Network Working Group                                   K. Zeilenga, Ed.
Request for Comments: 4616                           OpenLDAP Foundation
Updates: 2595                                                August 2006
Category: Standards Track

  The PLAIN Simple Authentication and Security Layer (SASL) Mechanism

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   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 that lack a simple password authentication

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RFC 4616                The PLAIN SASL Mechanism             August 2006

1.  Introduction

   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 connections where data confidentiality is not

   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 or 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])

   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 that 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
   client presents the authorization identity (identity to act as),
   followed by a NUL (U+0000) character, followed by the authentication
   identity (identity whose password will be used), followed by a NUL
   (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.

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RFC 4616                The PLAIN SASL Mechanism             August 2006

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

   message   = [authzid] UTF8NUL authcid UTF8NUL 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
   UTF8NUL   = %x00 ; UTF-8 encoded NUL character

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

   UTF1      = %x01-7F ;; except NUL
   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 NUL character deliminators SHALL be
   transferred as [UTF-8] encoded strings of [Unicode] characters.  As
   the NUL (U+0000) character is used as a deliminator, the NUL (U+0000)
   character 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 that 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 it will verify that the
   authentication credentials permit the client to act as the (presented
   or derived) authorization identity (authzid).  If both steps succeed,
   the user is authenticated.

   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

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RFC 4616                The PLAIN SASL Mechanism             August 2006

   recommended to improve the likelihood that comparisons behave in an
   expected manner.  The SASLprep preparation algorithm is not mandatory
   so as to allow the server to employ other preparation algorithms
   (including none) when appropriate.  For instance, use of a 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 of
   [StringPrep]) and hence unassigned code points are allowed to appear
   in their prepared output.  When preparing the database strings using
   [SASLPrep], the database strings are to be treated as "stored"
   strings (Section 7 of [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 produces the same authorization identity.

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

3.  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.

   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

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RFC 4616                The PLAIN SASL Mechanism             August 2006

     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.

   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.

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RFC 4616                The PLAIN SASL Mechanism             August 2006

4.  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.  "<NUL>" 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 authentication.

   C: a001 STARTTLS
   S: a001 OK "Begin TLS negotiation now"
   <TLS negotiation, further commands are under TLS layer>
   S: + ""
   C: {21}
   C: <NUL>tim<NUL>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 round-

   C: a001 STARTTLS
   S: a001 OK "Begin TLS negotiation now"
   <TLS negotiation, further commands are under TLS layer>
   C: a002 AUTHENTICATE "PLAIN" {20+}
   C: Ursel<NUL>Kurt<NUL>xipj3plmq
   S: a002 NO "Not authorized to requested authorization identity"

5.  Security Considerations

   As the PLAIN mechanism itself provided no integrity or
   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.

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RFC 4616                The PLAIN SASL Mechanism             August 2006

   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.  Whereas many other authentication mechanisms
   have similar weaknesses, stronger SASL mechanisms address this issue.
   Clients are encouraged to have an operational mode where all
   mechanisms that 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

6.  IANA Considerations

   The SASL Mechanism registry [IANA-SASL] entry for the PLAIN mechanism
   has been updated by the IANA to reflect that this document now
   provides its technical specification.

   Subject: Updated Registration of SASL mechanism PLAIN

   SASL mechanism name: PLAIN
   Security considerations: See RFC 4616.
   Published specification (optional, recommended): RFC 4616
   Person & email address to contact for further information:
        Kurt Zeilenga <>
        IETF SASL WG <>
   Intended usage: COMMON
   Author/Change controller: IESG <>
   Note: Updates existing entry for PLAIN

7.  Acknowledgements

   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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RFC 4616                The PLAIN SASL Mechanism             August 2006

8.  Normative References

   [ABNF]        Crocker, D., Ed. 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., Ed., and K. Zeilenga, Ed., "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"
                 ( and by the
                 "Unicode Standard Annex #28: Unicode 3.2"

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

   [TLS]         Dierks, T. and E. Rescorla, "The Transport Layer
                 Security (TLS) Protocol Version 1.1", RFC 4346, April

9.  Informative References

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

   [CRAM-MD5]    Nerenberg, L., Ed., "The CRAM-MD5 SASL Mechanism", Work
                 in Progress, June 2006.

   [DIGEST-MD5]  Melnikov, A., Ed., "Using Digest Authentication as a
                 SASL Mechanism", Work in Progress, June 2006.

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RFC 4616                The PLAIN SASL Mechanism             August 2006


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

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RFC 4616                The PLAIN SASL Mechanism             August 2006

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.

Editor's Address

   Kurt D. Zeilenga
   OpenLDAP Foundation


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RFC 4616                The PLAIN SASL Mechanism             August 2006

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   Copyright (C) The Internet Society (2006).

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