Revocation and transparency

A certificate may need to be invalidated before it expires — for example, if the private key has been stolen, the domain has been transferred, or the certificate was issued fraudulently. This is known as revocation, and it does not work properly.

Traditional mechanisms

The CRL (Certificate Revocation List) is a list published by the certificate authority. It has become enormous and takes a long time to download.

OCSP queries the certificate authority for a specific certificate. It is more lightweight, but has two serious flaws: every request tells the authority which site you are visiting, and if its server does not respond, the browser ignores the warning rather than blocking access.

This last point renders the mechanism ineffective: an attacker capable of intercepting the connection might just as easily block the OCSP request.

Stapling

OCSP stapling corrects this privacy leak: the server obtains a recent certificate itself and attaches it to the handshake. The browser no longer needs to query anyone.

The real solution: short validity periods

The web has finally acknowledged that revocation is unreliable and has changed its strategy: shortening the validity period.

autrefois  :  3 ans
aujourd hui :  quelques mois

A certificate that expires quickly automatically limits the damage. It’s less elegant than effective revocation, but it works — and automatic renewal makes the process painless.

Certificate transparency

There remains the problem of a certificate authority issuing a fraudulent certificate. The solution is called Certificate Transparency: every certificate issued must be recorded in public, read-only and verifiable registries.

It’s a clever idea. It doesn’t prevent fraudulent issuance, but it makes it impossible to conceal. The domain owner monitors the registers and quickly discovers any certificate issued in their name that they haven’t requested.

This is a strategic shift often seen in security: when prevention becomes too costly, fraud is made visible. Browsers now require this registration, and this is how several fraudulent issuances have been detected within a matter of hours.