A Method for Verifying Privacy-Type Properties: The Unbounded Case

In this paper, we consider the problem of verifying anonymity and unlinkability in the symbolic model, where protocols are represented as processes in a variant of the applied pi calculus notably used in the ProVerif tool. Existing tools and techniques do not allow one to verify directly these properties, expressed as behavioral equivalences. We propose a different approach: we design two conditions on protocols which are sufficient to ensure anonymity and unlinkability, and which can then be effectively checked automatically using ProVerif. Our two conditions correspond to two broad classes of attacks on unlinkability, corresponding to data and control-flow leaks. This theoretical result is general enough to apply to a wide class of protocols. In particular, we apply our techniques to provide the first formal security proof of the BAC protocol (e-passport). Our work has also lead to the discovery of new attacks, including one on the LAK protocol (RFID authentication) which was previously claimed to be unlinkable (in a weak sense) and one on the PACE protocol (e-passport).