1 2 3 4 5 6 7Network Working Group K. Zeilenga, Ed. 8Request for Comments: 4616 OpenLDAP Foundation 9Updates: 2595 August 2006 10Category: Standards Track 11 12 13 The PLAIN Simple Authentication and Security Layer (SASL) Mechanism 14 15Status of This Memo 16 17 This document specifies an Internet standards track protocol for the 18 Internet community, and requests discussion and suggestions for 19 improvements. Please refer to the current edition of the "Internet 20 Official Protocol Standards" (STD 1) for the standardization state 21 and status of this protocol. Distribution of this memo is unlimited. 22 23Copyright Notice 24 25 Copyright (C) The Internet Society (2006). 26 27Abstract 28 29 This document defines a simple clear-text user/password Simple 30 Authentication and Security Layer (SASL) mechanism called the PLAIN 31 mechanism. The PLAIN mechanism is intended to be used, in 32 combination with data confidentiality services provided by a lower 33 layer, in protocols that lack a simple password authentication 34 command. 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58Zeilenga Standards Track [Page 1] 59 60RFC 4616 The PLAIN SASL Mechanism August 2006 61 62 631. Introduction 64 65 Clear-text, multiple-use passwords are simple, interoperate with 66 almost all existing operating system authentication databases, and 67 are useful for a smooth transition to a more secure password-based 68 authentication mechanism. The drawback is that they are unacceptable 69 for use over network connections where data confidentiality is not 70 ensured. 71 72 This document defines the PLAIN Simple Authentication and Security 73 Layer ([SASL]) mechanism for use in protocols with no clear-text 74 login command (e.g., [ACAP] or [SMTP-AUTH]). This document updates 75 RFC 2595, replacing Section 6. Changes since RFC 2595 are detailed 76 in Appendix A. 77 78 The name associated with this mechanism is "PLAIN". 79 80 The PLAIN SASL mechanism does not provide a security layer. 81 82 The PLAIN mechanism should not be used without adequate data security 83 protection as this mechanism affords no integrity or confidentiality 84 protections itself. The mechanism is intended to be used with data 85 security protections provided by application-layer protocol, 86 generally through its use of Transport Layer Security ([TLS]) 87 services. 88 89 By default, implementations SHOULD advertise and make use of the 90 PLAIN mechanism only when adequate data security services are in 91 place. Specifications for IETF protocols that indicate that this 92 mechanism is an applicable authentication mechanism MUST mandate that 93 implementations support an strong data security service, such as TLS. 94 95 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 96 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 97 document are to be interpreted as described in [Keywords]. 98 992. PLAIN SASL Mechanism 100 101 The mechanism consists of a single message, a string of [UTF-8] 102 encoded [Unicode] characters, from the client to the server. The 103 client presents the authorization identity (identity to act as), 104 followed by a NUL (U+0000) character, followed by the authentication 105 identity (identity whose password will be used), followed by a NUL 106 (U+0000) character, followed by the clear-text password. As with 107 other SASL mechanisms, the client does not provide an authorization 108 identity when it wishes the server to derive an identity from the 109 credentials and use that as the authorization identity. 110 111 112 113 114Zeilenga Standards Track [Page 2] 115 116RFC 4616 The PLAIN SASL Mechanism August 2006 117 118 119 The formal grammar for the client message using Augmented BNF [ABNF] 120 follows. 121 122 message = [authzid] UTF8NUL authcid UTF8NUL passwd 123 authcid = 1*SAFE ; MUST accept up to 255 octets 124 authzid = 1*SAFE ; MUST accept up to 255 octets 125 passwd = 1*SAFE ; MUST accept up to 255 octets 126 UTF8NUL = %x00 ; UTF-8 encoded NUL character 127 128 SAFE = UTF1 / UTF2 / UTF3 / UTF4 129 ;; any UTF-8 encoded Unicode character except NUL 130 131 UTF1 = %x01-7F ;; except NUL 132 UTF2 = %xC2-DF UTF0 133 UTF3 = %xE0 %xA0-BF UTF0 / %xE1-EC 2(UTF0) / 134 %xED %x80-9F UTF0 / %xEE-EF 2(UTF0) 135 UTF4 = %xF0 %x90-BF 2(UTF0) / %xF1-F3 3(UTF0) / 136 %xF4 %x80-8F 2(UTF0) 137 UTF0 = %x80-BF 138 139 The authorization identity (authzid), authentication identity 140 (authcid), password (passwd), and NUL character deliminators SHALL be 141 transferred as [UTF-8] encoded strings of [Unicode] characters. As 142 the NUL (U+0000) character is used as a deliminator, the NUL (U+0000) 143 character MUST NOT appear in authzid, authcid, or passwd productions. 144 145 The form of the authzid production is specific to the application- 146 level protocol's SASL profile [SASL]. The authcid and passwd 147 productions are form-free. Use of non-visible characters or 148 characters that a user may be unable to enter on some keyboards is 149 discouraged. 150 151 Servers MUST be capable of accepting authzid, authcid, and passwd 152 productions up to and including 255 octets. It is noted that the 153 UTF-8 encoding of a Unicode character may be as long as 4 octets. 154 155 Upon receipt of the message, the server will verify the presented (in 156 the message) authentication identity (authcid) and password (passwd) 157 with the system authentication database, and it will verify that the 158 authentication credentials permit the client to act as the (presented 159 or derived) authorization identity (authzid). If both steps succeed, 160 the user is authenticated. 161 162 The presented authentication identity and password strings, as well 163 as the database authentication identity and password strings, are to 164 be prepared before being used in the verification process. The 165 [SASLPrep] profile of the [StringPrep] algorithm is the RECOMMENDED 166 preparation algorithm. The SASLprep preparation algorithm is 167 168 169 170Zeilenga Standards Track [Page 3] 171 172RFC 4616 The PLAIN SASL Mechanism August 2006 173 174 175 recommended to improve the likelihood that comparisons behave in an 176 expected manner. The SASLprep preparation algorithm is not mandatory 177 so as to allow the server to employ other preparation algorithms 178 (including none) when appropriate. For instance, use of a different 179 preparation algorithm may be necessary for the server to interoperate 180 with an external system. 181 182 When preparing the presented strings using [SASLPrep], the presented 183 strings are to be treated as "query" strings (Section 7 of 184 [StringPrep]) and hence unassigned code points are allowed to appear 185 in their prepared output. When preparing the database strings using 186 [SASLPrep], the database strings are to be treated as "stored" 187 strings (Section 7 of [StringPrep]) and hence unassigned code points 188 are prohibited from appearing in their prepared output. 189 190 Regardless of the preparation algorithm used, if the output of a 191 non-invertible function (e.g., hash) of the expected string is 192 stored, the string MUST be prepared before input to that function. 193 194 Regardless of the preparation algorithm used, if preparation fails or 195 results in an empty string, verification SHALL fail. 196 197 When no authorization identity is provided, the server derives an 198 authorization identity from the prepared representation of the 199 provided authentication identity string. This ensures that the 200 derivation of different representations of the authentication 201 identity produces the same authorization identity. 202 203 The server MAY use the credentials to initialize any new 204 authentication database, such as one suitable for [CRAM-MD5] or 205 [DIGEST-MD5]. 206 2073. Pseudo-Code 208 209 This section provides pseudo-code illustrating the verification 210 process (using hashed passwords and the SASLprep preparation 211 function) discussed above. This section is not definitive. 212 213 boolean Verify(string authzid, string authcid, string passwd) { 214 string pAuthcid = SASLprep(authcid, true); # prepare authcid 215 string pPasswd = SASLprep(passwd, true); # prepare passwd 216 if (pAuthcid == NULL || pPasswd == NULL) { 217 return false; # preparation failed 218 } 219 if (pAuthcid == "" || pPasswd == "") { 220 return false; # empty prepared string 221 } 222 223 224 225 226Zeilenga Standards Track [Page 4] 227 228RFC 4616 The PLAIN SASL Mechanism August 2006 229 230 231 storedHash = FetchPasswordHash(pAuthcid); 232 if (storedHash == NULL || storedHash == "") { 233 return false; # error or unknown authcid 234 } 235 236 if (!Compare(storedHash, Hash(pPasswd))) { 237 return false; # incorrect password 238 } 239 240 if (authzid == NULL ) { 241 authzid = DeriveAuthzid(pAuthcid); 242 if (authzid == NULL || authzid == "") { 243 return false; # could not derive authzid 244 } 245 } 246 247 if (!Authorize(pAuthcid, authzid)) { 248 return false; # not authorized 249 } 250 251 return true; 252 } 253 254 The second parameter of the SASLprep function, when true, indicates 255 that unassigned code points are allowed in the input. When the 256 SASLprep function is called to prepare the password prior to 257 computing the stored hash, the second parameter would be false. 258 259 The second parameter provided to the Authorize function is not 260 prepared by this code. The application-level SASL profile should be 261 consulted to determine what, if any, preparation is necessary. 262 263 Note that the DeriveAuthzid and Authorize functions (whether 264 implemented as one function or two, whether designed in a manner in 265 which these functions or whether the mechanism implementation can be 266 reused elsewhere) require knowledge and understanding of mechanism 267 and the application-level protocol specification and/or 268 implementation details to implement. 269 270 Note that the Authorize function outcome is clearly dependent on 271 details of the local authorization model and policy. Both functions 272 may be dependent on other factors as well. 273 274 275 276 277 278 279 280 281 282Zeilenga Standards Track [Page 5] 283 284RFC 4616 The PLAIN SASL Mechanism August 2006 285 286 2874. Examples 288 289 This section provides examples of PLAIN authentication exchanges. 290 The examples are intended to help the readers understand the above 291 text. The examples are not definitive. 292 293 "C:" and "S:" indicate lines sent by the client and server, 294 respectively. "<NUL>" represents a single NUL (U+0000) character. 295 The Application Configuration Access Protocol ([ACAP]) is used in the 296 examples. 297 298 The first example shows how the PLAIN mechanism might be used for 299 user authentication. 300 301 S: * ACAP (SASL "CRAM-MD5") (STARTTLS) 302 C: a001 STARTTLS 303 S: a001 OK "Begin TLS negotiation now" 304 <TLS negotiation, further commands are under TLS layer> 305 S: * ACAP (SASL "CRAM-MD5" "PLAIN") 306 C: a002 AUTHENTICATE "PLAIN" 307 S: + "" 308 C: {21} 309 C: <NUL>tim<NUL>tanstaaftanstaaf 310 S: a002 OK "Authenticated" 311 312 The second example shows how the PLAIN mechanism might be used to 313 attempt to assume the identity of another user. In this example, the 314 server rejects the request. Also, this example makes use of the 315 protocol optional initial response capability to eliminate a round- 316 trip. 317 318 S: * ACAP (SASL "CRAM-MD5") (STARTTLS) 319 C: a001 STARTTLS 320 S: a001 OK "Begin TLS negotiation now" 321 <TLS negotiation, further commands are under TLS layer> 322 S: * ACAP (SASL "CRAM-MD5" "PLAIN") 323 C: a002 AUTHENTICATE "PLAIN" {20+} 324 C: Ursel<NUL>Kurt<NUL>xipj3plmq 325 S: a002 NO "Not authorized to requested authorization identity" 326 3275. Security Considerations 328 329 As the PLAIN mechanism itself provided no integrity or 330 confidentiality protections, it should not be used without adequate 331 external data security protection, such as TLS services provided by 332 many application-layer protocols. By default, implementations SHOULD 333 NOT advertise and SHOULD NOT make use of the PLAIN mechanism unless 334 adequate data security services are in place. 335 336 337 338Zeilenga Standards Track [Page 6] 339 340RFC 4616 The PLAIN SASL Mechanism August 2006 341 342 343 When the PLAIN mechanism is used, the server gains the ability to 344 impersonate the user to all services with the same password 345 regardless of any encryption provided by TLS or other confidentiality 346 protection mechanisms. Whereas many other authentication mechanisms 347 have similar weaknesses, stronger SASL mechanisms address this issue. 348 Clients are encouraged to have an operational mode where all 349 mechanisms that are likely to reveal the user's password to the 350 server are disabled. 351 352 General [SASL] security considerations apply to this mechanism. 353 354 Unicode, [UTF-8], and [StringPrep] security considerations also 355 apply. 356 3576. IANA Considerations 358 359 The SASL Mechanism registry [IANA-SASL] entry for the PLAIN mechanism 360 has been updated by the IANA to reflect that this document now 361 provides its technical specification. 362 363 To: iana@iana.org 364 Subject: Updated Registration of SASL mechanism PLAIN 365 366 SASL mechanism name: PLAIN 367 Security considerations: See RFC 4616. 368 Published specification (optional, recommended): RFC 4616 369 Person & email address to contact for further information: 370 Kurt Zeilenga <kurt@openldap.org> 371 IETF SASL WG <ietf-sasl@imc.org> 372 Intended usage: COMMON 373 Author/Change controller: IESG <iesg@ietf.org> 374 Note: Updates existing entry for PLAIN 375 3767. Acknowledgements 377 378 This document is a revision of RFC 2595 by Chris Newman. Portions of 379 the grammar defined in Section 2 were borrowed from [UTF-8] by 380 Francois Yergeau. 381 382 This document is a product of the IETF Simple Authentication and 383 Security Layer (SASL) Working Group. 384 385 386 387 388 389 390 391 392 393 394Zeilenga Standards Track [Page 7] 395 396RFC 4616 The PLAIN SASL Mechanism August 2006 397 398 3998. Normative References 400 401 [ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for 402 Syntax Specifications: ABNF", RFC 4234, October 2005. 403 404 [Keywords] Bradner, S., "Key words for use in RFCs to Indicate 405 Requirement Levels", BCP 14, RFC 2119, March 1997. 406 407 [SASL] Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple 408 Authentication and Security Layer (SASL)", RFC 4422, 409 June 2006. 410 411 [SASLPrep] Zeilenga, K., "SASLprep: Stringprep Profile for User 412 Names and Passwords", RFC 4013, February 2005. 413 414 [StringPrep] Hoffman, P. and M. Blanchet, "Preparation of 415 Internationalized Strings ("stringprep")", RFC 3454, 416 December 2002. 417 418 [Unicode] The Unicode Consortium, "The Unicode Standard, Version 419 3.2.0" is defined by "The Unicode Standard, Version 420 3.0" (Reading, MA, Addison-Wesley, 2000. ISBN 0-201- 421 61633-5), as amended by the "Unicode Standard Annex 422 #27: Unicode 3.1" 423 (http://www.unicode.org/reports/tr27/) and by the 424 "Unicode Standard Annex #28: Unicode 3.2" 425 (http://www.unicode.org/reports/tr28/). 426 427 [UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO 428 10646", STD 63, RFC 3629, November 2003. 429 430 [TLS] Dierks, T. and E. Rescorla, "The Transport Layer 431 Security (TLS) Protocol Version 1.1", RFC 4346, April 432 2006. 433 4349. Informative References 435 436 [ACAP] Newman, C. and J. Myers, "ACAP -- Application 437 Configuration Access Protocol", RFC 2244, November 438 1997. 439 440 [CRAM-MD5] Nerenberg, L., Ed., "The CRAM-MD5 SASL Mechanism", Work 441 in Progress, June 2006. 442 443 [DIGEST-MD5] Melnikov, A., Ed., "Using Digest Authentication as a 444 SASL Mechanism", Work in Progress, June 2006. 445 446 447 448 449 450Zeilenga Standards Track [Page 8] 451 452RFC 4616 The PLAIN SASL Mechanism August 2006 453 454 455 [IANA-SASL] IANA, "SIMPLE AUTHENTICATION AND SECURITY LAYER (SASL) 456 MECHANISMS", 457 <http://www.iana.org/assignments/sasl-mechanisms>. 458 459 [SMTP-AUTH] Myers, J., "SMTP Service Extension for Authentication", 460 RFC 2554, March 1999. 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506Zeilenga Standards Track [Page 9] 507 508RFC 4616 The PLAIN SASL Mechanism August 2006 509 510 511Appendix A. Changes since RFC 2595 512 513 This appendix is non-normative. 514 515 This document replaces Section 6 of RFC 2595. 516 517 The specification details how the server is to compare client- 518 provided character strings with stored character strings. 519 520 The ABNF grammar was updated. In particular, the grammar now allows 521 LINE FEED (U+000A) and CARRIAGE RETURN (U+000D) characters in the 522 authzid, authcid, passwd productions. However, whether these control 523 characters may be used depends on the string preparation rules 524 applicable to the production. For passwd and authcid productions, 525 control characters are prohibited. For authzid, one must consult the 526 application-level SASL profile. This change allows PLAIN to carry 527 all possible authorization identity strings allowed in SASL. 528 529 Pseudo-code was added. 530 531 The example section was expanded to illustrate more features of the 532 PLAIN mechanism. 533 534Editor's Address 535 536 Kurt D. Zeilenga 537 OpenLDAP Foundation 538 539 EMail: Kurt@OpenLDAP.org 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562Zeilenga Standards Track [Page 10] 563 564RFC 4616 The PLAIN SASL Mechanism August 2006 565 566 567Full Copyright Statement 568 569 Copyright (C) The Internet Society (2006). 570 571 This document is subject to the rights, licenses and restrictions 572 contained in BCP 78, and except as set forth therein, the authors 573 retain all their rights. 574 575 This document and the information contained herein are provided on an 576 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 577 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET 578 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 579 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 580 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 581 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 582 583Intellectual Property 584 585 The IETF takes no position regarding the validity or scope of any 586 Intellectual Property Rights or other rights that might be claimed to 587 pertain to the implementation or use of the technology described in 588 this document or the extent to which any license under such rights 589 might or might not be available; nor does it represent that it has 590 made any independent effort to identify any such rights. Information 591 on the procedures with respect to rights in RFC documents can be 592 found in BCP 78 and BCP 79. 593 594 Copies of IPR disclosures made to the IETF Secretariat and any 595 assurances of licenses to be made available, or the result of an 596 attempt made to obtain a general license or permission for the use of 597 such proprietary rights by implementers or users of this 598 specification can be obtained from the IETF on-line IPR repository at 599 http://www.ietf.org/ipr. 600 601 The IETF invites any interested party to bring to its attention any 602 copyrights, patents or patent applications, or other proprietary 603 rights that may cover technology that may be required to implement 604 this standard. Please address the information to the IETF at 605 ietf-ipr@ietf.org. 606 607Acknowledgement 608 609 Funding for the RFC Editor function is provided by the IETF 610 Administrative Support Activity (IASA). 611 612 613 614 615 616 617 618Zeilenga Standards Track [Page 11] 619 620