Efficient private proximity testing with GSM location sketches

A protocol for private proximity testing allows two mobile users communicating through an untrusted third party to test whether they are in close physical proximity without revealing any additional information about their locations. At NDSS 2011, Narayanan and others introduced the use of unpredictable sets of "location tags" to secure these schemes against attacks based on guessing another user's location. Due to the need to perform privacy-preserving threshold set intersection, their scheme was not very efficient. We provably reduce threshold set intersection on location tags to equality testing using a de-duplication technique known as shingling. Due to the simplicity of private equality testing, our resulting scheme for location tag-based private proximity testing is several orders of magnitude more efficient than previous solutions. We also explore GSM cellular networks as a new source of location tags, and demonstrate empirically that our proposed location tag scheme has strong unpredictability and reproducibility.