to do this in a way that is easy to deploy, 232 even at the cost of absolute security. 234 o All encryption, integrity, and public key algorithms used are well- 235 known, well-established algorithms. 237 o All algorithms are used with cryptographically sound key sizes that 238 are believed to provide protection against even the strongest 239 cryptanalytic attacks for decades. 241 o All algorithms are negotiated, and in case some algorithm is broken, 242 it is easy to switch to some other algorithm without modifying the 243 base protocol. 245 Specific concessions were made to make wide-spread fast deployment 246 easier. The particular case where this comes up is verifying that the 247 server host key really belongs to the desired host; the protocol allows 248 the verification to be left out (but this is NOT RECOMMENDED). This is 249 believed to significantly improve usability in the short term, until 250 widespread Internet public key infrastructures emerge. 252 3.5. Packet Size and Overhead 254 Some readers will worry about the increase in packet size due to new 255 headers, padding, and MAC. The minimum packet size is in the order of 256 28 bytes (depending on negotiated algorithms). The increase is 257 negligible for large packets, but very significant for one-byte packets 258 (telnet-type sessions). There are, however, several factors that make 259 this a non-issue in almost all cases: 261 o The minimum size of a TCP/IP header is 32 bytes. Thus, the increase 262 is actually from 33 to 51 bytes (roughly). 264 o The minimum size of the data field of an Ethernet packet is 46 bytes 265 [RFC-894]. Thus, the increase is no more than 5 bytes. When Ethernet 266 headers are considered, the increase is less than 10 percent. 268 o The total fraction of telnet-type data in the Internet is negligible, 269 even with increased packet sizes. 271 The only environment where the packet size increase is likely to have a 272 significant effect is PPP [RFC-1134] over slow modem lines (PPP 273 compresses the TCP/IP headers, emphasizing the increase in packet size). 274 However, with modern modems, the time needed to transfer is in the order 275 of 2 milliseconds, which is a lot faster than people can type. 277 There are also issues related to the maximum packet size. To minimize 278 delays in screen updates, one does not want excessively large packets 279 for interactive sessions. The maximum packet size is negotiated 280 separately for each channel. 282 3.6. Localization and Character Set Support 284 For the most part, the Secure Shell protocols do not directly pass text 285 that would be displayed to the user. However, there are some places 286 where such data might be passed. When applicable, the character set for 287 the data MUST be explicitly specified. In most places, ISO 10646 with 288 UTF-8 encoding is used [RFC-2279]. When applicable, a field is also 289 provided for a language tag [RFC-1766]. 291 One big issue is the character set of the interactive session. There is 292 no clear solution, as different applications may display data in 293 different formats. Different types of terminal emulation may also be 294 employed in the client, and the character set to be used is effectively 295 determined by the terminal emulation. Thus, no place is provided for 296 directly specifying the character set or encoding for terminal session 297 data. However, the terminal emulation type (e.g. "vt100") is 298 transmitted to the remote site, and it implicitly specifies the 299 character set and encoding. Applications typically use the terminal 300 type to determine what character set they use, or the character set is 301 determined using some external means. The terminal emulation may also 302 allow configuring the default character set. In any case, the character 303 set for the terminal session is considered primarily a client local 304 issue. 306 Internal names used to identify algorithms or protocols are normally 307 never displayed to users, and must be in US-ASCII. 309 The client and server user names are inherently constrained by what the 310 server is prepared to accept. They might, however, occasionally be 311 displayed in logs, reports, etc. They MUST be encoded using ISO 10646 312 UTF-8, but other encodings may be required in some cases. It is up to 313 the server to decide how to map user names to accepted user names. 314 Straight bit-wise binary comparison is RECOMMENDED. 316 For localization purposes, the protocol attempts to minimize the number 317 of textual messages transmitted. When present, such messages typically 318 relate to errors, debugging information, or some externally configured 319 data. For data that is normally displayed, it SHOULD be possible to 320 fetch a localized message instead of the transmitted message by using a 321 numerical code. The remaining messages SHOULD be configurable. 323 4. Data Type Representations Used in the Secure Shell Protocols 325 byte 326 A byte represents an arbitrary 8-bit value (octet) [RFC-1700]. 327 Fixed length data is sometimes represented as an array of bytes, 328 written byte[n], where n is the number of bytes in the array. 330 boolean 331 A boolean value is stored as a single byte. The value 0 332 represents FALSE, and the value 1 represents TRUE. All non-zero 333 values MUST be interpreted as TRUE; however, applications MUST NOT 334 store values other than 0 and 1. 336 uint32 337 Represents a 32-bit unsigned integer. Stored as four bytes in the 338 order of decreasing significance (network byte order). 340 For example, the value 699921578 (0x29b7f4aa) is stored as 29 b7 341 f4 aa. 343 uint64 344 Represents a 64-bit unsigned integer. Stored as eight bytes in 345 the order of decreasing significance (network byte order). 347 string 348 Arbitrary length binary string. Strings are allowed to contain 349 arbitrary binary data, including null characters and 8-bit 350 characters. They are stored as a uint32 containing its length 351 (number of bytes that follow) and zero (= empty string) or more 352 bytes that are the value of the string. Terminating null 353 characters are not used. 355 Strings are also used to store text. In that case, US-ASCII is 356 used for internal names, and ISO-10646 UTF-8 for text that might 357 be displayed to the user. The terminating null character SHOULD 358 NOT normally be stored in the string. 360 For example, the US-ASCII string "testing" is represented as 00 00 361 00 07 t e s t i n g. The UTF8 mapping does not alter the encoding 362 of US-ASCII characters. 364 mpint 365 Represents multiple precision integers in two's complement format, 366 stored as a string, 8 bits per byte, MSB first. Negative numbers 367 have the value 1 as the most significant bit of the first byte of 368 the data partition. If the most significant bit would be set for a 369 positive number, the number MUST be preceded by a zero byte. 370 Unnecessary leading bytes with the value 0 or 255 MUST NOT be 371 included. The value zero MUST be stored as a string with zero 372 bytes of data. 374 By convention, a number that is used in modular computations in 375 Z_n SHOULD be represented in the range 0 <= x < n. 377 Examples: 379 value (hex) representation (hex) 380 --------------------------------------------------------------- 381 0 00 00 00 00 382 9a378f9b2e332a7 00 00 00 08 09 a3 78 f9 b2 e3 32 a7 383 80 00 00 00 02 00 80 384 -1234 00 00 00 02 ed cc 385 -deadbeef 00 00 00 05 ff 21 52 41 11 387 5. Algorithm Naming 389 The Secure Shell protocols refer to particular hash, encryption, 390 integrity, compression, and key exchange algorithms or protocols by 391 names. There are some standard algorithms that all implementations MUST 392 support. There are also algorithms that are defined in the protocol 393 specification but are OPTIONAL. Furthermore, it is expected that some 394 organizations will want to use their own algorithms. 396 In this protocol, all algorithm identifiers MUST be printable US-ASCII 397 strings no longer than 64 characters. Names MUST be case-sensitive. 399 There are two formats for algorithm names: 401 o Names that do not contain an at-sign (@) are reserved to be assigned 402 by IETF consensus (RFCs). Examples include `3des-cbc', `sha-1', 403 `hmac-sha1', and `zlib' (the quotes are not part of the name). Names 404 of this format MUST NOT be used without first registering them. 405 Registered names MUST NOT contain an at-sign (@) or a comma (,). 407 o Anyone can define additional algorithms by using names in the format 408 name@domainname, e.g. "ourcipher-cbc@ssh.com". The format of the part 409 preceding the at sign is not specified; it MUST consist of US-ASCII 410 characters except at-sign and comma. The part following the at-sign 411 MUST be a valid fully qualified internet domain name [RFC-1034] 412 controlled by the person or organization defining the name. It is up 413 to each domain how it manages its local namespace. 415 6. Message Numbers 416 Secure Shell protocol packets have message numbers in the range 1 to 417 255. These numbers have been allocated as follows: 419 Transport layer protocol: 421 1 to 19 Transport layer generic (e.g. disconnect, ignore, debug, 422 etc.) 423 20 to 29 Algorithm negotiation 424 30 to 49 Key exchange method specific (numbers can be reused for 425 different authentication methods) 427 User authentication protocol: 429 50 to 59 User authentication generic 430 60 to 79 User authentication method specific (numbers can be 431 reused for different authentication methods) 433 Connection protocol: 435 80 to 89 Connection protocol generic 436 90 to 127 Channel related messages 438 Reserved for client protocols: 440 128 to 191 Reserved 442 Local extensions: 444 192 to 255 Local extensions 446 7. IANA Considerations 448 Allocation of the following types of names in the Secure Shell protocols 449 is assigned by IETF consensus: 451 o encryption algorithm names, 453 o MAC algorithm names, 455 o public key algorithm names (public key algorithm also implies 456 encoding and signature/encryption capability), 458 o key exchange method names, and 460 o protocol (service) names. 462 These names MUST be printable US-ASCII strings, and MUST NOT contain the 463 characters at-sign ('@'), comma (','), or whitespace or control 464 characters (ASCII codes 32 or less). Names are case-sensitive, and MUST 465 NOT be longer than 64 characters. 467 Names with the at-sign ('@') in them are allocated by the owner of DNS 468 name after the at-sign (hierarchical allocation in [RFC-2343]), 469 otherwise the same restrictions as above. 470 Each category of names listed above has a separate namespace. However, 471 using the same name in multiple categories SHOULD be avoided to minimize 472 confusion. 474 Message numbers (see Section ``Message Numbers'') in the range of 0..191 475 should be allocated via IETF consensus; message numbers in the 192..255 476 range (the "Local extensions" set) are reserved for private use. 478 8. Security Considerations 480 Special care should be taken to ensure that all of the random numbers 481 are of good quality. The random numbers SHOULD be produced with safe 482 mechanisms discussed in [RFC-1750]. 484 When displaying text, such as error or debug messages to the user, the 485 client software SHOULD replace any control characters (except tab, 486 carriage return and newline) with safe sequences to avoid attacks by 487 sending terminal control characters. 489 Not using MAC or encryption SHOULD be avoided. The user authentication 490 protocol is subject to man-in-the-middle attacks if the encryption is 491 disabled. The Secure Shell protocol does not protect against message 492 alteration if no MAC is used. 494 9. Trademark Issues 496 "ssh" is a registered trademark of SSH Communications Security Corp in 497 the United States and/or other countries. 499 10. References 501 [FIPS-186] Federal Information Processing Standards Publication (FIPS 502 PUB) 186, Digital Signature Standard, 18 May 1994. 504 [RFC-854] Postel, J. and Reynolds, J: "Telnet Protocol Specification", 505 May 1983. 507 [RFC-894] Hornig, C: "A Standard for the Transmission of IP Datagrams 508 over Ethernet Networks", April 1984. 510 [RFC-1034] Mockapetris, P: "Domain Names - Concepts and Facilities", 511 November 1987. 513 [RFC-1134] Perkins, D: "The Point-to-Point Protocol: A Proposal for 514 Multi-Protocol Transmission of Datagrams Over Point-to-Point Links", 515 November 1989. 517 [RFC-1282] Kantor, B: "BSD Rlogin", December 1991. 519 [RFC-1700] Reynolds, J. and Postel, J: "Assigned Numbers", October 1994 520 (also STD 2). 522 [RFC-1750] Eastlake, D., Crocker, S., and Schiller, J: "Randomness 523 Recommendations for Security", December 1994. 525 [RFC-1766] Alvestrand, H: "Tags for the Identification of Languages", 526 March 1995. 528 [RFC-2279] Yergeau, F: "UTF-8, a transformation format of ISO 10646", 529 January 1998. 531 [RFC-2119] Bradner, S: "Key words for use in RFCs to indicate 532 Requirement Levels", March 1997. 534 [RFC-2343] Narten, T. and Alvestrand, H: "Guidelines for Writing an IANA 535 Considerations Section in RFCs", October 1998. 537 [SECSH-TRANS] Ylonen, T., et al: "Secure Shell Transport Layer 538 Protocol", Internet-Draft, draft-ietf-secsh-transport-10.txt 540 [SECSH-USERAUTH] Ylonen, T., et al: "Secure Shell Authentication 541 Protocol", Internet-Draft, draft-ietf-secsh-userauth-10.txt 543 [SECSH-CONNECT] Ylonen, T., et al: "Secure Shell Connection Protocol", 544 Internet-Draft, draft-ietf-secsh-connect-10.txt 546 11. Authors' Addresses 548 Tatu Ylonen 549 SSH Communications Security Corp 550 Fredrikinkatu 42 551 FIN-00100 HELSINKI 552 Finland 553 E-mail: ylo@ssh.com 555 Tero Kivinen 556 SSH Communications Security Corp 557 Fredrikinkatu 42 558 FIN-00100 HELSINKI 559 Finland 560 E-mail: kivinen@ssh.com 562 Markku-Juhani O. Saarinen 563 University of Jyvaskyla 565 Timo J. Rinne 566 SSH Communications Security Corp 567 Fredrikinkatu 42 568 FIN-00100 HELSINKI 569 Finland 570 E-mail: tri@ssh.com 572 Sami Lehtinen 573 SSH Communications Security Corp 574 Fredrikinkatu 42 575 FIN-00100 HELSINKI 576 Finland 577 E-mail: sjl@ssh.com