Network Working Group Robert Siemborski INTERNET-DRAFT Carnegie Mellon University Intended Category: Standards Track April, 2004 IMAP Extension for SASL Initial Client Response Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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. Siemborski Expires October, 2004 [Page 1] IMAP Extension for SASL Initial Client Response April, 2004 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 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-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] 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]. 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 encoded as defined in Section 3 of [BASE64]. 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] data in the initial client response, decoding errors MUST be treated as [IMAP4] 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 Siemborski Expires October, 2004 [Page 3] IMAP Extension for SASL Initial Client Response April, 2004 client response to servers which do not advertise the SASL-IR capability. In such a situation, clients MUST fall back to an [IMAP4] 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], with an initial zero-length server challenge. 4. Examples 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-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) Siemborski Expires October, 2004 [Page 4] IMAP Extension for SASL Initial Client Response April, 2004 The following is an example authentication using the EXTERNAL [SASL] mechanism (under a [TLS] protection layer) and an empty initial client response: ... 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 = 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 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-IR" authenticate = "AUTHENTICATE" SP auth-type [SP (base64 / "=")] *(CRLF base64) ;;redefine AUTHENTICATE command defined in [IMAP4] 6. Security Considerations The extension defined in this document is subject to many of the Security Considerations defined in [IMAP4] and [SASL]. Server implementations MUST treat the omission of an initial client response from the AUTHENTICATE command as defined by [IMAP4] (as if this extension did not exist). Although [IMAP4] has no express line length limitations, some Siemborski Expires October, 2004 [Page 5] IMAP Extension for SASL Initial Client Response April, 2004 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. 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. 8. 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 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 Siemborski Expires October, 2004 [Page 6] IMAP Extension for SASL Initial Client Response April, 2004 "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. 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 Carnegie Mellon, Andrew Systems Group Cyert Hall 207 5000 Forbes Avenue Pittsburgh, PA 15213 +1 412 268 7456 rjs3+@andrew.cmu.edu Siemborski Expires October, 2004 [Page 7] IMAP Extension for SASL Initial Client Response April, 2004 11. Acknowledgments: The author 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 12. Changes since -01 (RFC Editor: Remove this section before publication) - 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] - Add examples of an empty initial client response, and move examples into their own section - Update SASL reference to rfc2222bis Siemborski Expires October, 2004 [Page 8] IMAP Extension for SASL Initial Client Response April, 2004 Table of Contents 1. How to Read This Document . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Changes to the IMAP AUTHENTICATE Command . . . . . . . . . . . . 3 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 5 6. Security Considerations . . . . . . . . . . . . . . . . . . . . . 5 7. Intellectual Property Rights . . . . . . . . . . . . . . . . . . 6 8. Copyright . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 10. Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8 12. Changes since -01 . . . . . . . . . . . . . . . . . . . . . . . 8 13. Changes since -00 . . . . . . . . . . . . . . . . . . . . . . . 8 Siemborski Expires October, 2004 [Page 1]