Protocols & PKIAcceptable
Certificate revocation: OCSP and CRL
Checking whether a certificate has been revoked
How it works
A Certificate Revocation List (CRL) is a signed list of revoked certificate serial numbers that a CA publishes periodically. The Online Certificate Status Protocol (OCSP) lets a client ask the CA's responder about one certificate in real time. With OCSP stapling, the server fetches a signed OCSP response and sends it in the TLS handshake, saving the client a lookup.
Status: Required PKI function; the mechanisms have known practical limits.
Strengths
- Lets a CA invalidate certificates whose keys were compromised before they expire
- OCSP stapling reduces latency and privacy leakage
Weaknesses and known attacks
- CRLs can grow large and are only as fresh as their publication interval.
- Plain OCSP reveals to the CA which sites a user visits.
- Many clients 'soft fail' if the responder cannot be reached, so an attacker who blocks the check can bypass it.
- Some public CAs have moved away from OCSP in favour of CRLs, so behaviour varies by issuer.
Use it for
- Any PKI where certificates may need to be revoked before expiry
Don't use it for
- Relying on revocation as the only control; short certificate lifetimes reduce exposure too
Exam tip
CRL = downloaded list; OCSP = real-time query; OCSP stapling = the server attaches the signed status. Revoke a certificate when its private key is compromised.