GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc4616

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).

Abstract

 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
 command.

Zeilenga Standards Track [Page 1] 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
 ensured.
 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])
 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 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",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 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.

Zeilenga Standards Track [Page 2] RFC 4616 The PLAIN SASL Mechanism August 2006

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

Zeilenga Standards Track [Page 3] 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
 [DIGEST-MD5].

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
   }

Zeilenga Standards Track [Page 4] 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.

Zeilenga Standards Track [Page 5] 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
 examples.
 The first example shows how the PLAIN mechanism might be used for
 user authentication.
 S: * ACAP (SASL "CRAM-MD5") (STARTTLS)
 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"
 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-
 trip.
 S: * ACAP (SASL "CRAM-MD5") (STARTTLS)
 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<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.

Zeilenga Standards Track [Page 6] 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
 apply.

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.
 To: iana@iana.org
 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 <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

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.

Zeilenga Standards Track [Page 7] 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"
               (http://www.unicode.org/reports/tr27/) and by the
               "Unicode Standard Annex #28: Unicode 3.2"
               (http://www.unicode.org/reports/tr28/).
 [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
               2006.

9. Informative References

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

Zeilenga Standards Track [Page 8] RFC 4616 The PLAIN SASL Mechanism August 2006

 [IANA-SASL]   IANA, "SIMPLE AUTHENTICATION AND SECURITY LAYER (SASL)
               MECHANISMS",
               <http://www.iana.org/assignments/sasl-mechanisms>.
 [SMTP-AUTH]   Myers, J., "SMTP Service Extension for Authentication",
               RFC 2554, March 1999.

Zeilenga Standards Track [Page 9] 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
 EMail: Kurt@OpenLDAP.org

Zeilenga Standards Track [Page 10] RFC 4616 The PLAIN SASL Mechanism August 2006

Full Copyright Statement

 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
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 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.
 Copies of IPR disclosures made to the IETF Secretariat and any
 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
 http://www.ietf.org/ipr.
 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
 ietf-ipr@ietf.org.

Acknowledgement

 Funding for the RFC Editor function is provided by the IETF
 Administrative Support Activity (IASA).

Zeilenga Standards Track [Page 11]

/data/webs/external/dokuwiki/data/pages/rfc/rfc4616.txt · Last modified: 2006/08/28 17:43 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki