Traffic analysis for instant messaging (IM) applications continues to pose an important privacy challenge. In particular, transport-level data can leak unintentional information about IM – such as who communicates with whom. Existing tools for metadata privacy have adoption obstacles, including the risks of being scrutinized for having a particular app installed, and performance overheads incompatible with mobile devices. We posit that resilience to traffic analysis must be directly supported by major IM services themselves, and must be done in a low-cost manner without breaking existing features. As a first step in this direction, we propose a hybrid messaging model that combines regular and deniable messages. We present a novel protocol for deniable instant messaging, which we call DenIM. DenIM is built on the principle that deniable messages can be made indistinguishable from regular messages with a little help from a user’s friends. Deniable messages’ network traffic can then be explained by a plausible cover story. DenIM achieves overhead proportional to the messages sent, as opposed to scaling with time or number of users. To show the effectiveness of DenIM, we implement a trace simulator, and show that DenIM’s deniability guarantees hold against strong adversaries such as internet service providers. Joint work in progress with Aslan Askarov.
Boel Nelson is a postdoc at Aarhus University. Her research interests include privacy enhancing technologies, data privacy, and real-world applications of security and privacy techniques. Boel graduated with a PhD (topic: differential privacy) from Chalmers University of Technology in 2021. She holds a MSc in Computer Science and Engineering, and a BSc in Software Engineering, also from Chalmers University of Technology.