We uncover privacy vulnerabilities in the ICAO 9303 standard implemented by ePassports worldwide. These vulnerabilities, confirmed by ICAO, enable an ePassport holder who recently passed through a checkpoint to be reidentified without opening their ePassport. This paper explains how bisimilarity was used to discover these vulnerabilities, which exploit the BAC protocol - the original ICAO 9303 standard ePassport authentication protocol - and remains valid for the PACE protocol, which improves on the security of BAC in the latest ICAO 9303 standards. In order to tackle such bisimilarity problems, we develop here a chain of methods for the applied $\pi$-calculus including a symbolic under-approximation of bisimilarity, called open bisimilarity, and a modal logic, called classical FM, for describing and certifying attacks. Evidence is provided to argue for a new scheme for specifying such unlinkability problems that more accurately reflects the capabilities of an attacker.

@article{paperbot1304,
  title = {Discovering ePassport Vulnerabilities using Bisimilarity},
  author = {Ross Horne and Sjouke Mauw},
  journal = {Logical Methods in Computer Science},
  volume = {Volume 17, Issue 2},
  year = {2021},
  doi = {10.23638/lmcs-17(2:24)2021}
}