RFC 4959

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

📅 published September 2007, from ietf

RFC 4959: IMAP Extension for SASL Initial Client Response 💖✨

okay bestie, let's get into this one 🎀 It's an IMAP extension from September 2007, written by Robert Siemborski (Google, Inc.) and Arnt Gulbrandsen (Oryx Mail Systems GmbH). It's a Standards Track doc 🔥, and it doesn't obsolete or update any other RFCs 🌸

The whole vibe is about skipping a round trip 🦋 Normally, IMAP authentication with SASL always ate at least one full round trip at the start of a session, because there was no way to send an initial client response with the command. That extra trip is a total drag when round-trip costs are high 💀 So this extension lets clients and servers cut it out by putting an initial client response right in the AUTHENTICATE command 🧚

Status of This Memo 📜

This document is an Internet standards track protocol for the Internet community, and it's asking for discussion and suggestions for improvements 🫶 For the standardization state and status, check the current edition of the "Internet Official Protocol Standards" (STD 1) ✨ Distribution is unlimited, so share the tea 🍓

Abstract 🌈

So, the Internet Message Access Protocol (IMAP) has been using a Simple Authentication and Security Layer (SASL) profile that always required at least one complete round trip for authentication, since it had no initial client response argument 😭 That extra trip at the start of the session is undesirable, especially when round-trip costs are high 💅

This document defines an extension to IMAP that lets clients and servers skip that round trip by allowing an initial client response argument to the IMAP AUTHENTICATE command 🦋✨

1. Introduction 🎀

The SASL initial client response extension is present in any IMAP [RFC3501] server implementation that returns "SASL-IR" as one of its supported capabilities in its CAPABILITY response 💖

Servers that support this extension will accept an optional initial client response with the AUTHENTICATE command for any SASL [RFC4422] mechanisms that 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 [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] 🫶

3. IMAP Changes to the IMAP AUTHENTICATE Command 🦋

This extension adds an optional second argument to the AUTHENTICATE command, which is defined in Section 6.2.2 of [RFC3501] 💜 If this second argument is present, it holds 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) 🙈

⚠️ Heads up: 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 💀

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 🫶 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 [RFC3501] compatible mode 🦋

If either the client or the server does not support the SASL-IR capability, a mechanism that 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 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 [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
        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 how the situation is handled 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 🌈

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 [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 ✨

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 help reviewing this document 💖

Alexey Melnikov and Cyrus Daboo also had some early discussions about this extension 🌸

9. References 📚

9.1. Normative References 🔥

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003.

[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005.

[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and Security Layer (SASL)", RFC 4422, June 2006.

[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, October 2006.

9.2. Informative References 💜

[RFC4616] Zeilenga, K., "The PLAIN Simple Authentication and Security Layer (SASL) Mechanism", RFC 4616, August 2006.

[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, April 2006.

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

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.

📦 where this page comes from

ietf: rfc4959 2007-09
💖 girlified by claude-haiku-5-5 on 2026-10-17: a language model's rewrite of the original, under its terms