$3.$ In credential systems, there are three players: the issuer $($who issues the credential to the prover$),$ the prover $($who proves possession of the credential to the verifier$)$ and the verifier $($who checks possession$).$ However:

$($a$)$ The one$-$time credentials protocol based on blind signatures only specifies the prover and the issuer. Specify in detail what the verifier does.

$($b$)$ The multiple$-$use credentials protocol based on zero$-$knowledge proofs only specifies the prover and the verifier. Specify in detail what the issuer does.