7Network Working Group K. Zeilenga, Ed.
8Request for Comments: 4616 OpenLDAP Foundation
9Updates: 2595 August 2006
10Category: Standards Track
13 The PLAIN Simple Authentication and Security Layer (SASL) Mechanism
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.
25 Copyright (C) The Internet Society (2006).
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
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60RFC 4616 The PLAIN SASL Mechanism August 2006
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
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
78 The name associated with this mechanism is "PLAIN".
80 The PLAIN SASL mechanism does not provide a security layer.
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])
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.
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].
992. PLAIN SASL Mechanism
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.
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116RFC 4616 The PLAIN SASL Mechanism August 2006
119 The formal grammar for the client message using Augmented BNF [ABNF]
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
128 SAFE = UTF1 / UTF2 / UTF3 / UTF4
129 ;; any UTF-8 encoded Unicode character except NUL
131 UTF1 = %x01-7F ;; except NUL
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) /
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.
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
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.
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.
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
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172RFC 4616 The PLAIN SASL Mechanism August 2006
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.
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.
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.
194 Regardless of the preparation algorithm used, if preparation fails or
195 results in an empty string, verification SHALL fail.
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.
203 The server MAY use the credentials to initialize any new
204 authentication database, such as one suitable for [CRAM-MD5] or
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.
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
219 if (pAuthcid == "" || pPasswd == "") {
220 return false; # empty prepared string
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228RFC 4616 The PLAIN SASL Mechanism August 2006
231 storedHash = FetchPasswordHash(pAuthcid);
232 if (storedHash == NULL || storedHash == "") {
233 return false; # error or unknown authcid
236 if (!Compare(storedHash, Hash(pPasswd))) {
237 return false; # incorrect password
240 if (authzid == NULL ) {
241 authzid = DeriveAuthzid(pAuthcid);
242 if (authzid == NULL || authzid == "") {
243 return false; # could not derive authzid
247 if (!Authorize(pAuthcid, authzid)) {
248 return false; # not authorized
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.
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.
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.
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.
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284RFC 4616 The PLAIN SASL Mechanism August 2006
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.
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
298 The first example shows how the PLAIN mechanism might be used for
301 S: * ACAP (SASL "CRAM-MD5") (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"
309 C: <NUL>tim<NUL>tanstaaftanstaaf
310 S: a002 OK "Authenticated"
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-
318 S: * ACAP (SASL "CRAM-MD5") (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"
3275. Security Considerations
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.
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340RFC 4616 The PLAIN SASL Mechanism August 2006
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
352 General [SASL] security considerations apply to this mechanism.
354 Unicode, [UTF-8], and [StringPrep] security considerations also
3576. IANA Considerations
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.
364 Subject: Updated Registration of SASL mechanism PLAIN
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
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
382 This document is a product of the IETF Simple Authentication and
383 Security Layer (SASL) Working Group.
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396RFC 4616 The PLAIN SASL Mechanism August 2006
3998. Normative References
401 [ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for
402 Syntax Specifications: ABNF", RFC 4234, October 2005.
404 [Keywords] Bradner, S., "Key words for use in RFCs to Indicate
405 Requirement Levels", BCP 14, RFC 2119, March 1997.
407 [SASL] Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple
408 Authentication and Security Layer (SASL)", RFC 4422,
411 [SASLPrep] Zeilenga, K., "SASLprep: Stringprep Profile for User
412 Names and Passwords", RFC 4013, February 2005.
414 [StringPrep] Hoffman, P. and M. Blanchet, "Preparation of
415 Internationalized Strings ("stringprep")", RFC 3454,
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
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/).
427 [UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
428 10646", STD 63, RFC 3629, November 2003.
430 [TLS] Dierks, T. and E. Rescorla, "The Transport Layer
431 Security (TLS) Protocol Version 1.1", RFC 4346, April
4349. Informative References
436 [ACAP] Newman, C. and J. Myers, "ACAP -- Application
437 Configuration Access Protocol", RFC 2244, November
440 [CRAM-MD5] Nerenberg, L., Ed., "The CRAM-MD5 SASL Mechanism", Work
441 in Progress, June 2006.
443 [DIGEST-MD5] Melnikov, A., Ed., "Using Digest Authentication as a
444 SASL Mechanism", Work in Progress, June 2006.
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452RFC 4616 The PLAIN SASL Mechanism August 2006
455 [IANA-SASL] IANA, "SIMPLE AUTHENTICATION AND SECURITY LAYER (SASL)
457 <http://www.iana.org/assignments/sasl-mechanisms>.
459 [SMTP-AUTH] Myers, J., "SMTP Service Extension for Authentication",
460 RFC 2554, March 1999.
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508RFC 4616 The PLAIN SASL Mechanism August 2006
511Appendix A. Changes since RFC 2595
513 This appendix is non-normative.
515 This document replaces Section 6 of RFC 2595.
517 The specification details how the server is to compare client-
518 provided character strings with stored character strings.
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.
529 Pseudo-code was added.
531 The example section was expanded to illustrate more features of the
539 EMail: Kurt@OpenLDAP.org
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564RFC 4616 The PLAIN SASL Mechanism August 2006
567Full Copyright Statement
569 Copyright (C) The Internet Society (2006).
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.
575 This document and the information contained herein are provided on an
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577 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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579 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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