Beyond the ratchet: practical challenges in secure messaging

Speaker: Simone Colombo, King’s College London

Abstract: Private digital communication relies critically on secure messaging. Despite major advances such as the widespread adoption of end-to-end encryption, existing systems still face key unresolved challenges. In this talk, I present two such challenges and explain how my research addresses them through a combination of cryptographic, security, and system-level techniques. First, we examine the privacy risks associated with public key retrieval, particularly the exposure of users’ social graphs and the threat posed by malicious service providers. To address these risks, we introduce authenticated private information retrieval, a new cryptographic primitive that conceals metadata and ensures clients either retrieve the correct key or abort. Second, we assess the real-world relevance of deniability in secure messaging. Although often promoted as a privacy feature by messaging applications, our technical modeling and legal analysis reveal that deniability typically fails in practice. I conclude with an overview of my broader research in this space and outline future directions aimed at closing the gap between theoretical guarantees and the practical demands of secure messaging.

Biography: Simone is a postdoctoral researcher in cryptography at King’s College London, where he primarily works on the social foundations of cryptography. His broader research interests span computer systems, security, and cryptography, with a particular focus on secure messaging. He completed his PhD under the supervision of Bryan Ford at EPFL, where he also earned his BSc and MSc degrees.

Host: Prof. Marc Langheinrich