A Revocation Registry is a large number residing on the ledger representing a cryptographic product of all the “serial numbers” of validly issued credentials based on a specific credential definition. This number enables a credential holder to prove a credential they have been issued hasn’t been revoked by the issuer. A revocation registry is an optional part of a credential definition and must be included in the credential definition for any credential that is revocable. The only time this ledger write is necessary is when the revocation registry that was created as a part of the credential definition is insufficient and an additional one is required.