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Versions: 00 01 02 03 04 05 06 RFC 4959

Network Working Group                                  Robert Siemborski
INTERNET-DRAFT                                              Google, Inc.
Intended Category: Standards Track                      Arnt Gulbrandsen
                                                  Oryx Mail Systems GmbH
                                                           November 2006


            IMAP Extension for SASL Initial Client Response
           draft-siemborski-imap-sasl-initial-response-06.txt


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Abstract

    To date, the Internet Message Access Protocol (IMAP) has used a
    Simple Authentication and Security Layer (SASL) profile which always
    required at least one complete round trip for an authentication, as
    it did not support an initial client response argument.  This
    additional round trip at the beginning of the session is
    undesirable, especially when round trip costs are high.

    This document defines an extension to IMAP which allows clients and
    servers to avoid this round trip by allowing an initial client
    response argument to the IMAP AUTHENTICATE command.



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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] as extended by [RFC3501].


2.  Introduction

    The SASL initial client response extension is present in any IMAP
    [RFC3501] server implementation which returns "SASL-IR" as one of
    the supported capabilities in its CAPABILITY response.

    Servers which support this extension will accept an optional initial
    client response with the AUTHENTICATE command for any SASL [RFC4422]
    mechanisms which support it.


3.  IMAP Changes to the IMAP AUTHENTICATE Command

    This extension adds an optional second argument to the AUTHENTICATE
    command that is defined in Section 6.2.2 of [RFC3501].  If this
    second argument is present, it represents the contents of the
    "initial client response" defined in section 5.1 of [RFC4422].

    As with any other client response, this initial client response MUST
    be encoded as defined in Section 4 of [RFC4648].  It also MUST be
    transmitted outside of a quoted string or literal.  To send a zero-
    length initial response, the client MUST send a single pad character
    ("=").  This indicates that the response is present, but is a zero-
    length string.

    When decoding the BASE64 [RFC4648] data in the initial client
    response, decoding errors MUST be treated as IMAP [RFC3501] would
    handle them in any normal SASL client response.  In particular, the
    server should check for any characters not explicitly allowed by the
    BASE64 alphabet, as well as 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: support and use of the initial client response is optional for
    both clients and servers.  Servers which implement this extension
    MUST support clients which omit the initial client response, and



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    clients which implement this extension MUST NOT send an initial
    client response to servers which do not advertise the SASL-IR
    capability.  In such a situation, clients MUST fall back to an IMAP
    [RFC3501] compatible mode.

    If either the client or the server do not support the SASL-IR
    capability, a mechanism which uses an initial client response is
    negotiated using the challenge/response exchange described in
    [RFC3501], with an initial zero-length server challenge.


4.  Examples

    The following is an example authentication using the PLAIN (see
    [RFC4616]) SASL mechanism (under a TLS protection layer, see
    [RFC4346]) and an initial client response:

             ... client connects to server and negotiates a TLS
            protection layer ...
         C: C01 CAPABILITY
         S: * CAPABILITY IMAP4 IMAP4rev1 SASL-IR AUTH=PLAIN
         S: C01 OK Completed
         C: A01 AUTHENTICATE PLAIN dGVzdAB0ZXN0AHRlc3Q=
         S: A01 OK Success (tls protection)

    Note that even when a server supports this extension, the following
    negotiation (which does not use the initial response) is still valid
    and MUST be supported by the server:

             ... client connects to server and negotiates a TLS
            protection layer ...
         C: C01 CAPABILITY
         S: * CAPABILITY IMAP4 IMAP4rev1 SASL-IR AUTH=PLAIN
         S: C01 OK Completed
         C: A01 AUTHENTICATE PLAIN
             (note that there is a space following the "+" in the
            following line)
         S: +
         C: dGVzdAB0ZXN0AHRlc3Q=
         S: A01 OK Success (tls protection)

    The following is an example authentication using the SASL EXTERNAL
    mechanism (defined in [RFC4422]) under a TLS protection layer (see
    [RFC4346]) and an empty initial client response:

             ... client connects to server and negotiates a TLS
            protection layer ...
         C: C01 CAPABILITY



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         S: * CAPABILITY IMAP4 IMAP4rev1 SASL-IR AUTH=PLAIN
            AUTH=EXTERNAL
         S: C01 OK Completed
         C: A01 AUTHENTICATE EXTERNAL =
         S: A01 OK Success (tls protection)

    This is in contrast with the handling of such a situation when an
    initial response is omitted:

          ... client connects to server and negotiates a TLS protection
            layer ...
         C: C01 CAPABILITY
         S: * CAPABILITY IMAP4 IMAP4rev1 SASL-IR AUTH=PLAIN
            AUTH=EXTERNAL
         S: C01 OK Completed
         C: A01 AUTHENTICATE EXTERNAL
             (note that there is a space following the "+" in the
            following line)
         S: +
         C:
         S: A01 OK Success (tls protection)


5.  IANA Considerations

    The IANA is requested to add SASL-IR to the list of IMAP extensions.


6.  Security Considerations

    The extension defined in this document is subject to many of the
    Security Considerations defined in [RFC3501] and [RFC4422].

    Server implementations MUST treat the omission of an initial client
    response from the AUTHENTICATE command as defined by [RFC3501] (as
    if this extension did not exist).

    Although [RFC3501] has no express line length limitations, some
    implementations choose to enforce them anyway.  Such implementations
    MUST be aware that the addition of the initial response parameter to
    AUTHENTICATE may increase the maximum line length that IMAP parsers
    may expect to support.  Server implementations MUST be able to
    receive the largest possible initial client response that their
    supported mechanisms might receive.







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7.  Formal Syntax

    The following syntax specification uses the Augmented Backus-Naur
    Form [RFC4234] notation.  [RFC3501] defines the non-terminals
    capability, auth-type and base64.

        capability    =/ "SASL-IR"

        authenticate  = "AUTHENTICATE" SP auth-type [SP (base64 / "=")]
                        *(CRLF base64)
                                          ;;redefine AUTHENTICATE from
                        [RFC3501]


8.  Acknowledgments

    The authors would like to acknowledge the contributions of Ken
    Murchison and Mark Crispin, along with the rest of the IMAPEXT
    Working Group for their assistance in reviewing this document.

    Alexey Melnikov and Cyrus Daboo also had some early discussions
    about this extension.


9.  References


9.1. Normative References

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

[RFC3501]  Crispin, "Internet Message Access Protocol - Version 4rev1",
           RFC 3501, University of Washington, June 2003.

[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.







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9.2. Informative References

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

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


10. Authors' addresses.

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

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


    Arnt Gulbrandsen
    Oryx Mail Systems GmbH
    Schweppermannstr. 8
    D-81671 Muenchen
    Germany

    Email: arnt@oryx.com























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Acknowledgment

    Funding for the RFC Editor function is currently provided by the
    Internet Society.







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             (RFC Editor: Remove everything after this point.)


Changes since -05

    - Updated the references

    - Reworded around some references for clearer prose


Changes since -04

    - Reformatting to make idnits 1.103 as happy as I can get it

    - Update references, at least RFC4422.


Changes since -03

    (Not known)


Changes since -02

    (Not known)


Changes since -01

    - Remove implicit SHOULD behavior for clients.

    - Indicate that the Section 3 BASE64 encoding should be used.


Changes since -00

    - Add missing CAPABILITY OK responses in examples

    - Change capability string to "SASL-IR"

    - Fix a nit regarding an "=" in the initial response ABNF

    - Clean up wording of BASE64 decoding requirements to be more in
    line with [RFC3501]

    - Add examples of an empty initial client response, and move
    examples into their own section




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    - Update SASL reference to rfc2222bis


















































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