SMTP REMEMBERME extension for quick reauthentication token generation
draft-melnikov-smtp-rememberme-00

Network Working Group                                        A. Melnikov
Internet-Draft                                                 Isode Ltd
Intended status: Standards Track                             29 May 2026
Expires: 30 November 2026

 SMTP REMEMBERME extension for quick reauthentication token generation
                   draft-melnikov-smtp-rememberme-00

Abstract

   This document specifies an SMTP extension for generating quick
   reauthentication tokens that allow clients to re-login without user
   interaction, once authentication using a strong SASL mechanism is
   completed.

Status of This Memo

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   This Internet-Draft will expire on 30 November 2026.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  REMEMBERME command  . . . . . . . . . . . . . . . . . . . . .   3
   4.  Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   This document specifies an SMTP [RFC5321] (update to rfc5321bis, once
   published) extension which is a protocol specific extension to Simple
   Authentication and Security Layer (SASL) [RFC4422] framework for
   generation of proof-of-posession reauthentication tokens.  Such
   tokens can be used for subsequent 1 roundtrip reauthentication using
   SASL mechanisms such as REMEMBERME and HT2-*.

   The typical sequence of events is going to be like this:

   1.  Client establishes SMTP connection protected by TLS on Connection
       1.

   2.  On Connection 1 the client authenticates using a strong SASL
       mechanism, which might be CPU intensive, and most likely requires
       user interaction, e.g., SCRAM with 2FA extension, PASSKEY, OAUTH
       with 2FA.

   3.  On Connection 1 the client requests reauthentication token using
       REMEMBERME command.

   4.  <Connection gets interrupted or closed due to inactivity>

   5.  Client establishes another SMTP connection protected by TLS on
       Connection N.  The client then uses a previous issues quick
       reauthentication token with one of 1 round trip SASL mechanisms
       such as REMEMBERME and HT2-*.  The same token is reusable on
       other SMTP connections until it is replaced or revoked.

   SMTP servers advertise support for this extension by returning one or
   more <token-type> in the EHLO REMEMBERME response.

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2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  REMEMBERME command

   The REMEMBERME command is used to request a reauthentication token
   that is suitable for 1 round trip SASL authentication mechanisms.
   The REMEMBERME command takes 1 parameter which is token type name.
   See Section 5 for the list of registered tokens types.

   This command is only allowed after a successfully completed AUTH
   command [RFC4954].

   Upon receipt of REMEMBERME command the SMTP server checks that the
   specified token type is recognized and supported.  If it is, it
   generates a new token of the requested type and returns it in the 235
   response, that might also contain the 2.7.X enhanced status code.

     S: 220-smtp.example.com ESMTP Server
     C: EHLO client.example.com
     S: 250-smtp.example.com Hello client.example.com
     S: 250-AUTH GSSAPI PASSKEY SCRAM-SHA-256
     S: 250-ENHANCEDSTATUSCODES
     S: 250 STARTTLS
     C: STARTTLS
     S: 220 Ready to start TLS
       ... TLS negotiation proceeds, further commands
           protected by TLS layer ...
     C: EHLO client.example.com
     S: 250-smtp.example.com Hello client.example.com
     S: 250-AUTH GSSAPI PASSKEY SCRAM-SHA-256 PLAIN
     S: 250 REMEMBERME JWT RANDOM
     C: AUTH SCRAM-SHA-256 ...
      <SASL exchange>
     S: 235 2.7.0 Authentication successful
        ... later in the same session ...
     C: REMEMBERME JWT
     S: 235 2.7.X <base64-encoded token>

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   Note that in the above example list of capabilities returned in EHLO
   response changes after STARTTLS.  Also note that the order of AUTH
   and REMEMBERME lines in EHLO response can be arbitrary.

4.  Formal Syntax

   rememberme-cmd = "REMEMBERME" SP token-type

   token-type     = atom
                       ;; SHOULD be registered with IANA

   rememberme-rsp = "235" [SP 2.7.X] SP base64-token
                       ;; Successful response to REMEMBERME command
                       ;; contains a base64-encoded token.
                       ;; An optional enhanced status code might preceed the token.

   base64-token        = base64

5.  IANA Considerations

   TBD.  Register the SMTP capabilities and create a separate registry
   of token types?  The separate registry will be shared with the IMAP
   REMEMBERME document.

   Also request assignment of 2.7.X from
   https://www.iana.org/assignments/smtp-enhanced-status-codes/smtp-
   enhanced-status-codes.xhtml

6.  Security Considerations

   TBD.

7.  Normative References

   [ABNF]     Crocker, D., Ed. and P. Overell, Ed., "Augmented BNF for
              Syntax Specifications: ABNF", RFC 5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4422]  Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
              Authentication and Security Layer (SASL)", RFC 4422,
              DOI 10.17487/RFC4422, June 2006,
              <https://www.rfc-editor.org/info/rfc4422>.

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   [RFC4954]  Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service
              Extension for Authentication", RFC 4954,
              DOI 10.17487/RFC4954, July 2007,
              <https://www.rfc-editor.org/info/rfc4954>.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <https://www.rfc-editor.org/info/rfc5321>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

Author's Address

   Alexey Melnikov
   Isode Ltd
   14 Castle Mews
   Hampton
   TW12 2NP
   United Kingdom
   Email: alexey.melnikov@isode.com

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