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INTERNET-DRAFT                           Editor: Kurt D. Zeilenga
Intended Category: Standards Track            OpenLDAP Foundation
Expires in six months                                  4 May 2003
Updates: RFC 2595

                         The Plain SASL Mechanism

Status of Memo

  This document is an Internet-Draft and is in full conformance with all
  provisions of Section 10 of RFC2026.

  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>.

  Internet-Drafts are working documents of the Internet Engineering Task
  Force (IETF), its areas, and its working groups.  Note that other
  groups may also distribute working documents as Internet-Drafts.
  Internet-Drafts are draft documents valid for a maximum of six months
  and may be updated, replaced, or obsoleted by other documents at any
  time.  It is inappropriate to use Internet-Drafts as reference
  material or to cite them other than as ``work in progress.''

  The list of current Internet-Drafts can be accessed at
  <http://www.ietf.org/ietf/1id-abstracts.txt>. The list of
  Internet-Draft Shadow Directories can be accessed at

  Copyright 2003, The Internet Society.  All Rights Reserved.

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


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

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

1. Background and Intended Usage

  Clear-text 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 an unencrypted
  network connection.

  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]).

  The name associated with this mechanism is "PLAIN".

  The PLAIN SASL mechanism does not provide a security layer.  This
  mechanism MUST NOT be used without adequate security protection as the
  mechanism affords no integrity nor confidentiality protection itself.
  The PLAIN SASL mechanism MUST NOT be advertised unless a strong
  encryption layer, such as provided by Transport Layer Security
  ([TLS]), is active.

  This document updates RFC 2595, replacing Section 6.  Changes since
  RFC 2595 are detailed in Appendix A.

2. PLAIN SASL mechanism

  The mechanism consists of a single message from the client to the
  server.  The client sends the authorization identity (identity to
  login as), followed by a NUL character, followed by the authentication
  identity (identity whose password will be used), followed by a NUL
  character, followed by the clear-text password.   The client leaves
  the authorization identity empty if wishes the server to derive the
  authorization identity from the provided authentication identity.

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

      message   = [authzid] NUL authcid NUL passwd
      authcid   = 1*SAFE ; MUST accept up to 255 octets
      authzid   = 1*SAFE ; MUST accept up to 255 octets

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      passwd    = 1*SAFE ; MUST accept up to 255 octets
      NUL       = %x00

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

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

  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 6 octets.

  Upon receipt of the message, the server will verify presented
  authentication identity (authcid) and password (passwd) with the
  system authentication database and that the authentication credentials
  permit the client to login as the (presented or derived) authorization
  identity.  If both steps succeed, the user is authenticated.

  The presented authentication identity and password strings are not be
  compared directly with stored strings.  The server SHALL first prepare
  authentication identity strings and password strings using the
  [SASLPrep] profile of the [StringPrep] algorithm.  If preparation
  fails or results in an empty string, verification SHALL fail.  If the
  server stores only the hash of expected string, that string MUST be
  prepared before generation of the hash.

  The verification function (using hashed password) can be written (in

      boolean Verify(string authzid, string authcid, string passwd) {

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        string pAuthcid = SASLprep(authcid); # prepare authcid
        string pPasswd = SASLprep(passwd);   # 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(pPassword))) {
          return false;     # incorrect password

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

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

        return true;

  Note that the parameters provided to the Authorize function are not
  prepared.  The application-level SASL profile should be consulted to
  determine what, if any, preparation is necessary.

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

4. Example

  Here is an example of how this might be used to initialize a CRAM-MD5
  authentication database using the Application Configuration Access
  Protocol ([ACAP]).  "C:" and "S:" indicate lines sent by the client
  and server respectively and <NUL> represents a single NUL (U+0000)

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      S: + "<1896.697170952@postoffice.reston.mci.net>"
      C: "tim b913a602c7eda7a495b4e6e7334d3890"
         "Please change your password, or use TLS to login"
      C: a002 STARTTLS
      S: a002 OK "Begin TLS negotiation now"
      <TLS negotiation, further commands are under TLS layer>
      C: a003 AUTHENTICATE "PLAIN" {21+}
      C: <NUL>tim<NUL>tanstaaftanstaaf
      S: a003 OK CRAM-MD5 password initialized

5. Security Considerations

  The PLAIN mechanism relies on the TLS encryption layer for security.
  When used without TLS, it is vulnerable to a common network
  eavesdropping attack.  Therefore PLAIN MUST NOT be advertised or used
  unless a suitable TLS encryption layer is active or backwards
  compatibility dictates otherwise.

  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 network privacy mechanisms.
  While many other authentication mechanisms have similar weaknesses,
  stronger SASL mechanisms such as the Kerberos-based GSSAPI mechanism
  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.  It is RECOMMENDED that this mode
  be the default.

  General SASL security considerations apply to this mechanism.
  "stringprep" and Unicode security considerations also apply.

6. 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

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      Security considerations: See RFC XXXX.
      Published specification (optional, recommended): RFC XXXX
      Person & email address to contact for further information:
          Kurt Zeilenga <kurt@openldap.org>
          Chris Neuman <chris.newman@innosoft.com>
      Intended usage: COMMON
      Author/Change controller: IESG <iesg@ietf.org>
      Note: Updates existing entry for PLAIN

7. Acknowledgement

  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.

8. Normative References

  [ABNF]       D. Crocker, P. Overell, "Augmented BNF for Syntax
               Specifications: ABNF", RFC 2234, November 1997.

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

  [SASL]       J. Myers, "Simple Authentication and Security Layer
               (SASL)", RFC 2222bis (a work in progress).

  [SASLPrep]   K. Zeilenga, "SASL String Preparation Profiles", draft-
               ietf-sasl-saslprep (a work in progress).

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

  [Unicode]    The Unicode Consortium, "The Unicode Standard, Version
               3.2.0", 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 #28: Unicode 3.2

  [UTF-8]      F. Yergeau, "UTF-8, a transformation format of ISO
               10646", RFC 2279, January 1998.

  [TLS]        T. Dierks, C. Allen, "The TLS Protocol Version 1.0", RFC
               2246, January 1999.

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9. Informative References

  [ACAP]       C. Newman, and J. Myers, "ACAP -- Application
               Configuration Access Protocol", RFC 2244, November 1997.

  [CRAM-MD5]   J. Klensin, R. Catoe, and P. Krumviede, "IMAP/POP
               AUTHorize Extension for Simple Challenge/Response", RFC
               2195, September 1997.

  [DIGEST-MD5] P. Leach, C. Newman, "Using Digest Authentication as a
               SASL Mechanism", RFC 2831, May 2000.

               MECHANISMS", http://www.iana.org/assignments/sasl-

10. Editor's Address

  Kurt 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.

Full Copyright Statement

  Copyright 2003, The Internet Society.  All Rights Reserved.

  This document and translations of it may be copied and furnished to

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  others, and derivative works that comment on or otherwise explain it
  or assist in its implementation may be prepared, copied, published and
  distributed, in whole or in part, without restriction of any kind,
  provided that the above copyright notice and this paragraph are
  included on all such copies and derivative works.  However, this
  document itself may not be modified in any way, such as by removing
  the copyright notice or references to the Internet Society or other
  Internet organizations, except as needed for the  purpose of
  developing Internet standards in which case the procedures for
  copyrights defined in the Internet Standards process must be followed,
  or as required to translate it into languages other than English.

  The limited permissions granted above are perpetual and will not be
  revoked by the Internet Society or its successors or assigns.

  This document and the information contained herein is provided on an

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