To use a service or a resource that the issuer of an AC controls, a user presents both the PKC and the AC to a part of the service or resource that functions as an AC verifier. The verifier will first check the identity of the user using the PKC, for example, by asking the user to decrypt a message encrypted by the user's public key in the PKC. If the authentication is successful, the verifier will use the preinstalled public key of the AC issuer to check the validity of the presented AC.