Speaker: Rolfe Schmidt (Signal)

Title: Private Groups in Signal Messenger

Abstract: End-to-end encrypted [E2EE] messaging has become mainstream, with over 2 billion people worldwide using E2EE apps. While this is immense progress, privacy requires more than encryption and almost every feature in a messaging app has the potential to leak sensitive information. In this talk we will look at one feature in particular - group chats - and show how the Signal Messenger app implements it so with provable guarantees that nobody outside a group chat can learn anything about the membership of the group. To this end we will cover key cryptographic primitives including Elliptic Curve Cryptography, Zero Knowledge Proofs and Verifiable Encryption, then show how they can be assembled into a production system. We will close with a pressing open problem: how can we get the same features in a way that is post-quantum secure?