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INTERNET-DRAFT                           Editor: Kurt D. Zeilenga
Intended Category: Standards Track            OpenLDAP Foundation
Expires September 2005                              18 March 2005
Updates: RFC 2595



                         The PLAIN SASL Mechanism
                      <draft-ietf-sasl-plain-08.txt>



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
  <address@hidden>.  Please send editorial comments directly to the
  document editor <address@hidden>.

  By submitting this Internet-Draft, I accept the provisions of Section
  4 of RFC 3667.  By submitting this Internet-Draft, I certify that any
  applicable patent or other IPR claims of which I am aware have been
  disclosed, or will be disclosed, and any of which I become aware will
  be disclosed, in accordance with RFC 3668.

  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/1id-abstracts.html

  The list of Internet-Draft Shadow Directories can be accessed at
  http://www.ietf.org/shadow.html


  Copyright (C) The Internet Society (2005).  All Rights Reserved.

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




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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 which lack a simple password authentication command.


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] 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",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
  document are to be interpreted as described in [Keywords].









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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 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]
  follows.

      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
  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
  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.



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  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
  credentials permit the client to act as the (presented or derived)
  authorization identity.  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
  recommended to improve the likelihood that comparisons behave in an
  expected manner.  The SASLprep preparation algorithm is not mandatory
  to allow, when appropriate, the server to employ other preparation
  algorithms (including none).  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
  [DIGEST-MD5].








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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.

      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.




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  It is noted that the DerivateAuthzid and Authorize functions (whether
  implemented as one function or two, whether designed in a manner in
  which these functions or 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.

  It is also noted 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 NULL (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: <NULL>tim<NULL>tanstaaftanstaaf
      S: a002 OK "Authenticated"

  The second example shows how the PLAIN mechanism might be used 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+}



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      C: Ursel<NULL>Kurt<NULL>xipj3plmq
      S: a002 NO "Not authorized to assume asserted identity"


6. Security Considerations

  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 network privacy 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.
  "stringprep" and Unicode [StringPrep] security considerations also
  apply, as do [UTF-8] security considerations.


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: address@hidden
      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 <address@hidden>
           IETF SASL WG <address@hidden>
      Intended usage: COMMON
      Author/Change controller: IESG <address@hidden>
      Note: Updates existing entry for PLAIN



8. Acknowledgment



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  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.

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