Privately (and Unlinkably) Exchanging Messages Using a Public Bulletin Board
Jaap-Henk Hoepman · 2015
We implement a secure and privacy friendly asynchronous unidirectional message transmission protocol using a public bulletin board that makes individual send or receive events unlinkable to one another. While the clients must securely run in the user's endpoint device, the bulletin board can be hosted on an arbitrary public cloud at no additional risk. In fact the protocol provides the same unlinkability guarantees as the underlying mixing network. The protocol is efficient, and the central bulletin board can adaptively be scaled and distributed depending on the load. We show how the basic unidirectional protocol can be used to hide the metadata in bidirectional message exchange applications like WhatsApp, and how it can be used to implement a private presence service efficiently. In these applications our protocol protects the so-called social graph.