TASP: Towards Anonymity Sets That Persist

Anonymous communication systems are vulnerable to long term passive "intersection attacks". Not all users of an anonymous communication system will be online at the same time, this leaks some information about who is talking to who. A global passive adversary observing all communications can learn the set of potential recipients of a message with more and more confidence over time. Nearly all deployed anonymous communication tools offer no protection against such attacks. In this work, we introduce TASP, a protocol used by an anonymous communication system that mitigates intersection attacks by intelligently grouping clients together into anonymity sets. We find that with a bandwidth overhead of just 8% we can dramatically extend the time necessary to perform a successful intersection attack.