When A Certificate Goes Bad
A TLS certificate has an expiry date, but sometimes it must be invalidated early, for example if its private key leaks. Revocation is how a certificate authority declares a certificate no longer trustworthy before it would naturally expire.
Certificate Revocation Lists
The original approach is a certificate revocation list, a signed list of revoked serial numbers the authority publishes.
- Clients download the list and check whether a certificate appears on it.
- The list grows over time and can become large.
- It can be stale between updates, leaving a window of risk.
Online Status With OCSP
The online certificate status protocol improves freshness. Instead of a whole list, the client asks the authority about one certificate and gets a signed yes or no.
- It returns a small, current answer.
- It can add latency and leak which sites a user visits.
Stapling For Speed And Privacy
OCSP stapling fixes those drawbacks. The server itself fetches a recent signed status and staples it into the TLS handshake. The client gets proof of validity without contacting the authority, improving both speed and privacy.
Key idea
Revocation invalidates a certificate before expiry through published lists or online status queries, and OCSP stapling lets the server attach a fresh signed status for speed and privacy.