Methods and systems are provided for demonstrating authorization to access a resource to a verifier computer controlling access to the resource. The method comprises, at a user computer, storing an attribute credential certifying a set of attributes; and communicating with a revocation authority computer to obtain an auxiliary credential, bound to the attribute credential, certifying a validity status for each attribute in the attribute credential. The method further comprises, at the user computer, communicating with the verifier computer to prove possession of the attribute credential and the auxiliary credential such that the verifier computer can determine whether at least one attribute in the attribute credential, certified as valid by the auxiliary credential, satisfies an access condition for the resource.