Catching Bandits and Only Bandits: Privacy-Preserving Intersection Warrants for Lawful Surveillance

Motivated in part by the Snowden revelations, we address the question of whether intelligence and lawenforcement agencies can gather actionable, relevant information about unknown electronic targets without conducting dragnet surveillance. We formulate principles that we believe effective, lawful surveillance protocols should adhere to in an era of big data and global communication networks. We then focus on intersection of cell-tower dumps, a specific surveillance operation that the FBI has used effectively. As a case study, we present a system that computes such intersections in a privacy-preserving, accountable fashion. Preliminary experiments indicate that such a system could be efficient and usable, suggesting that privacy and accountability need not be barriers to effective intelligence gathering.