Anonymous credential system promise efficient, ubiquitous access to digital services while preserving user privacy. However, their diffusion is impaired by the lack of efficient revocation techniques. Traditional credential revocation measures based on certificate revocation lists or online certification authorities do not provide privacy and cannot be used in privacy-sensitive contexts. Existing revocation techniques specifically geared towards anonymous credential systems are more involved – for the credential issuer, users, as wells as credential consumers – as users have to prove that their credential is still valid, e.g., not included in a revocation list.
We introduce a novel, non-interactive technique to update issuer-controlled attributes of anonymous credentials. Revocation is implemented by encoding the validity time of a credential into one of these attributes. With the proposed protocol, credential issuers can periodically update valid credentials off-line and publish a small per-credential update value on a public bulletin-board. Users can later download their values and re-validate their credentials to prove possession of a valid credential for the current time period. Our solution outperforms all prior solutions for credential revocation in terms of communication and computational costs for the users and credentials consumers and the issuer’s effort is comparable to the best prior proposals.