A method for unbounded verification of privacy-type properties

TL;DR

This paper introduces a novel method for verifying privacy properties like anonymity and unlinkability in protocols, using conditions that can be automatically checked with ProVerif, enabling formal proofs and attack discovery.

Contribution

It proposes two sufficient conditions for verifying unlinkability and anonymity in protocols, applicable to a wide range of cryptographic primitives, and implements them in the ProVerif-based tool UKano.

Findings

First formal security proofs for protocols like BAC and PACE.

Discovered new attacks on RFID protocols previously considered unlinkable.

Provided a general, effective approach for verifying privacy properties in symbolic models.

Abstract

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 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, i.e. data and control-flow leaks. This theoretical result is general enough that it applies to a wide class of protocols based on a variety of cryptographic primitives. In particular, using our tool, UKano, we provide the first formal security proofs of protocols such as BAC and PACE (e-passport), Hash-Lock (RFID authentication), etc. 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).