Léo Robert. On The Post-Compromise Security of Messaging Protocols

Post-Compromise Security (PCS) is a property of secure-channel
establishment schemes, which limits the security breach of an
adversary that has compromised one of the endpoint to a certain
number of messages, after which the channel heals. An attractive
property, especially in view of Snowden's revelation of
mass-surveillance, PCS features in prominent messaging protocols
such as Signal. In this talk, we first present a variant of
Signal which improves PCS property. Since the PCS is not a binary
property but rather a spectrum, we then introduce a framework for
quantifying and comparing PCS security, with respect to a broad
taxonomy of adversaries. The generality and flexibility of our
approach allows us to model the healing speed of a broad class of
protocols, including Signal and our variant, but also an
identity-based messaging protocol named SAID, and even a
composition of 5G handover protocols. We also apply the results
obtained for this latter example in order to provide a quick fix,
which massively improves its post-compromise security.