The Online Certificate Status Protocol (OCSP) is a technology designed to check the revocation status of digital certificates in real-time without requiring the client to download entire revocation lists. Unlike Certificate Revocation Lists (CRLs), which are periodically updated and can be large, OCSP queries an OCSP responder to receive the status of a specific certificate.
OCSP is considered passive verification because it allows clients to check a certificate's current validity status on-demand without maintaining local copies of revocation data. The OCSP responder returns whether the certificate is valid, revoked, or expired.
Trusted Platform Module (TPM) is hardware for secure key storage, and Certificate Signing Request (CSR) is a request for certificate issuance; neither is used for verifying certificate expiration status.
The differences and roles of OCSP and CRLs are thoroughly covered in the Cryptography and PKI chapter of the SY0-701, where OCSP is highlighted as the more efficient and real-time method to verify certificate status passively【6:Chapter 7†CompTIA Security+ Study Guide】.
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