Network Working Group Robert Siemborski
INTERNET-DRAFT Carnegie Mellon University
Intended Category: Standards Track October, 2003
IMAP Extension for SASL Initial Client Response
Status of this Memo
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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.
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
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IMAP Extension for SASL Initial Client Response October, 2003
Table of Contents
1. How to Read This Document . . . . . . . . . . . . . . . . . . . . 1
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Changes to the IMAP AUTHENTICATE Command . . . . . . . . . . . . 3
4. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. Intellectual Property Rights . . . . . . . . . . . . . . . . . . 5
7. Copyright . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
9. Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
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Levels" [KEYWORDS]
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
Formal syntax is defined using [ABNF] as modified by [IMAP4].
2. Introduction
The [SASL] initial client response extension is present in any
[IMAP4] server implementation which returns "SASL" 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]
mechanisms which support it. Clients which support this extension
SHOULD make use of the initial response whenever they attempt to
authenticate using a SASL mechanism which supports 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 [IMAP4]. If this second
argument is present, it represents the contents of the "initial
client response" defined in section 5.1 of [SASL].
As with any other client response, this initial client response MUST
be [BASE64] encoded. It also MUST be transmitted outside of a
quoted string or literal. If the client needs to transmit an
initial response which is a zero-length string, it MUST transmit a
single equals sign ("="). This indicates that the response is
present, but is a zero-length string.
If the mechanism chosen by the client does not support an initial
client response, the server MUST fail the authentication with a
tagged NO response. If the server cannot decode the [BASE64] data
transmitted by the client, the server MUST fail the authentication
with a tagged BAD response. In particular, the server MUST reject
(and not ignore) any characters not explicitly allowed by the BASE64
alphabet, and should 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: 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
clients which implement this extension MUST NOT send an initial
client response to servers which do not advertise the SASL
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capability. If either the client or the server do not support the
SASL capability, a mechanism which uses an initial client response
is negotiated using the challenge/response exchange described in
[IMAP4], with an initial zero-length server challenge.
The following is an example authentication using the [PLAIN] SASL
mechanism (under a [TLS] protection layer) and an initial client
response:
... client connects to server and negotiates a TLS protection layer ...
C: C01 CAPABILITY
S: * CAPABILITY IMAP4 IMAP4rev1 SASL AUTH=PLAIN
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 AUTH=PLAIN
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)
Clients which support this extension MUST fall back to an [IMAP4]
compatible mode when they encounter a server which does not
advertise this extension.
4. Formal Syntax
The following syntax specification uses the Augmented Bakus-Naur
Form [ABNF] notation. Non-terminals referenced but not defined
below are as defined by [IMAP4].
capability =/ "SASL"
authenticate = "AUTHENTICATE" SP auth-type [SP base64] *(CRLF base64)
;;redefine AUTHENTICATE command defined in [IMAP4]
5. Security Considerations
The extension defined in this document is subject to many of the
Security Considerations defined in [IMAP4] and [SASL].
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Server implementations MUST treat the omission of an initial client
response from the AUTHENTICATE command as defined by [IMAP4] (as if
this extention did not exist).
Although [IMAP4] 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.
6. 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
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.
7. 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
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IMAP Extension for SASL Initial Client Response October, 2003
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.
8. References
The following documents contain normative definitions or
specifications that are necessary for correct understanding of this
protocol:
[ABNF] Crocker, D., "Augmented BNF for Syntax Specifications:
ABNF", RFC 2234, November, 1997.
[BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[IMAP4] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[SASL] Myers, J., "Simple Authentication and Security Layer
(SASL)", RFC 2222, October 1997.
The following references are for informational purposes only:
[PLAIN] Newman, C. "Using TLS with IMAP, POP3, and ACAP", RFC 2595,
June 1999.
[TLS] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
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9. 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
10. Acknowledgments:
The author would like to acknowledge the contributions of Ken
Murchison and Mark Crispin and their assistance in reviewing this
document.
Alexey Melnikov and Cyrus Daboo also had some early discussions
about this extension
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