Paper 2026/2438

Censorship Resistance in Credential Systems and Application to the EUDI Wallet

Abstract

Privacy-preserving credential systems typically focus on data protection during the presentation phase, overlooking potential privacy threats during issuance. A malicious issuer could use the non-minimal information it obtains to deny issuance selectively. Such a denial, going beyond the eligibility policy, is a form of censorship. In this work, we revisit the security of anonymous credential systems to protect users from malicious issuers during credential issuance. We first highlight the limitations of existing definitions and introduce a security model capturing censorship resistance. Next, we present a round-optimal (i.e., one message per party) construction built from blind signatures and specifically tailored for selective disclosure in the presentation phase. Finally, we present CRACS, an instantiation of our construction, deployable within the European Digital Identity Wallet (EUDIW) framework. CRACS improves on the current reference implementation of the EUDIW, maintaining selective disclosure and yielding unlinkability between presentation and issuance sessions in addition to censorship resistance. Unlike prior work, CRACS confines advanced cryptography to issuance, leaving secure elements, the standardized credential formats, signature schemes, and presentation mechanisms untouched. It can therefore be deployed incrementally alongside current EUDIW implementations, issuer by issuer and user by user, with no change for verifiers.