IMAP REMEMBERME extension for quick reauthentication token generation

Abstract

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

1. Introduction

This document specifies an IMAP [RFC9051] 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 [draft-ietf-kitten-sasl-rememberme] or HT2-* [draft-ietf-kitten-sasl-ht].¶

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

1. Client establishes IMAP 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, GSSAPI.¶

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 IMAP 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 or HT2-*. The same token is reusable on other IMAP connections until it is replaced or revoked.¶

IMAP servers advertise support for this extension by returning one or more TOKEN=<token-type> capabilities in the CAPABILITY response.¶

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

Arguments:

Token type¶

Responses:

REQUIRED TOKEN response code¶

Result:

OK - new token successfully issued¶

NO - authentication mechanism too week, unrecognized token type¶

BAD - command unknown or arguments invalid,¶

invalid state¶

This command is only valid in authenticated and selected states.¶

Upon receipt of REMEMBERME command the IMAP 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 TOKEN response code in the tagged OK response.¶

  S: * OK ACME IMAP Server v1.23 is ready
  C: 22 CAPABILITY
  S: * CAPABILITY IMAP4rev1 IMAP4rev2 STARTTLS AUTH=PASSKEY AUTH=REMEMBERME
   AUTH=SCRAM-SHA-256 AUTH=REMEMBERME TOKEN=JWT TOKEN=RANDOM
  S: 22 CAPABILITY completed
  C: 23 STARTTLS
  S: 23 OK Completed
  C: 24 AUTHENTICATE ...
   <SASL exchange>
  S: 24 OK Completed
  C: 25 REMEMBERME JWT
  S: 26 OK [TOKEN <base64-encoded-token>] Completed

4. Formal Syntax

The following syntax specification uses the Augmented Backus-Naur Form (ABNF) notation as specified in [ABNF].¶

capability     =/ "TOKEN=" token-type
                    ;; <capability> from [RFC3501]

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

resp-text-code =/ "TOKEN" SP base64-token

base64-token   = base64

5. IANA Considerations

TBD. Register the IMAP capabilities and create a separate registry of token types.¶

Register "JWT" [RFC7519] in the newly created registry of token types. Do we need to also define which claims are to be included in JWT payload?¶

Also, do we need a recommended and/or "mandatory to implement" token type?¶

7. References