OCSP
acronymcryptographysecurity
Stands for: Online Certificate Status Protocol
A protocol to check in real time whether a certificate has been revoked.
Online Certificate Status Protocol (RFC 6960) lets a client ask a responder whether a certificate is still valid, an alternative to downloading large revocation lists. OCSP stapling lets the server present the proof itself to avoid a separate round trip.
OCSP answers a question certificates cannot answer themselves: has this certificate been revoked since it was issued. A certificate carries its own expiry, but revocation is a decision made afterwards, so the client has to ask somewhere, and OCSP is the protocol for asking.
The naive design had two serious problems. Every client contacting the certificate authority for every connection is a privacy leak, since the authority learns which sites you visit, and a performance dependency, since a slow or unreachable responder delays or breaks the handshake. Worse, clients that fail open, which most did, gain no security at all against an attacker who can also block the responder.
Stapling is the fix that stuck. The server periodically fetches its own signed status and includes it in the handshake, so the client gets a fresh answer with no extra request, no privacy leak and no third-party dependency in the connection path. Must-staple goes further by marking the certificate as requiring it, which closes the fail-open hole. The broader direction of travel is shorter certificate lifetimes, which reduce the window revocation is needed for in the first place.
Also known as: ocsp