Cooperative Location Privacy in Vehicular Networks: Why Simple Mix Zones are Not Enough

Vehicular communications disclose rich information about the vehicles and their whereabouts. Pseudonymous authentication secures communication while enhancing user privacy. To enhance location privacy, cryptographic mix zones were proposed to facilitate vehicles covertly transition to new ephemeral credentials. The resilience to (syntactic and semantic) pseudonym linking (attacks) highly depends on the geometry of the mix zones, mobility patterns, vehicle density, and arrival rates. We introduce a tracking algorithm for linking pseudonyms before and after a cryptographically protected mix zone. Our experimental results show that an eavesdropper, leveraging standardized vehicular communication messages and road layout, could successfully link approximate to 73% of pseudonyms during nonrush hours and approximate to 62% of pseudonyms during rush hours after vehicles change their pseudonyms in a mix zone. To mitigate such inference attacks, we present a novel cooperative mix zone scheme that enhances user privacy regardless of the vehicle mobility patterns, vehicle density, and arrival rate to the mix zone. A subset of vehicles, termed relaying vehicles, is selected to be responsible for emulating nonexisting vehicles. Such vehicles cooperatively disseminate decoy traffic without affecting safety-critical operations: with 50% of vehicles as relaying vehicles, the probability of linking pseudonyms (for the entire interval) drops from approximate to 68% to approximate to 18%. On average, this imposes 28 ms extra computation overhead, per second, on the roadside unit (RSU) and 4.67 ms extra computation overhead, per second, on the (relaying) vehicle side; it also introduces 1.46 kB/s extra communication overhead by (relaying) vehicles and 45 kB/s by RSUs for the dissemination of decoy traffic. Thus, user privacy is enhanced at the cost of low computation and communication overheads.