OCSP
Online Certificate Status Protocol
A real-time protocol for checking whether a certificate is revoked.
Online Certificate Status Protocol checks in real time whether a specific certificate has been revoked, querying a responder rather than downloading an entire revocation list. This is timelier and more bandwidth efficient than a Certificate Revocation List, which can grow large and become stale between publications. A privacy and performance concern is that the responder learns which sites a client visits, which OCSP stapling mitigates by having the server present a recent, signed status to the client. Reliable revocation checking is essential to trusting a PKI.
Online Certificate Status Protocol: asks the CA about one certificate status in real time (lighter than a CRL).
Looking OCSP up is step one. Getting tested on it is step two.
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