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Versions: 00 01 02 03 04 05 06 07 08 09 rfc4616          Standards Track
INTERNET-DRAFT                           Editor: Kurt D. Zeilenga
Intended Category: Standards Track            OpenLDAP Foundation
Expires December 2006                                16 June 2006
Updates: RFC 2595

                         The PLAIN SASL Mechanism

Status of Memo

  This document is intended to be, after appropriate review and
  revision, submitted to the RFC Editor as a Standards Track document.
  Distribution of this memo is unlimited.  Technical discussion of this
  document will take place on the IETF SASL mailing list
  <ietf-sasl@imc.org>.  Please send editorial comments directly to the
  document editor <Kurt@OpenLDAP.org>.

  By submitting this Internet-Draft, each author represents that any
  applicable patent or other IPR claims of which he or she is aware have
  been or will be disclosed, and any of which he or she becomes aware
  will be disclosed, in accordance with Section 6 of BCP 79.

  Internet-Drafts are working documents of the Internet Engineering Task
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  Copyright (C) The Internet Society (2006).  All Rights Reserved.

  Please see the Full Copyright section near the end of this document
  for more information.

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  This document defines a simple clear-text user/password Simple
  Authentication and Security Layer (SASL) mechanism called the PLAIN
  mechanism.  The PLAIN mechanism is intended to be used, in combination
  with data confidentiality services provided by a lower layer, in
  protocols which lack a simple password authentication command.

1. Background and Intended Usage

  Clear-text, multiple-use passwords are simple, interoperate with
  almost all existing operating system authentication databases, and are
  useful for a smooth transition to a more secure password-based
  authentication mechanism. The drawback is that they are unacceptable
  for use over network connection where data confidentiality is not
  assured (by encryption or other means).

  This document defines the PLAIN Simple Authentication and Security
  Layer ([SASL]) mechanism for use in protocols with no clear-text login
  command (e.g., [ACAP] or [SMTP-AUTH]).  This document updates RFC
  2595, replacing Section 6.  Changes since RFC 2595 are detailed in
  Appendix A.

  The name associated with this mechanism is "PLAIN".

  The PLAIN SASL mechanism does not provide a security layer.

  The PLAIN mechanism should not be used without adequate data security
  protection as this mechanism affords no integrity nor confidentiality
  protections itself.  The mechanism is intended to be used with data
  security protections provided by application layer protocol, generally
  through its use of Transport Layer Security ([TLS]) services.

  By default, implementations SHOULD advertise and make use of the PLAIN
  mechanism only when adequate data security services are in place.
  Specifications for IETF protocols which indicate that this mechanism
  is an applicable authentication mechanism MUST mandate that
  implementations support an strong data security service, such as TLS.

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  document are to be interpreted as described in [Keywords].

2. PLAIN SASL mechanism

  The mechanism consists of a single message, a string of [UTF-8]
  encoded [Unicode] characters, from the client to the server.  The

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  client presents the authorization identity (identity to act as),
  followed by a NULL (U+0000) character, followed by the authentication
  identity (identity whose password will be used), followed by a NULL
  (U+0000) character, followed by the clear-text password.  As with
  other SASL mechanisms, the client does not provide an authorization
  identity when it wishes the server to derive an identity from the
  credentials and use that as the authorization identity.

  The formal grammar for the client message using Augmented BNF [ABNF]

      message   = [authzid] UTF8NULL authcid UTF8NULL passwd
      authcid   = 1*SAFE ; MUST accept up to 255 octets
      authzid   = 1*SAFE ; MUST accept up to 255 octets
      passwd    = 1*SAFE ; MUST accept up to 255 octets
      UTF8NULL  = %x00 ; UTF-8 encoded NULL character

      SAFE      = UTF1 / UTF2 / UTF3 / UTF4
                  ;; any UTF-8 encoded Unicode character except NULL

      UTF1      = %x01-7F ;; except NULL
      UTF2      = %xC2-DF UTF0
      UTF3      = %xE0 %xA0-BF UTF0 / %xE1-EC 2(UTF0) /
                  %xED %x80-9F UTF0 / %xEE-EF 2(UTF0)
      UTF4      = %xF0 %x90-BF 2(UTF0) / %xF1-F3 3(UTF0) /
                  %xF4 %x80-8F 2(UTF0)
      UTF0      = %x80-BF

  The authorization identity (authzid), authentication identity
  (authcid), password (passwd), and NULL character deliminators SHALL be
  transferred as [UTF-8] encoded strings of [Unicode] characters.  As
  the NULL (U+0000) character is used as a deliminator, the NULL
  (U+0000) character MUST NOT appear in authzid, authcid, or passwd

  The form of the authzid production is specific to the
  application-level protocol's SASL profile [SASL].  The authcid and
  passwd productions are form-free.  Use of non-visible characters or
  characters which a user may be unable to enter on some keyboards is

  Servers MUST be capable of accepting authzid, authcid, and passwd
  productions up to and including 255 octets.  It is noted that the
  UTF-8 encoding of a Unicode character may be as long as 4 octets.

  Upon receipt of the message, the server will verify the presented (in
  the message) authentication identity (authcid) and password (passwd)
  with the system authentication database, and verify the authentication

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  credentials permit the client to act as the (presented or derived)
  authorization identity.  If both steps succeed, the user is

  The presented authentication identity and password strings, as well as
  the database authentication identity and password strings, are to be
  prepared before being used in the verification process.  The
  [SASLPrep] profile of the [StringPrep] algorithm is the RECOMMENDED
  preparation algorithm.  The SASLprep preparation algorithm is
  recommended to improve the likelihood that comparisons behave in an
  expected manner.  The SASLprep preparation algorithm is not so as to
  allow the server to employ other preparation algorithms (including
  none) when appropriate.  For instance, use of different preparation
  algorithm may be necessary for the server to interoperate with an
  external system.

  When preparing the presented strings using [SASLPrep], the presented
  strings are to be treated as "query" strings [Section 7, Stringprep]
  and hence unassigned code points are allowed appear in their prepared
  output.  When preparing the database strings using [SASLprep], the
  database strings are to be treated as "stored" strings [Section 7,
  Stringprep] and hence unassigned code points are prohibited from
  appearing in their prepared output.

  Regardless of the preparation algorithm used, if the output of a non-
  invertible function (e.g., hash) of the expected string is stored, the
  string MUST be prepared before input to that function.

  Regardless of the preparation algorithm used, if preparation fails or
  results in an empty string, verification SHALL fail.

  When no authorization identity is provided, the server derives an
  authorization identity from the prepared representation of the
  provided authentication identity string.  This ensures that the
  derivation of different representations of the authentication identity
  produce the same authorization identity.

  The server MAY use the credentials to initialize any new
  authentication database, such as one suitable for [CRAM-MD5] or

4. Pseudo-Code

  This section provides pseudo-code illustrating the verification
  process (using hashed passwords and the SASLprep preparation function)
  discussed above.  This section is not definitive.

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      boolean Verify(string authzid, string authcid, string passwd) {
        string pAuthcid = SASLprep(authcid, true); # prepare authcid
        string pPasswd = SASLprep(passwd, true);   # prepare passwd
        if (pAuthcid == NULL || pPasswd == NULL) {
          return false;     # preparation failed
        if (pAuthcid == "" || pPasswd == "") {
          return false;     # empty prepared string

        storedHash = FetchPasswordHash(pAuthcid);
        if (storedHash == NULL || storedHash == "") {
          return false;     # error or unknown authcid

        if (!Compare(storedHash, Hash(pPasswd))) {
          return false;     # incorrect password

        if (authzid == NULL ) {
          authzid = DeriveAuthzid(pAuthcid);
          if (authzid == NULL || authzid == "") {
              return false; # could not derive authzid

        if (!Authorize(pAuthcid, authzid)) {
          return false;     # not authorized

        return true;

  The second parameter of the SASLprep function, when true, indicates
  that unassigned code points are allowed in the input.  When the
  SASLprep function is called to prepare the password prior to computing
  the stored hash, the second parameter would be false.

  The second parameter provided to the Authorize function is not
  prepared by this code.  The application-level SASL profile should be
  consulted to determine what, if any, preparation is necessary.

  Note that the DeriveAuthzid and Authorize functions (whether
  implemented as one function or two, whether designed in a manner in
  which these functions or whether the mechanism implementation can be
  reused elsewhere) require knowledge and understanding of mechanism and
  the application-level protocol specification and/or implementation
  details to implement.

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  Note that the Authorize function outcome is clearly dependent on
  details of the local authorization model and policy.  Both functions
  may be dependent on other factors as well.

5. Examples

  This section provides examples of PLAIN authentication exchanges.  The
  examples are intended to help the readers understand the above text.
  The examples are not definitive.

  "C:" and "S:" indicate lines sent by the client and server
  respectively.  "<NULL>" represents a single NUL (U+0000) character.
  The Application Configuration Access Protocol ([ACAP]) is used in the

  The first example shows how the PLAIN mechanism might be used for user

      C: a001 STARTTLS
      S: a001 OK "Begin TLS negotiation now"
      <TLS negotiation, further commands are under TLS layer>
      S: * ACAP (SASL "CRAM-MD5" "PLAIN")
      S: + ""
      C: {21}
      C: <NULL>tim<NULL>tanstaaftanstaaf
      S: a002 OK "Authenticated"

  The second example shows how the PLAIN mechanism might be used to
  attempt to assume the identity of another user.  In this example, the
  server rejects the request.  Also, this example makes use of the
  protocol optional initial response capability to eliminate a

      C: a001 STARTTLS
      S: a001 OK "Begin TLS negotiation now"
      <TLS negotiation, further commands are under TLS layer>
      S: * ACAP (SASL "CRAM-MD5" "PLAIN")
      C: a002 AUTHENTICATE "PLAIN" {20+}
      C: Ursel<NULL>Kurt<NULL>xipj3plmq
      S: a002 NO "Not authorized to requested authorization identity"

6. Security Considerations

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  As the PLAIN mechanism itself provided no integrity nor
  confidentiality protections, it should not be used without adequate
  external data security protection, such as TLS services provided by
  many application layer protocols.  By default, implementations SHOULD
  NOT advertise and SHOULD NOT make use of the PLAIN mechanism unless
  adequate data security services are in place.

  When the PLAIN mechanism is used, the server gains the ability to
  impersonate the user to all services with the same password regardless
  of any encryption provided by TLS or other confidentiality protection
  mechanisms.  While many other authentication mechanisms have similar
  weaknesses, stronger SASL mechanisms address this issue.  Clients are
  encouraged to have an operational mode where all mechanisms which are
  likely to reveal the user's password to the server are disabled.

  General [SASL] security considerations apply to this mechanism.

  Unicode, [UTF-8], and [StringPrep] security considerations also apply.

7. IANA Considerations

  It is requested that the SASL Mechanism registry [IANA-SASL] entry for
  the PLAIN mechanism be updated to reflect that this document now
  provides its technical specification.

      To: iana@iana.org
      Subject: Updated Registration of SASL mechanism PLAIN

      SASL mechanism name: PLAIN
      Security considerations: See RFC XXXX.
      Published specification (optional, recommended): RFC XXXX
      Person & email address to contact for further information:
           Kurt Zeilenga <kurt@openldap.org>
           IETF SASL WG <ietf-sasl@imc.org>
      Intended usage: COMMON
      Author/Change controller: IESG <iesg@ietf.org>
      Note: Updates existing entry for PLAIN

8. Acknowledgment

  This document is a revision of RFC 2595 by Chris Newman.  Portions of
  the grammar defined in Section 2 were borrowed from [UTF-8] by
  Francois Yergeau.

  This document is a product of the IETF Simple Authentication and
  Security Layer (SASL) Working Group.

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9. Normative References

  [ABNF]        Crocker, D. and P. Overell, "Augmented BNF for Syntax
                Specifications: ABNF", RFC 4234, October 2005.

  [Keywords]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997

  [SASL]        Melnikov, A. (Editor), K. Zeilenga (Editor), "Simple
                Authentication and Security Layer (SASL)", RFC 4422,
                June 2006.

  [SASLPrep]    Zeilenga, K., "SASLprep: Stringprep Profile for User
                Names and Passwords", RFC 4013, February 2005.

  [StringPrep]  Hoffman, P. and M. Blanchet, "Preparation of
                Internationalized Strings ('stringprep')", RFC 3454,
                December 2002.

  [Unicode]     The Unicode Consortium, "The Unicode Standard, Version
                3.2.0" is defined by "The Unicode Standard, Version 3.0"
                (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5),
                as amended by the "Unicode Standard Annex #27: Unicode
                3.1" (http://www.unicode.org/reports/tr27/) and by the
                "Unicode Standard Annex #28: Unicode 3.2"

  [UTF-8]       Yergeau, F., "UTF-8, a transformation format of ISO
                10646", RFC 3629 (also STD 63), November 2003.

  [TLS]         Dierks, T. and, E. Rescorla, "The TLS Protocol Version
                1.1", RFC 4346, April 2006.

10. Informative References

  [ACAP]        Newman, C. and J. Myers, "ACAP -- Application
                Configuration Access Protocol", RFC 2244, November 1997.
  [CRAM-MD5]    Nerenberg, L., "The CRAM-MD5 SASL Mechanism",
                draft-ietf-sasl-crammd5-xx.txt, a work in progress.

  [DIGEST-MD5]  Leach, P., C. Newman, and A. Melnikov, "Using Digest
                Authentication as a SASL Mechanism",
                draft-ietf-sasl-rfc2831bis-xx.txt, a work in progress.


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  [SMTP-AUTH]   Myers, J., "SMTP Service Extension for Authentication",
                RFC 2554, March 1999.

11. Editor's Address

  Kurt D. Zeilenga
  OpenLDAP Foundation

  Email: Kurt@OpenLDAP.org

Appendix A.  Changes since RFC 2595

  This appendix is non-normative.

  This document replaces Section 6 of RFC 2595.

  The specification details how the server is to compare client-provided
  character strings with stored character strings.

  The ABNF grammar was updated.  In particular, the grammar now allows
  LINE FEED (U+000A) and CARRIAGE RETURN (U+000D) characters in the
  authzid, authcid, passwd productions.   However, whether these control
  characters may be used depends on the string preparation rules
  applicable to the production.   For passwd and authcid productions,
  control characters are prohibited.  For authzid, one must consult the
  application-level SASL profile.  This change allows PLAIN to carry all
  possible authorization identity strings allowed in SASL.

  Pseudo-code was added.

  The example section was expanded to illustrate more features of the
  PLAIN mechanism.

Intellectual Property

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  Intellectual Property Rights or other rights that might be claimed to
  pertain to the implementation or use of the technology described in
  this document or the extent to which any license under such rights
  might or might not be available; nor does it represent that it has
  made any independent effort to identify any such rights.  Information
  on the procedures with respect to rights in RFC documents can be found
  in BCP 78 and BCP 79.

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  assurances of licenses to be made available, or the result of an
  attempt made to obtain a general license or permission for the use of
  such proprietary rights by implementers or users of this specification
  can be obtained from the IETF on-line IPR repository at

  The IETF invites any interested party to bring to its attention any
  copyrights, patents or patent applications, or other proprietary
  rights that may cover technology that may be required to implement
  this standard.  Please address the information to the IETF at

Full Copyright

  Copyright (C) The Internet Society (2006).

  This document is subject to the rights, licenses and restrictions
  contained in BCP 78, and except as set forth therein, the authors
  retain all their rights.

  This document and the information contained herein are provided on an

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