How does certificate revocation work, and what is OCSP stapling?
The mechanism for saying "this certificate is no longer valid before its expiry date" — and it is the weakest part of the web's trust system, because the obvious designs all conflict with either privacy or availability.
Why revocation is needed. A certificate is valid for a fixed window. If the private key is compromised, the domain changes hands, or it was issued in error, it must be repudiated before that window closes.
Certificate Revocation Lists (CRLs). The original design: the issuer publishes a list of revoked serial numbers and clients download it. The problem is size — a list covering a large issuer becomes enormous, and downloading it on every connection is untenable.
OCSP (Online Certificate Status Protocol). The client asks the issuer directly about one certificate. This fixed the size problem and created two worse ones:
Privacy. The client tells the issuer which site it is visiting, every time. The certificate authority acquires a browsing history.
Availability. If the responder is unreachable, what should the browser do? Hard-fail makes every issuer outage a global outage. Soft-fail — proceed anyway — is what browsers actually chose, which makes the check useless against an attacker, since anyone able to present a stolen certificate can also block the status request.
OCSP stapling. The elegant fix: the server fetches a signed, time-stamped status response for its own certificate and attaches it to the TLS handshake. The client gets proof of validity without contacting the issuer at all — privacy preserved, latency removed, no dependency on the responder during the connection.
Its weakness is that a server can simply omit the staple, and the client cannot distinguish that from a server that does not support it. Must-staple addresses this by marking the certificate as requiring one.
Where it has landed. Browsers increasingly rely on pre-compiled revocation sets pushed to the client, and OCSP is being retired. Short certificate lifetimes are the real answer: a certificate valid for days barely needs revoking.