IMAP SASL-IR: IMAP Extension for Simple Authentication and Security Layer (SASL) Initial Client Response

Network Working Group                                      R. Siemborski
Request for Comments: 4959                                  Google, Inc.
Category: Standards Track                                 A. Gulbrandsen
                                                  Oryx Mail Systems GmbH
                                                          September 2007


   IMAP Extension for Simple Authentication and Security Layer (SASL)
                        Initial Client Response

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

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.

1.  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.

2.  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].

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).

   If the client uses an initial response with a SASL mechanism that
   does not support an initial response, the server MUST reject the
   command with a tagged BAD response.

   Note: support and use of the initial client response is optional for
   both clients and servers.  Servers that implement this extension MUST
   support clients that omit the initial client response, and clients
   that implement this extension MUST NOT send an initial client
   response to servers that 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 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 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
        S: * CAPABILITY 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 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 has added SASL-IR to the IMAP4 Capabilities Registry.

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.

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

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.

   [RFC4234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

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

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

9.2.  Informative References

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

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

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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