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

                   POP3 SASL Authentication Mechanism


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
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    Distribution of this memo is unlimited.


    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.

    This document obsoletes RFC 1734 and replaces it as a Proposed
    Standard.  It also updates information contained in Section 6.3 of
    RFC 2449.

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

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Table of Contents

1. How to Read This Document . . . . . . . . . . . . . . . . . . . .   1
2. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
3. The SASL Capability . . . . . . . . . . . . . . . . . . . . . . .   3
4. The AUTH Command  . . . . . . . . . . . . . . . . . . . . . . . .   4
4.1. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . .   2
5. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .   8
6. Security Considerations . . . . . . . . . . . . . . . . . . . . .   9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . .   9
8. Intellectual Property Rights  . . . . . . . . . . . . . . . . . .   9
9. Copyright . . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
11. Changes From RFC 1734 and RFC 2449 . . . . . . . . . . . . . . .  11
12. Author's Address . . . . . . . . . . . . . . . . . . . . . . . .  12
13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . .  12

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    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 AUTH command [POP3-AUTH] in [POP3] has suffered several
    deficiencies 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 specification of how the initial
    client response was to be handled remained.  This document attempts
    to combine all of the POP3 SASL authentication related details into
    a single document.

3.  The SASL Capability

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

    CAPA tag:

        Supported SASL Mechanisms

    Standard Commands Affected

    Announced states / possible differences:
        both / no

    Commands valid in states:

    Specification Reference:
        This Document, [SASL]

        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.

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        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.


        S: +OK BlurdyBlurp POP3 server ready
        C: CAPA
        S: +OK List of capabilities follows
        S: STLS
        S: IMPLEMENTATION BlurdyBlurp POP3 server
        S: .

4.  The AUTH Command

    AUTH mechanism [initial-response]

          mechanism: A string identifying a [SASL] authentication

          initial-response: An optional initial client response.  If
          present, this response MUST be [BASE64] encoded.

          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

          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

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          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 [BASE64] encoded
          string supplied by the SASL mechanism.  This challenge MUST
          NOT contain any text other than the BASE64 encoded challenge.

          A client response consists of a line containing a [BASE64]
          encoded string.  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
          parameter (and instead proceed as defined in section 5.1 of

          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

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          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.

          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
          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.

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          To ensure interoperability, client and server implementations
          of this extension MUST implement the STLS extension, and the
          PLAIN SASL mechanism [POP3-TLS].  Implementations MUST support
          a configuration where [SASL] mechanisms that are vulnerable to
          passive eavesdropping attacks are not advertised or used
          without the presence of an external security layer such as

4.1.    Examples

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

     S: +OK BlurdyBlurp POP3 server ready
     C: CAPA
     S: +OK List of capabilities follows
     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: IMPLEMENTATION BlurdyBlurp POP3 server
     S: .
     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: IMPLEMENTATION BlurdyBlurp POP3 server
     S: .
       (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

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    initial client send, and includes server challenges:

     S: +OK BlurdyBlurp POP3 server ready
     C: CAPA
     S: +OK List of capabilities follows
     S: STLS
     S: IMPLEMENTATION BlurdyBlurp POP3 server
     S: .
     S: + ezLUFA==
        (the following lines are broken for editorial clarity only)
     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.  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
    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

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        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

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.

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.

8.  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

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    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

9.  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

    This document and the information contained herein is provided on an

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10.  References

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

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

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

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

[POP3-EXT]  Gellens, R., Newman, C. & 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]      Myers, J., "Simple Authentication and Security Layer
            (SASL)", RFC 2222, October 1997.

The following references are for informational purposes only:

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

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

11.  Changes From RFC 1734 and RFC 2449

            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 TLS+PLAIN.

            4.   Clarify ordering of TLS and SASL security layers.

            5.   Update references to newer versions of various

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            6.   Clarify that the mechanism list can change.

            7.   General other editorial clarifications.

12.  Author's    Address:

    Robert Siemborski
    Carnegie Mellon, Andrew Systems Group
    Cyert Hall 207
    5000 Forbes Avenue
    Pittsburgh, PA  15213
    +1 412 268 7456

13.  Acknowledgments:

    The author would like to acknowledge the contributions of John Myers
    and other contributors to RFC 1734, on which this document draws

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

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