International Association
for Cryptologic Research

Secure messaging schemes such as the Signal protocol rely on out-of-band channels to verify the authenticity of long-running communication. Such out-of-band checks however are only rarely actually performed by users in practice.

In this paper, we propose a new method for performing continuous authentication during a secure messaging session, without the need for an out-of-band channel. Leveraging the users' long-term secrets, our Authentication Steps extension guarantees authenticity as long as long-term secrets are not compromised, strengthening Signal's post-compromise security. Our mechanism further allows to detect a potential compromise of long-term secrets after the fact via an out-of-band channel.

Our protocol comes with a novel, formal security definition capturing continuous authentication, a general construction for Signal-like protocols, and a security proof for the proposed instantiation. We further provide a prototype implementation which seamlessly integrates on top of the official Signal Java library, together with bandwidth and storage overhead benchmarks.