Poster: Tracking Vehicles Through Encrypted Mix-Zones Using Physical Layer Properties

The main objective of Vehicular Ad-hoc Networks (VANETs) is to enable a safer and more efficient driving experience by augmenting situational awareness on the road. Therefore, vehicles participating in a VANET continuously broadcast their state and location in Cooperative Awareness Messages (CAMs). While this information can be of great value to all road users, the transmission of such sensitive data poses a huge threat to privacy if messages can be linked to the sender. Signing messages using ephemeral pseudonyms is a widely accepted mitigation strategy that provides the security properties to guarantee the safe operation of VANET applications while offering conditional privacy to users. Pseudonym schemes allow On-Board Units (OBUs) to acquire a set of credentials and periodically exchange the signing material, dividing a journey into smaller, less meaningful segments. However, it is crucial to prevent an attacker from observing when pseudonym changes occur. In this poster, we investigate the resilience of a pseudonym change strategy based on encrypted mix-zones against the tracking of the physical properties inherent in the message transmission. By encrypting vehicle messages, Cryptographic mix-zones (CMIXs) offer protection against an eavesdropper external to the VANET, while at the same time preserving the integrity of safety applications. Even though CAMs containing location information are illegible for the attacker, the physical properties of radio signals, such as time of flight, cannot be obfuscated using encryption. To evaluate these properties, we acquired raw In-phase and Quadrature (I/Q) samples from transmissions of a NEXCOM OBU using Software Defined Radios (SDRs). Our results indicate that physical signal properties could be used to track vehicles through a CMIX.