Christoph Coijanovic

Daniel Schadt, Christoph Coijanovic, Thorsten Strufe

Apps such as Firechat and Bridgefy have been used during recent protests in Hong Kong and Iran, as they allow communication over ad-hoc wireless networks even when internet access is restricted. However, these apps do not provide sufficient protection as they do not achieve forward secrecy in unreliable networks. Without forward secrecy, caught protesters' devices will disclose all previous messages to the authorities, putting them and others at great risk. In this paper, we introduce FoSAM, the first protocol to provide proven anonymous and forward secret messaging in unreliable ad-hoc networks. Communication in FoSAM requires only the receiver's public key, rather than an interactive handshake. We evaluate the performance of FoSAM using a large-scale simulation with different user movement patterns, showing that it achieves between 92% and 99% successful message delivery. We additionally implement a FoSAM prototype for Android.

Sarah Abdelwahab Gaballah, Christoph Coijanovic, Thorsten Strufe et al.

Publish/Subscribe systems like Twitter and Reddit let users communicate with many recipients without requiring prior personal connections. The content that participants of these systems publish and subscribe to is typically public, but they may nevertheless wish to remain anonymous. While many existing systems allow users to omit explicit identifiers, they do not address the obvious privacy risks of being associated with content that may contain a wide range of sensitive information. We present 2PPS (Twice-Private Publish-Subscribe), the first pub/sub protocol to deliver strong provable privacy protection for both publishers and subscribers, leveraging Distributed Point Function-based secret sharing for publishing and Private Information Retrieval for subscribing. 2PPS does not require trust in other clients and its privacy guarantees hold as long as even a single honest server participant remains. Furthermore, it is scalable and delivers latency suitable for microblogging applications. A prototype implementation of 2PPS can handle 100,000 concurrent active clients with 5 seconds end-to-end latency and significantly lower bandwidth requirements than comparable systems.