Patent ID: 8499149

Claim:
A cryptographic method in which an Issuer, on assessing a Signer as a currently legitimate member of a group associated with a public/private key pair of the Issuer, uses a disguised secret of the Signer and the group-associated private key to generate a credential, in the form of a signature of the Signer's secret, which the Issuer provides to the Signer as an Issuer attestation of the Signer's group membership, the Signer subsequently keeping the credential as a secret and using a signature proof of knowledge to prove, on the basis of the group-associated public key, its possession of the credential to a Verifier without the identity of the Signer being revealed; the method comprising the Issuer, acting through Issuer computing apparatus, at intervals: updating at least the public key of the public/private key pair associated with the group, and effecting a complementary updating to the Signer's credential to take account of the updating of the group-associated key pair unless the Signer has ceased to be a legitimate group member; a non-updated credential being inadequate to enable the Signer to prove its credential possession to a Verifier on the basis of the updated Issuer public key.