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 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering 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. This Internet-Draft will expire in May 2007. 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, et al. Expires July, 2005 [Page 1] IMAP Extension for SASL Initial Client Response November 2006 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 Siemborski, et al. Expires July, 2005 [Page 2] IMAP Extension for SASL Initial Client Response November 2006 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 Siemborski, et al. Expires July, 2005 [Page 3] IMAP Extension for SASL Initial Client Response November 2006 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. Siemborski, et al. Expires July, 2005 [Page 4] IMAP Extension for SASL Initial Client Response November 2006 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. Siemborski, et al. Expires July, 2005 [Page 5] IMAP Extension for SASL Initial Client Response November 2006 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 Siemborski, et al. Expires July, 2005 [Page 6] IMAP Extension for SASL Initial Client Response November 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any 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; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made 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 implementers or users of this specification can be obtained from the IETF 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 that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org. Full Copyright Statement Copyright (C) The Internet Society (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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Siemborski, et al. Expires July, 2005 [Page 7] IMAP Extension for SASL Initial Client Response November 2006 (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 Siemborski, et al. Expires July, 2005 [Page 8] IMAP Extension for SASL Initial Client Response November 2006 - Update SASL reference to rfc2222bis Siemborski, et al. Expires July, 2005 [Page 9]