SMTP REMEMBERME extension for quick reauthentication token generation

This document specifies an SMTP (update to rfc5321bis, once published) extension which is a protocol specific extension to Simple Authentication and Security Layer (SASL) 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: Client establishes SMTP connection protected by TLS on Connection 1. 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. On Connection 1 the client requests reauthentication token using REMEMBERME command. <Connection gets interrupted or closed due to inactivity> 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.

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 when, and only when, they appear in all capitals, as shown here.

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 for the list of registered tokens types. This command is only allowed after a successfully completed AUTH command . 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: 235 2.7.0 Authentication successful ... later in the same session ... C: REMEMBERME JWT S: 235 2.7.X ]]> 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.

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

TBD.