The Post Office Protocol (POP3) Simple Authentication and Security Layer (SASL) Authentication Mechanism
draft-siemborski-rfc1734bis-11

Network Working Group                                  Robert Siemborski
INTERNET-DRAFT                                              Google, Inc.
Intended status: Proposed Standard                     Abhijit Menon-Sen
Obsoletes: RFC 1734                               Oryx Mail Systems GmbH
Updates: RFC 2449                                           January 2007
Expires: August 26, 2007

                   POP3 SASL Authentication Mechanism
                   draft-siemborski-rfc1734bis-11.txt

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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.

    This document seeks to consolidate the information related to POP3
    AUTH into a single document.  To this end, this document obsoletes

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    RFC 1734, replacing it as a Proposed Standard, and updates
    information contained in Section 6.3 of RFC 2449.

1.  Conventions Used in This Document

    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 [RFC2119].

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

    Formal syntax is defined by [RFC4234].

2.  Introduction

    The POP3 (see [RFC1939]) AUTH command (see [RFC1734]) has suffered
    several problems in its specification.  The first is that it was
    very similar to a SASL framework defined by [RFC4422], 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, [RFC2449] attempted to remedy this situation by adding the
    CAPA command and allowing an initial client response with the AUTH
    command, but problems remained in the clarity of the specification
    of how the initial client response was to be handled.

    Together, this means creating a full POP3 AUTH implementation
    requires an understanding of material in at least five different
    documents (and [RFC3206] provides additional response codes that are
    useful during authentication).

    This document attempts to combine the information in [RFC1734] and
    [RFC2449] to simplify this situation.  Additionally, it aims to
    clarify and update the older specifications where appropriate.

3.  The SASL Capability

    This section supersedes the definition of the SASL Capability in
    section 6.3 of [RFC2449].

    CAPA tag:
        SASL

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    Arguments:
        Supported SASL Mechanisms

    Added commands:
        AUTH

    Standard Commands Affected
        None

    Announced states / possible differences:
        both / no

    Commands valid in states:
        AUTHORIZATION

    Specification Reference:
        This Document, [RFC4422]

    Discussion
        The SASL capability permits the use of the AUTH command (as
        defined in section 4 of this document) to begin a SASL
        negotiation (as defined in [RFC4422]).  The argument 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 [RFC1734]).

        Note that the list of available mechanisms MAY change after a
        successful STLS command (see [RFC2595]).  However, as required
        by [RFC2449], implementations MUST continue to include the SASL
        capability even after a successful AUTH command has been
        completed (even though no further AUTH commands may be issued).

    Example
        S: +OK pop.example.com BlurdyBlurp POP3 server ready
        C: CAPA
        S: +OK List of capabilities follows
        S: SASL PLAIN DIGEST-MD5 GSSAPI ANONYMOUS
        S: STLS
        S: IMPLEMENTATION BlurdyBlurp POP3 server
        S: .

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4.  The AUTH Command

    AUTH mechanism [initial-response]

      Arguments:

          mechanism: A string identifying a SASL authentication
          mechanism.

          initial-response: An optional initial client response, as
          defined in section 3 of [RFC4422].  If present, this response
          MUST be encoded as Base64 (specified in Section 4 of
          [RFC4648]), or consist only of the single character "=", which
          represents an empty initial response.

      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 an -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 an -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
          using Base64 as specified in Section 4 of [RFC4648].  This
          line MUST NOT contain any text other than the BASE64 encoded
          challenge.

          A client response consists of a line containing a string

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          encoded as 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 an -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 by issuing
          an empty challenge, as defined in section 3 of [RFC4422].  In
          POP3, an empty server challenge 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 255-octet
          limit on the length of a single command, defined in section 4
          of [RFC2449], still applies.  If specifying an initial
          response would cause the AUTH command to exceed this length,
          the client MUST NOT use the initial-response parameter (and
          must proceed instead by sending its initial response after an
          empty challenge from the server, as in section 3 of
          [RFC4422]).

          If the client needs to send a zero-length initial response, it
          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 an
          -ERR reply.

          If the server cannot Base64 decode a client response, it MUST
          reject the AUTH command with an -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

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          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 an -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 the SASL exchange is a
          simple username, and SHOULD use the SASLprep profile (see
          [RFC4013]) of the StringPrep algorithm (see [RFC3454]) 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
          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.

          When both TLS (see [RFC4346]) and SASL security layers are in
          effect, the TLS encoding MUST be applied after the SASL
          encoding when sending data. (According to [RFC2595], STLS can
          only be issued before AUTH in any case.)

          Note that POP3 does not allow for additional data to be sent
          with a message indicating a successful outcome (see section
          3.6 of [RFC4422]).

          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 PLAIN SASL mechanism,
          defined in [RFC4616].

          A server implementation MUST implement a configuration in
          which it does NOT permit any plaintext password mechanisms,
          unless either the STLS command has been used to negotiate a
          TLS session (see [RFC2595]), or some other mechanism that
          protects the session from password snooping has been provided.
          Server sites SHOULD NOT use any configuration which permits a
          plaintext password mechanism without such a protection
          mechanism against password snooping. Client and server
          implementations SHOULD implement additional SASL mechanisms
          that do not send plaintext passwords, such as the [DIGEST-MD5]
          mechanism.

5.  Formal Syntax

    The following syntax specification uses the Augmented Backus-Naur
    Form notation as specified in [RFC4234]. The rules CRLF, ALPHA and
    DIGIT are imported from [RFC4234]. The sasl-mech rule is from
    [RFC4422].

    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.

      auth-command    = "AUTH" SP sasl-mech [SP initial-response]
                        *(CRLF [base64]) [CRLF cancel-response] CRLF

      initial-response= base64 / "="

      cancel-response = "*"

      base64          = base64-terminal /
                        ( 1*(4base64-CHAR) [base64-terminal] )

      base64-char     = ALPHA / DIGIT / "+" / "/"
                        ;; Case-sensitive

      base64-terminal = (2base64-char "==") / (3base64-char "=")

      continue-req    = "+" SP [base64] CRLF

    Additionally, the ABNF specified in [RFC2449] is updated as follows:

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      response        =/ continue-req

6.  Examples

    Here is an example of a client attempting AUTH PLAIN (see [RFC4616])
    under TLS and making use of the initial client response:

        S: +OK pop.example.com BlurdyBlurp POP3 server ready
        C: CAPA
        S: +OK List of capabilities follows
        S: SASL DIGEST-MD5 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 DIGEST-MD5 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 DIGEST-MD5 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 in which the exchange begins
    with a server challenge (the long lines are broken for editorial
    clarity only):

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        S: +OK pop.example.com BlurdyBlurp POP3 server ready
        C: CAPA
        S: +OK List of capabilities follows
        S: SASL DIGEST-MD5 GSSAPI ANONYMOUS
        S: STLS
        S: IMPLEMENTATION BlurdyBlurp POP3 server
        S: .
        C: AUTH DIGEST-MD5
        S: + cmVhbG09ImVsd29vZC5pbm5vc29mdC5jb20iLG5vbmNlPSJPQTZNRzl0
             RVFHbTJoaCIscW9wPSJhdXRoIixhbGdvcml0aG09bWQ1LXNlc3MsY2hh
             cnNldD11dGYtOA==
        C: Y2hhcnNldD11dGYtOCx1c2VybmFtZT0iY2hyaXMiLHJlYWxtPSJlbHdvb2
           QuaW5ub3NvZnQuY29tIixub25jZT0iT0E2TUc5dEVRR20yaGgiLG5jPTAw
           MDAwMDAxLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLGRpZ2VzdC11cmk9In
           BvcC9lbHdvb2QuaW5ub3NvZnQuY29tIixyZXNwb25zZT1iMGQ1NmQyZjA1
           NGMyNGI2MjA3MjMyMjEwNjQ2OGRiOSxxb3A9YXV0aA==
        S: + cnNwYXV0aD0wYjk3MTQ2MmNlZjVlOGY5MzBkYjlhMzNiMDJmYzlhMA==
        C:
        S: +OK Maildrop locked and ready

7.  Security Considerations

    Security issues are discussed throughout this document.

8.  IANA Considerations

    The IANA is requested to refer to this RFC instead of [RFC1734] in
    http://www.iana.org/assignments/pop3-extension-mechanism (the POP3
    extension registry), and also in
    http://www.iana.org/assignments/gssapi-service-names (the
    GSSAPI/SASL service name registry).

9.  Acknowledgments

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

    The authors would also like to thank Ken Murchison, Randall Gellens,
    Alexey Melnikov, Mark Crispin, Arnt Gulbrandsen, Lisa Dusseault,
    Frank Ellermann, and Philip Guenther for their reviews of this
    document.

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10.  Changes From RFC 1734, 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 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 much applicable information into a single
        document.

    10. CR is no longer (incorrectly) defined here.

    12. Explicitly mention that "=" means a zero-length initial
        response.

    13. Change MUST to SHOULD use SASLprep, because nobody does.

    14. Clarify that the TLS encoding should be applied after any SASL
        one.

    15. Note that POP3 doesn't allow additional data to be sent with
        +OK.

    16. Change "_" to "-" in the ABNF, and use the sasl-mech rule
        instead of AUTH_CHAR.

    17. Change the KERBEROS_V4 example to DIGEST-MD5 for now; remove
        KERBEROS_V4.

    18. Reword the reference to [RFC3206] to make it clearer that it is
        not mandatory.

    19. Define the initial-response by reference to SASL.

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    20. Add continue-req to the response production from [RFC2449].

    21. Add initial-response and cancel-response productions to the
        ABNF.

11. Normative References

    [RFC1939]  Myers, Rose, "Post Office Protocol - Version 3", STD 53,
               RFC 1939, May 1996.

    [RFC2119]  Bradner, "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

    [RFC2449]  Gellens, Newman, Lundblade, "POP3 Extension Mechanism",
               RFC 2449, November 1998.

    [RFC2595]  Newman, "Using TLS with IMAP, POP3, and ACAP", RFC 2595,
               June 1999.

    [RFC3454] Hoffman, Blanchet, "Preparation of Internationalized
               Strings ("stringprep")", RFC 3454, December 2002.

    [RFC4013]  Zeilenga, "SASLprep: Stringprep Profile for User Names
               and Passwords", RFC 4013, OpenLDAP Foundation, February
               2005.

    [RFC4234]  Crocker, Overell, "Augmented BNF for Syntax
               Specifications: ABNF", RFC 4234, Brandenburg
               Internetworking, Demon Internet Ltd, October 2005.

    [RFC4422]  Melnikov, Zeilenga, "Simple Authentication and Security
               Layer (SASL)", RFC 4422, June 2006.

    [RFC4648]  Josefsson, "The Base16, Base32, and Base64 Data
               Encodings", RFC 4648, October 2003.

    [RFC4616]  Zeilenga, "The PLAIN Simple Authentication and Security
               Layer (SASL) Mechanism", RFC 4616, OpenLDAP Foundation,
               August 2006.

12. Informative References

    [RFC1734]  Myers, "POP3 AUTHentication Command", RFC 1734, January
               1994.

    [RFC3206]  Gellens, "The SYS and AUTH POP Response Codes", RFC 3206,

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               February 2002.

    [RFC4346]  Dierks, Rescorla, "The Transport Layer Security (TLS)
               Protocol, Version 1.1", RFC 4346, April 2006.

    [DIGEST-MD5] Melnikov, "Using Digest Authentication as a SASL
               Mechanism", draft-ietf-sasl-rfc2831bis-11.txt, Isode
               Ltd., November 2006

13. Authors' Addresses

    Robert Siemborski
    Google, Inc.
    1600 Ampitheatre Parkway
    Mountain View, CA 94043

    Phone: +1 650 623 6925
    Email: robsiemb@google.com

    Abhijit Menon-Sen
    Oryx Mail Systems GmbH

    Email: ams@oryx.com

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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).

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

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