Metadata Protection in Instant Messaging Applications: a Review

Twelve years after the public specification of the Signal protocol, almost all instant messaging protocols have embraced the ratchet construct, granting perfect forward secrecy and post-compromise security. Whatsapp, Signal, OMEMO-based applications, Olm and Megolm-based applications, or SimpleX Chat all use the Double Ratchet protocol. Olvid also uses a ratchet protocol, although the construct is a bit different. And there are the stragglers who insist on not using any form of perfect forward secrecy, such as Session or Delta Chat. Of those, we will talk no more. But since then, we have learned the hard way from some NSA executive that metadata gets you arrested or killed. And so begs the question: how well are protected our metadata by the various instant messaging infrastructures? Signal claims one cannot hand over data one doesn't have. But how honest are they about the metadata they do have, and that could be requested from them or their hosting provider by a subpoena and sealed orders. In this talk, we will explore some metadata available to Signal servers, Olvid servers, Matrix/Element home servers and SimpleX Chat SMP queue servers. We will then discuss the strategies that some of these applications have deployed to limit metadata exposition, including those leveraging external transport security, such as the use of Tor.