Revocation List - Revocation Vs. Expiration

Revocation Vs. Expiration

Certificate expiration dates are not a substitute for a CRL. While all expired certificates are considered invalid, not all unexpired certificates are necessarily valid. CRLs or other certificate validation techniques are a necessary part of any properly operated PKI, as mistakes in certificate vetting and key management are expected to occur in real world operations.

In a noteworthy example, a certificate for Microsoft was mistakenly issued to an unknown individual, who had successfully posed as Microsoft to the CA contracted to maintain the ActiveX 'publisher certificate' system (VeriSign). Microsoft saw the need to patch their cryptography subsystem so it would check the status of certificates before trusting them. As a short-term fix, a patch was issued for the relevant Microsoft software (most importantly Windows) specifically listing the two certificates in question as "revoked".