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draft-siemborski-imap-sasl-initial-response-04.txt --> draft-siemborski-imap-sasl-initial-response-05.txt

INTERNET-DRAFT Google, Inc.
Intended Category: Standards Track February, 2005 Arnt Gulbrandsen
Oryx Mail Systems GmbH
November 2006

IMAP Extension for SASL Initial Client Response
<draft-siemborski-imap-sasl-initial-response-04.txt>
draft-siemborski-imap-sasl-initial-response-05.txt

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aware will be disclosed, in accordance with RFC 3668.

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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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Table of Contents

1. How to Read This Document . . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Changes to the IMAP AUTHENTICATE Command . . . . . . . . . . . . 4
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Intellectual Property Rights . . . . . . . . . . . . . . . . . . 7
8. Copyright . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
10. Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8
12. Changes since -01 . . . . . . . . . . . . . . . . . . . . . . . 9
13. Changes since -00 . . . . . . . . . . . . . . . . . . . . . . . 9

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1. How to Read Conventions Used in This Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "MAY" "OPTIONAL" in this
document are to be interpreted as defined in "Key words for use described in RFCs to Indicate
Requirement Levels" [KEYWORDS] [RFC2119].

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

Formal syntax is defined using [ABNF] by [RFC4234] as modified extended by [IMAP4]. [RFC3501].

2. Introduction

The [SASL] [RFC4422] initial client response extension is present in any
[IMAP4]
[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 [IMAP4]. [RFC3501]. If this
second argument is present, it represents the contents of the
"initial client response" defined in section 5.1 of [SASL]. [RFC4422].

As with any other client response, this initial client response MUST
be encoded as defined in Section 3 of [BASE64]. [RFC3548]. 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] [RFC3548] data in the initial client response,
decoding errors MUST be treated as [IMAP4] [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
clients which implement this extension MUST NOT send an initial

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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
[IMAP4]
[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
[IMAP4],
[RFC3501], with an initial zero-length server challenge.

4. Examples

The following is an example authentication using the [PLAIN] PLAIN (see
[RFC2595]) SASL mechanism (under a [TLS] TLS protection layer) layer, see
[RFC2246]) 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)

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The following is an example authentication using the SASL EXTERNAL [SASL]
mechanism (under (defined in [RFC4422]) under a [TLS] TLS protection layer) layer (see
[RFC2246]) 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. Formal Syntax IANA Considerations

The following syntax specification uses IANA is requested to add SASL-IR to the Augmented Bakus-Naur
Form [ABNF] notation. Non-terminals referenced but not defined
below are as defined by [IMAP4].

capability =/ "SASL-IR"

authenticate = "AUTHENTICATE" SP auth-type [SP (base64 / "=")]
*(CRLF base64)
;;redefine AUTHENTICATE command defined in [IMAP4] list of IMAP extensions.

6. Security Considerations

The extension defined in this document is subject to many of the
Security Considerations defined in [IMAP4] [RFC3501] and [SASL]. [RFC4422].

Server implementations MUST treat the omission of an initial client
response from the AUTHENTICATE command as defined by [IMAP4] [RFC3501] (as
if this extension did not exist).

Although [IMAP4] [RFC3501] has no express line length limitations, some

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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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IMAP Extension for SASL Initial Client Response November 2006

7. Formal Syntax

The following syntax specification uses the Augmented Bakus-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.

[RFC3548] Josefsson, "The Base16, Base32, and Base64 Data Encodings",
RFC 3548, July 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.

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IMAP Extension for SASL Initial Client Response November 2006

9.2. Informative References

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

[RFC2246] Dierks, Allen, "The TLS Protocol Version 1.0", RFC 2246,
January 1999.

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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Intellectual Property Rights Statement

The IETF takes no position regarding the validity or scope of any
intellectual property
Intellectual Property Rights 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 nor does it represent that
it has made any independent effort to identify any such rights.
Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation RFC
documents can be found in BCP-11. BCP 78 and BCP 79.

Copies of
claims of rights IPR disclosures made available for publication to the IETF Secretariat 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 implementers or users of this
specification can be obtained from the IETF Secretariat. on-line IPR repository
at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which that may cover technology that may be required to practice implement
this standard. Please address the information to the IETF Executive
Director.

8. at ietf-
ipr@ietf.org.

Full Copyright Statement

Copyright (C) The Internet Society (2005). (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIM 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.

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IMAP Extension

Acknowledgment

Funding for SASL Initial Client Response February, 2005

9. 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] Melnikov, A., "Simple Authentication and Security Layer
(SASL)", draft-ietf-sasl-rfc2222bis-*.txt, a work in
progress.

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.

10. Author's Address:

Robert Siemborski
Google, Inc.
1600 Ampitheatre Parkway
Mountain View, CA 94043
+1 650 623 6925
robsiemb@google.com

11. Acknowledgments:

The author would like to acknowledge the contributions of Ken
Murchison and Mark Crispin, along with the rest of RFC Editor function is currently provided by the IMAPEXT
Working Group for their assistance in reviewing this document.

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

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12. Changes since -01 November 2006

(RFC Editor: Remove everything after this section before publication) point.)

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.

13.

Changes since -00

(RFC Editor: Remove this section before publication)

- 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 [IMAP4] [RFC3501]

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

- Update SASL reference to rfc2222bis

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