The Hashed Token SASL Mechanism

This document specifies the family of Hashed Token SASL mechanisms, which enable a proof-of-possession-based authentication scheme and are meant to quickly re-authenticate a previous session. The Hashed Token SASL mechanism's authentication sequence consists of only one round-trip. The usage of short-lived, exclusively ephemeral hashed tokens is achieving the single round-trip property. The SASL mechanism specified herein further provides hash agility, mutual authentication, support for channel binding, and the capability to exchange authenticated key/value pairs.