Quarantined-TreeKEM: A Continuous Group Key Agreement for MLS, Secure in Presence of Inactive Users

The recently standardized secure group messaging protocol Messaging Layer Security (MLS) is designed to ensure asynchronous communications within large groups, with an almost-optimal communication cost and the same security level as point-to-point secure messaging protocols such as Signal. In particular, the core sub-protocol of MLS, a Continuous Group Key Agreement (CGKA) called TreeKEM, must generate a common group key that respects the fundamental security properties of post-compromise security and forward secrecy which mitigate the effects of user corruption over time