When a Certificate Goes Bad
A certificate is valid until its expiry, but a key may be stolen or issued in error before then. Revocation is how an authority declares a certificate untrustworthy ahead of expiry, and clients must be able to learn about it.
Two Mechanisms
There are two classic ways to publish revocation status.
- A certificate revocation list is a signed list of revoked serial numbers a client downloads.
- The online certificate status protocol lets a client ask about one certificate and get a fresh signed answer.
The status protocol avoids downloading a huge list, but it adds a request and can leak which sites a user visits to the responder.
OCSP Stapling
To fix the privacy and latency cost, stapling has the server fetch its own status proof and attach it to the handshake. The client gets a recent, signed status without contacting the authority itself.
- The server queries the responder periodically.
- It staples the signed response into the TLS handshake.
- The client trusts the stapled proof because it is signed and time stamped.
The Soft Fail Problem
Many clients treat an unreachable responder as acceptable, a soft fail that an attacker can exploit by blocking the check. This weakness pushes the industry toward short lived certificates that limit the window of risk.
Key idea
Revocation lets an authority distrust a certificate early, the status protocol checks one at a time, stapling delivers a fresh signed proof in the handshake, and soft fail behavior drives the move to short lived certificates.