OCSP (Online Certificate Status Protocol) refers to a protocol that provides automatic verification of a certificate’s status (in terms of its validity – the absence of revocation/cancellation of the certificate by the issuer).

Using the OCSP protocol is a form of certificate validity verification that yields a result faster than CRLs of revoked certificates.

The CRL (Certificate Revocation List) is a list of certificates, updated from time to time, or more precisely the serial numbers of certificates that have been revoked or suspended by their issuer.