SCRAM with Modular Crypt Format (SCRAM-MCF)

This document specifies SCRAM-MCF, an extension to the Salted Challenge Response Authentication Mechanism (SCRAM) family of SASL mechanisms ([RFC5802]) and HTTP Digest extensions ([RFC7616], [RFC7677]). The extension replaces the PBKDF2-specific iteration count attributes i= and s= in the server-first-message with a generic Modular Crypt Format (MCF) descriptor f=. This allows servers to use modern memory- hard key derivation functions such as Argon2, SCrypt, or bcrypt while preserving the full security properties and message flow of SCRAM. The change is fully backward compatible: servers can continue sending i= and s= for legacy clients and only send f= (or both) when the client advertises support. This document is intended to be discussed and potentially adopted by the KITTEN working group. Feedback from the KITTEN WG is welcome on the [email protected] mailing list.