Bump versions.

[gsasl.git] / doc / specification / draft-siemborski-rfc1734bis-02.txt

Network Working Group                                  Robert Siemborski
INTERNET-DRAFT                                Carnegie Mellon University
Intended Category: Proposed Standard                      December, 2003


                   POP3 SASL Authentication Mechanism

                  <draft-siemborski-rfc1734bis-02.txt>


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

    Internet-Drafts are working documents of the Internet 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
    http://www.ietf.org/shadow.html.

    Distribution of this memo is unlimited.

Abstract

    This document defines a profile of the Simple Authentication and
    Security Layer (SASL) for the Post Office Protocol (POP3).  This
    extension allows a POP3 client to indicate an authentication
    mechanism to the server, perform an authentication protocol
    exchange, and optionally negotiate a security layer for subsequent
    protocol interactions during this session.

    In order to consolidate all of the authentication related
    information for POP3 into a single document, this document obsoletes
    RFC 1734 and RFC 3206, replacing them as a Proposed Standard. It
    also updates information contained in Section 6.3 and Section 8 of
    RFC 2449.






Siemborski                  Expires May, 2004                   [Page 1]





POP3 SASL Authentication Mechanism                        December, 2003


Table of Contents


1. How to Read This Document . . . . . . . . . . . . . . . . . . . .   3
2. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
3. The SASL Capability . . . . . . . . . . . . . . . . . . . . . . .   3
4. The AUTH Command  . . . . . . . . . . . . . . . . . . . . . . . .   4
4.1. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . .   7
4.2. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
5. Extended POP3 Response Codes  . . . . . . . . . . . . . . . . . .  10
5.1. The LOGIN-DELAY Response Code . . . . . . . . . . . . . . . . .  10
5.2. The IN-USE Response Code  . . . . . . . . . . . . . . . . . . .  10
5.3. The AUTH Response Code  . . . . . . . . . . . . . . . . . . . .  11
5.3.1. The AUTH-RESP-CODE Capability . . . . . . . . . . . . . . . .  11
5.4. The SYS Response Code . . . . . . . . . . . . . . . . . . . . .  12
6. Security Considerations . . . . . . . . . . . . . . . . . . . . .  12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . .  13
8. Protocol Actions  . . . . . . . . . . . . . . . . . . . . . . . .  13
9. Intellectual Property Rights  . . . . . . . . . . . . . . . . . .  13
10. Copyright  . . . . . . . . . . . . . . . . . . . . . . . . . . .  14
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . .  14
12. Changes From RFC 1734, RFC 2449, and RFC 3206  . . . . . . . . .  16
13. Author's Address . . . . . . . . . . . . . . . . . . . . . . . .  16
14. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . .  16



























Siemborski                  Expires May, 2004                   [Page 2]





POP3 SASL Authentication Mechanism                        December, 2003


1.  How to Read This Document

    The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD
    NOT", "RECOMMENDED", and "MAY" in this document are to be
    interpreted as defined in "Key words for use in RFCs to Indicate
    Requirement Levels" [KEYWORDS]

    In examples, "C:" and "S:" indicate lines sent by the client and
    server respectively.

2.  Introduction

    The [POP3] AUTH command [POP3-AUTH] in has suffered several problems
    in its specification.  The first is that it was very similar to a
    [SASL] framework, but pre-dated the initial SASL specification.  It
    was therefore missing some key components, such as a way to list the
    available authentication mechanisms.

    Later, [POP3-EXT] attempted to remedy this situation by adding the
    CAPA command and allowing an initial client response to the AUTH
    command, however problems in the clarity of the specification of how
    the initial client response was to be handled remained.

    Additionally, there is yet another document, [POP3-CODES], that
    provides additional response codes that are useful during
    authentication.  Together, this means creating a full POP3 AUTH
    implementaiton requires an understanding of material in atleast six
    different documents.

    This document attempts to combine all of the POP3 SASL
    authentication related details into a single document, in addition
    to clarifying and updating the older specifications where
    appropriate.

3.  The SASL Capability

    This section supercedes the definition of the SASL Capability in
    section 6.3 of [POP3-EXT].

    CAPA tag:
        SASL

    Arguments:
        Supported SASL Mechanisms

    Standard Commands Affected
        None




Siemborski                  Expires May, 2004                   [Page 3]





POP3 SASL Authentication Mechanism                        December, 2003


    Announced states / possible differences:
        both / no

    Commands valid in states:
        AUTHORIZATION

    Specification Reference:
        This Document, [SASL]

    Discussion
        The SASL capability permits the use of the AUTH command (as
        defined in section 4 of this document) to begin a [SASL]
        negotiation.  The arguments to the SASL capability is a space-
        separated list of SASL mechanisms which are supported.

        If a server either does not support the CAPA command or does not
        advertise the SASL capability, clients SHOULD NOT attempt the
        AUTH command.  If a client does attempt the AUTH command in such
        a situation, it MUST NOT supply the client initial response
        parameter (for backwards compatibility with [POP3-AUTH]).

        Note that the list of available mechanisms MAY change after a
        successful STLS command [POP3-TLS].  Additionally,
        implementations MAY choose to omit the SASL capability after a
        successful AUTH command has been completed.

    Example

        S: +OK pop.example.com BlurdyBlurp POP3 server ready
        C: CAPA
        S: +OK List of capabilities follows
        S: SASL KERBEROS_V4 GSSAPI ANONYMOUS
        S: STLS
        S: IMPLEMENTATION BlurdyBlurp POP3 server
        S: .


4.  The AUTH Command

    AUTH mechanism [initial-response]

      Arguments:
          mechanism: A string identifying a [SASL] authentication
          mechanism.

          initial-response: An optional initial client response.  If
          present, this response MUST be encoded as specified in Section
          3 of [BASE64].



Siemborski                  Expires May, 2004                   [Page 4]





POP3 SASL Authentication Mechanism                        December, 2003


      Restrictions:
          After an AUTH command has been successfully completed, no more
          AUTH commands may be issued in the same session.  After a
          successful AUTH command completes, a server MUST reject any
          further AUTH commands with a -ERR reply.

          The AUTH command may only be given during the AUTHORIZATION
          state.

      Discussion:
          The AUTH command initiates a [SASL] authentication exchange
          between the client and the server.  The client identifies the
          SASL mechanism to use with the first parameter of the AUTH
          command.  If the server supports the requested authentication
          mechanism, it performs the SASL exchange to authenticate the
          user.  Optionally, it also negotiates a security layer for
          subsequent protocol interactions during this session.  If the
          requested authentication mechanism is not supported, the
          server rejects the AUTH command with a -ERR reply.

          The authentication protocol exchange consists of a series of
          server challenges and client responses that are specific to
          the chosen [SASL] mechanism.

          A server challenge is sent as a line consisting of a "+"
          character followed by a single space and a string encoded as
          specified in Section 3 of [BASE64].  This challenge MUST NOT
          contain any text other than the BASE64 encoded challenge.

          A client response consists of a line containing a string
          encoded as defined in Section 3 of [BASE64].  If the client
          wishes to cancel the authentication exchange, it issues a line
          with a single "*".  If the server receives such a response, it
          MUST reject the AUTH command by sending a -ERR reply.

          The optional initial response argument to the AUTH command is
          used to save a round trip when using authentication mechanisms
          that support an initial client response.  If the initial
          response argument is omitted and the chosen mechanism requires
          an initial client response, the server MUST proceed as defined
          in section 5.1 of [SASL].  In POP3, a server challenge with no
          data is defined as line with only a "+" followed by a single
          space.  It MUST NOT contain any other data.

          For the purposes of the initial client response, the line
          length limitation defined in [POP3-EXT] still applies.  If a
          client initial send would cause the AUTH command to exceed
          this length, the client MUST NOT use the initial response



Siemborski                  Expires May, 2004                   [Page 5]





POP3 SASL Authentication Mechanism                        December, 2003


          parameter (and instead proceed as defined in section 5.1 of
          [SASL]).

          If the client needs to send a zero-length initial response,
          the client MUST transmit the response as a single equals sign
          ("=").  This indicates that the response is present, but
          contains no data.

          If the client uses an initial-response argument to the AUTH
          command with a SASL mechanism that does not support an initial
          client send, the server MUST reject the AUTH command with a
          -ERR reply.

          If the server cannot [BASE64] decode any client response, it
          MUST reject the AUTH command with a -ERR reply.  If the client
          cannot BASE64 decode any of the server's challenges, it MUST
          cancel the authentication using the "*" response.  In
          particular, servers and clients MUST reject (and not ignore)
          any character not explicitly allowed by the BASE64 alphabet,
          and MUST reject any sequence of BASE64 characters that
          contains the pad character ('=') anywhere other than the end
          of the string (e.g. "=AAA" and "AAA=BBB" are not allowed).

          Note that these [BASE64] strings (excepting the initial client
          response) may be of arbitrarily length.  Clients and servers
          MUST be able to handle the maximum encoded size of challenges
          and responses generated by their supported authentication
          mechanisms.  This requirement is independent of any line
          length limitations the client or server may have in other
          parts of its protocol implementation.

          If the server is unable to authenticate the client, it MUST
          reject the AUTH command with a -ERR reply.  Should the client
          successfully complete the exchange, the server issues a +OK
          reply.  Additionally, upon success, the POP3 session enters
          the TRANSACTION state.

          The authorization identity generated by this [SASL] exchange
          is a simple username, and MUST use the [SASLprep] profile of
          the [StringPrep] algorithm to prepare these names for
          matching.  If preparation of the authorization identity fails
          or results in an empty string (unless it was transmitted as
          the empty string), the server MUST fail the authentication.

          If a security layer is negotiated during the SASL exchange, it
          takes effect for the client on the octet immediately following
          the CRLF that concludes the last response generated by the
          client.  For the server, it takes effect immediately following



Siemborski                  Expires May, 2004                   [Page 6]





POP3 SASL Authentication Mechanism                        December, 2003


          the CRLF of its success reply.

          When a security layer takes effect, the server MUST discard
          any knowledge previously obtained from the client, which was
          not obtained from the SASL negotiation itself.  Likewise, the
          client MUST discard any knowledge obtained from the server,
          such as the list of available POP3 service extensions.  After
          a security layer is established, the server SHOULD NOT
          advertise either the SASL or the STLS extension.

          When both [TLS] and SASL security layers are in effect, the
          TLS encoding MUST be applied after the SASL encoding,
          regardless of the order in which the layers were negotiated.

          The service name specified by this protocol's profile of SASL
          is "pop".

          If an AUTH command fails, the client may try another
          authentication mechanism or present different credentials by
          issuing another AUTH command (or by using one of the other
          [POP3] authentication mechanisms).  Likewise, the server MUST
          behave as if the client had not issued the AUTH command.

          To ensure interoperability, client and server implementations
          of this extension MUST implement the [DIGEST-MD5] SASL
          mechanism.


     <<Open Issue: Is this the best choice of mandatory-to-implement
       mechanism for POP3?  IMAP arrived at a choice that equates
       to STLS+PLAIN, and therefore is likely to be implemented in
       clients already.  Is there really a compelling reason to
       choose something else?

       DIGEST-MD5 has been suggested as a choice that does
       not require servers to implement TLS, which is desireable from a
       code complexity/deployability standpoint.  However, DIGEST-MD5 also
       requires the storage of (essentially) plaintext equivilent passwords
       which also may not be acceptable in some enviornments.>>


4.1.    Formal Syntax

    The following syntax specification uses the Augmented Backus-Naur
    Form notation as specified in [ABNF].

    Except as noted otherwise, all alphabetic characters are case-
    insensitive.  The use of upper or lower case characters to define



Siemborski                  Expires May, 2004                   [Page 7]





POP3 SASL Authentication Mechanism                        December, 2003


    token strings is for editorial clarity only.  Implementations MUST
    accept these strings in a case-insensitive fashion.


        UPALPHA         = %x41-5A            ;; Uppercase: A-Z

        LOALPHA         = %x61-7A            ;; Lowercase: a-z

        ALPHA           = UPALPHA / LOALPHA  ;; case insensitive

        DIGIT           = %x30-39            ;; Digits 0-9

        AUTH_CHAR       = ALPHA / DIGIT / "-" / "_"

        auth_type       = 1*20AUTH_CHAR

        auth_command    = "AUTH" SPACE auth_type [SPACE (base64 / "=")]
                          *(CRLF [base64]) CRLF

        base64          = base64_terminal /
                          ( 1*(4base64_CHAR) [base64_terminal] )

        base64_char     = UPALPHA / LOALPHA / DIGIT / "+" / "/"
                          ;; Case-sensitive

        base64_terminal = (2base64_char "==") / (3base64_char "=")

        continue_req    = "+" SPACE [base64] CRLF

        CR              = %x0C           ;; ASCII CR, carriage return

        CRLF            = CR LF

        LF              = %x0A           ;; ASCII LF, line feed

        SPACE           = %x20           ;; ASCII SP, space


4.2.    Examples

    Here is an example of a client attempting AUTH PLAIN under TLS and
    making use of the initial client response:









Siemborski                  Expires May, 2004                   [Page 8]





POP3 SASL Authentication Mechanism                        December, 2003


     S: +OK pop.example.com BlurdyBlurp POP3 server ready
     C: CAPA
     S: +OK List of capabilities follows
     S: SASL KERBEROS_V4 GSSAPI ANONYMOUS
     S: STLS
     S: IMPLEMENTATION BlurdyBlurp POP3 server
     S: .
     C: STLS
     S: +OK Begin TLS negotiation now
       ... TLS negotiation proceeds, further commands protected by TLS layer ...
     C: CAPA
     S: +OK List of capabilities follows
     S: SASL PLAIN KERBEROS_V4 GSSAPI ANONYMOUS
     S: IMPLEMENTATION BlurdyBlurp POP3 server
     S: .
     C: AUTH PLAIN dGVzdAB0ZXN0AHRlc3Q=
     S: +OK Maildrop locked and ready

    Here is another client that is attempting AUTH PLAIN under a TLS
    layer, this time without the initial response.  Parts of the
    negotiation before the TLS layer was established have been omitted:

       ... TLS negotiation proceeds, further commands protected by TLS layer ...
     C: CAPA
     S: +OK List of capabilities follows
     S: SASL PLAIN KERBEROS_V4 GSSAPI ANONYMOUS
     S: IMPLEMENTATION BlurdyBlurp POP3 server
     S: .
     C: AUTH PLAIN
       (note that there is a space following the '+' on the following line)
     S: +
     C: dGVzdAB0ZXN0AHRlc3Q=
     S: +OK Maildrop locked and ready

    Here is an example using a mechanism which does not support an
    initial client send, and includes server challenges:















Siemborski                  Expires May, 2004                   [Page 9]





POP3 SASL Authentication Mechanism                        December, 2003


     S: +OK pop.example.com BlurdyBlurp POP3 server ready
     C: CAPA
     S: +OK List of capabilities follows
     S: SASL KERBEROS_V4 GSSAPI ANONYMOUS
     S: STLS
     S: IMPLEMENTATION BlurdyBlurp POP3 server
     S: .
     C: AUTH KERBEROS_V4
     S: + ezLUFA==
        (the following lines are broken for editorial clarity only)
     C: BAYFQU5EUkVXLkNNVS5FRFUAOCCXeMiVyFe9K6Nwne7+sPLgIoF9YQ5ePfxUsMlJAf
        C7aoNySU8nrqS9m8JAddsUeuyc5HFXXovaKLrZNo2bTLH0Lyolwy0W9ryJDojbKmHy
        zSMqFsGD4EL0
     S: + Z74fTwDw7KQ=
     C: vSAF7ha6qotK2UHUgKlsEA==
     S: +OK Maildrop locked and ready
        ... at this point a security layer has been established and additional
            commands and responses proceed within it ...


5.  Extended POP3 Response Codes

    This section defines four POP3 response codes which can be used to
    determine the reason for a failed login (provided that the server
    advertises the RESP-CODES capability [POP3-EXT]).  These definitions
    supercede those in [POP3-EXT] and [POP3-CODES].

    It is RECOMMENDED that server applications use these codes when
    possible to allow clients a straightforward, interoperable way to
    determine the cause of an authentication failure (as opposed to
    parsing error text).

5.1.    The LOGIN-DELAY Response Code

    This occurs on an -ERR response to an AUTH, USER (see note), PASS or
    APOP command and indicates that the user has logged in recently and
    will not be allowed to login again until the login delay period has
    expired.

    Please see the Security Considerations section of this document for
    an important note about returning this code in response to the USER
    command.

5.2.    The IN-USE Response Code

    This occurs on an -ERR response to an AUTH, APOP, or PASS command.
    It indicates the authentication was successful, but the user's
    maildrop is currently in use (probably by another POP3 client).



Siemborski                  Expires May, 2004                  [Page 10]





POP3 SASL Authentication Mechanism                        December, 2003


5.3.    The AUTH Response Code

    The AUTH response code informs the client that there is a problem
    with the user's credentials.  This might be an incorrect password,
    an unknown user name, an expired account, an attempt to authenticate
    in violation of policy (such as from an invalid location or during
    an unauthorized time), or some other problem.

    The AUTH response code is valid with an -ERR response to any
    authentication command including AUTH, USER (see the note in the
    Security Considerations section of this document), PASS, or APOP.

    Servers which include the AUTH response code with any authentication
    failure SHOULD support the CAPA command [POP3-EXT] and SHOULD
    include the AUTH-RESP-CODE capability (defined in the next section)
    in the CAPA response.  AUTH-RESP-CODE assures the client that only
    errors with the AUTH code are caused by credential problems.

5.3.1.  The AUTH-RESP-CODE Capability

    CAPA tag:
        AUTH-RESP-CODE

    Arguments:
        none

    Added commands:
        none

    Standard commands affected:
        none

    Announced states / possible differences:
        both / no

    Commands valid in states:
        n/a

    Specification reference:
        this document

    Discussion:
        The AUTH-RESP-CODE capability indicates that the server includes
        the AUTH response code with any authentication error caused by a
        problem with the user's credentials.






Siemborski                  Expires May, 2004                  [Page 11]





POP3 SASL Authentication Mechanism                        December, 2003


5.4.    The SYS Response Code

    The SYS response code announces that a failure is due to a system
    error, as opposed to the user's credentials or an external
    condition.  It is hierarchical, with two possible second-level
    codes: TEMP and PERM.  (Case is not significant at any level of the
    hierarchy.)

    SYS/TEMP indicates a problem which is likely to be temporary in
    nature, and therefore there is no need to alarm the user, unless the
    failure persists.  Examples might include a central resource which
    is currently locked or otherwise temporarily unavailable,
    insufficient free disk or memory, etc.

    SYS/PERM is used for problems which are unlikely to be resolved
    without intervention.  It is appropriate to alert the user and
    suggest that the organization's support or assistance personnel be
    contacted.  Examples include corrupted mailboxes, system
    configuration errors, etc.

    The SYS response code is valid with an -ERR response to any command.

6.  Security Considerations

    Security issues are discussed throughout this memo.

    Before the [SASL] negotiation has begun, any protocol interactions
    are performed in the clear and may be modified by an active
    attacker.  For this reason, clients and servers MUST discard any
    knowledge obtained prior to the start of the SASL negotiation upon
    the establishment of a security layer.

    Servers MAY implement a policy whereby the connection is dropped
    after a number of failed authentication attempts.  If they do so,
    they SHOULD NOT drop the connection until atleast 3 attempts to
    authenticate have failed.

    Implementations MUST support a configuration where [SASL] mechanisms
    that are vulnerable to passive eavesdropping attacks (such as
    [PLAIN]) are not advertised or used without the presence of an
    external security layer such as [TLS].

    Returning the LOGIN-DELAY or AUTH response codes to the USER command
    avoids the work of authenticating the user but is likely to reveal
    information to the client about the existence of the account in
    question.  Unless the server is operating in an environment where
    user names are not secret (for example, many popular email clients
    advertise the POP server and user name in an outgoing mail header),



Siemborski                  Expires May, 2004                  [Page 12]





POP3 SASL Authentication Mechanism                        December, 2003


    or where server access is restricted, or the server can verify that
    the connection is to the same user, the the server SHOULD NOT issue
    this response code to the USER command.  The server still saves the
    cost of opening the maildrop, which in some environments is the most
    expensive step.

7.  IANA Considerations

    This document requests that the IANA update the entry for the "pop"
    SASL protocol name to point at this document.

    This document requests that the IANA update the entry for the SASL
    POP3 capability to be as defined in Section 3 of this document.

    This document requests that the IANA update the entry for the LOGIN-
    DELAY, IN-USE, AUTH, SYS/TEMP, and SYS/PERM POP3 response codes to
    this document.

    This document requests that the IANA update the entry for the AUTH-
    RESP-CODE capability to be as defined in Section 5.3.1 of this
    document.

8.  Protocol Actions

    [RFC Editor: Remove this section before publication]

    This document obsoletes RFC 1734 and replaces it as a Proposed
    Standard.  By moving RFC 1734 to Historic, RFC 1731 can also be
    moved to Historic (as RFC 1734 was the last document to have a
    normative reference).

    This document obsoletes RFC 3206 and replaces it as a Proposed
    Standard.

    It also updates information contained in Section 6.3 and Section 8
    of RFC 2449.

9.  Intellectual Property Rights

    The IETF takes no position regarding the validity or scope of any
    intellectual property 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; neither does it represent that it
    has made any effort to identify any such rights.  Information on the
    IETF's procedures with respect to rights in standards-track and
    standards-related documentation can be found in BCP-11.  Copies of
    claims of rights made available for publication and any assurances



Siemborski                  Expires May, 2004                  [Page 13]





POP3 SASL Authentication Mechanism                        December, 2003


    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 implementors or users of this specification
    can be obtained from the IETF Secretariat.

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

10.  Copyright

    Copyright (C) The Internet Society (2003). All Rights Reserved.

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

    This document and the information contained herein is provided on an
    "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
    TASK FORCE DISCLAIMS 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.
















Siemborski                  Expires May, 2004                  [Page 14]





POP3 SASL Authentication Mechanism                        December, 2003


11.  References

     The following documents contain normative definitions or
specifications that are necessary for correct understanding of this
protocol:

[BASE64]    Josefsson, S., "The Base16, Base32, and Base64 Data
            Encodings", RFC 3548, July 2003.

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

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

[POP3]      Myers, J. and Rose, M., "Post Office Protocol - Version 3",
            STD 53, RFC 1939, May 1996.

[POP3-EXT]  Gellens, R., Newman, C., and Lundblade, L., "POP3 Extension
            Mechanism", RFC 2449, November 1998.

[POP3-TLS]  Newman, C., "Using TLS with IMAP, POP3, and ACAP", RFC 2595,
            June 1999.

[SASL]      Melnikov, A., "Simple Authentication and Security Layer
            (SASL)", draft-ietf-sasl-rfc2222bis-*.txt, a work in
            progress.

[SASLprep]  Zeilega, K., "SASLprep: Stringprep profile for user names
            and passwords", draft-ietf-sasl-saslprep-*.txt, a work in
            progress

[StringPrep]
            Hoffman, P. and Blanchet, M., "Preparation of
            Internationalized Strings ("stringprep")", draft-hoffman-
            rfc3454bis-*.txt, a work in progress

The following references are for informational purposes only:

[PLAIN]     Zeilenga, K., "The Plain SASL Mechanism", draft-ietf-sasl-
            plain-*.txt, a work in progress.

[POP3-AUTH] Myers, J., "POP3 AUTHentication Command", RFC 1734, January
            1994.

[POP3-CODES]



Siemborski                  Expires May, 2004                  [Page 15]





POP3 SASL Authentication Mechanism                        December, 2003


            Gellens, R., "The SYS and AUTH POP Response Codes", RFC
            3206, February 2002.

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

12.  Changes From RFC 1734, RFC 2449, and RFC 3206

            1.   The SASL-based semantics defined in RFC 2449 are now
                 normative for the AUTH extension.

            2.   Clarifications and examples of the proper behavior of
                 initial client response handling.

            3.   Minimum requirement of support for DIGEST-MD5.

            4.   Clarify ordering of TLS and SASL security layers.

            5.   Update references to newer versions of various
                 specifications.

            6.   Clarify that the mechanism list can change.

            7.   Add the use of the SASLprep profile for preparing
                 authorization identities.

            8.   General other editorial clarifications.

            9.   Consolidation of all applicable information into a
                 single document.

13.  Author's    Address:

    Robert Siemborski
    Carnegie Mellon, Andrew Systems Group
    Cyert Hall 207
    5000 Forbes Avenue
    Pittsburgh, PA  15213
    +1 412 268 7456
    rjs3+@andrew.cmu.edu

14.  Acknowledgments:

    The author would like to acknowledge the contributions of John
    Myers, Randall Gellens, Chris Newman, Laurence Lundblade,  and other
    contributors to RFC 1734, RFC 2554, and RFC 3206, on which this
    document draws heavily.




Siemborski                  Expires May, 2004                  [Page 16]





POP3 SASL Authentication Mechanism                        December, 2003


    The author would also like to thank Ken Murchison, Alexey Melnikov,
    and Mark Crispin for the time they devoted to reviewing early drafts
    of this document.
















































Siemborski                  Expires May, 2004                  [Page 17]