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IETF RFC 4959

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

Last modified on Friday, September 21st, 2007

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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.





















Siemborski & Gulbrandsen  Standards Track                    PAGE 1 top


RFC 4959 IMAP Ext for SASL Initial Client Response September 2007 1. Introduction The SASL initial client response extension is present in any IMAP [RFC 3501] 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 [RFC 4422] 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 [RFC 2119]. In examples, "C:" and "S:" indicate lines sent by the client and server, respectively. Formal syntax is defined by [RFC 4234] as extended by [RFC 3501]. 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 [RFC 3501]. If this second argument is present, it represents the contents of the "initial client response" defined in Section 5.1 of [RFC 4422]. As with any other client response, this initial client response MUST be encoded as defined in Section 4 of [RFC 4648]. 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 [RFC 4648] data in the initial client response, decoding errors MUST be treated as IMAP [RFC 3501] 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. Siemborski & Gulbrandsen Standards Track PAGE 2 top

RFC 4959 IMAP Ext for SASL Initial Client Response September 2007 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 [RFC 3501] 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 [RFC 3501], with an initial zero-length server challenge. 4. Examples The following is an example authentication using the PLAIN (see [RFC 4616]) SASL mechanism (under a TLS protection layer, see [RFC 4346]) 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 [RFC 4422]) under a TLS protection layer (see [RFC 4346]) and an empty initial client response: Siemborski & Gulbrandsen Standards Track PAGE 3 top

RFC 4959 IMAP Ext for SASL Initial Client Response September 2007 ... 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 [RFC 3501] and [RFC 4422]. Server implementations MUST treat the omission of an initial client response from the AUTHENTICATE command as defined by [RFC 3501] (as if this extension did not exist). Although [RFC 3501] 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 & Gulbrandsen Standards Track PAGE 4 top

RFC 4959 IMAP Ext for SASL Initial Client Response September 2007 7. Formal Syntax The following syntax specification uses the Augmented Backus-Naur Form [RFC 4234] notation. [RFC 3501] defines the non-terminals capability, auth-type, and base64. capability =/ "SASL-IR" authenticate = "AUTHENTICATE" SP auth-type [SP (base64 / "=")] *(CRLF base64) ;;redefine AUTHENTICATE from [RFC 3501] 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 [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC 3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003. [RFC 4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005. [RFC 4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and Security Layer (SASL)", RFC 4422, June 2006. [RFC 4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, October 2006. 9.2. Informative References [RFC 4616] Zeilenga, K., "The PLAIN Simple Authentication and Security Layer (SASL) Mechanism", RFC 4616, August 2006. [RFC 4346] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, April 2006. Siemborski & Gulbrandsen Standards Track PAGE 5 top

RFC 4959 IMAP Ext for SASL Initial Client Response September 2007 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 & Gulbrandsen Standards Track PAGE 6 top

RFC 4959 IMAP Ext for SASL Initial Client Response September 2007 Full Copyright Statement Copyright © The IETF Trust (2007). 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, THE IETF TRUST 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. Intellectual Property 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. Siemborski & Gulbrandsen Standards Track PAGE 7 top

IMAP Extension for Simple Authentication and Security Layer (SASL) Initial Client Response RFC TOTAL SIZE: 12284 bytes PUBLICATION DATE: Friday, September 21st, 2007 LEGAL RIGHTS: The IETF Trust (see BCP 78)


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