What is CRL (Certificate Revocation List)?

A CRL (Certificate Revocation List) is a digitally signed file containing a list of revoked X.509 digital certificates.

After a CA (Certificate Authority) has issued a digital certificate for a public key of a given entity, it may decide to revoke this certificate for some reason.

This can be done by publish a CRL (Certificate Revocation List) to include the serial number of this revoked certificate.

When a client system receives a certificate from a server, the client system should first make sure the certificate has not been revoked by the CA.

This can be done by comparing the serial number of the certificate with the CRL published by the CA. If the serial number is in the CRL, then this certificate has been revoked, and it should be rejected.

Here is a diagram showing how a client system receives a server certificate and validates it against CA's CRL (source: venafi.com).

Validating certificate with CA's CRL is not very efficient, because you need to download the CRL and compare certificate's serial number each time when using a certificate.

A more efficient way is to use the OCSP (Online Certificate Status Protocol) technology, see other tutorials for more information on OCSP.

 

⇒ Retrieve CRL File from Certificate CA

⇐ CRL (Certificate Revocation List)

⇑ CRL (Certificate Revocation List)

⇑⇑ Revoked Certificates - CRL and OCSP