CertiGuide to Security+  9  Chapter 4:  Basics of Cryptography (Domain 4.0; 15%)       9  4.5  Key Management and Certificate Lifecycles            9  4.5.5  Revocation

4.5.5  Revocation 1 4.5.5.1  Status Checking

Several methods for certificate revocation by managing entities are currently in place. The older method employs a list of keys that cannot be trusted being held in a CRL (Certificate Revocation List), which is a time-stamped list of no-longer-valid certificates signed by the CA’s private key. One of the issues with this is that the CA may generate a new CRL only once a day, so revocations do not happen immediately. If an e-commerce site’s private key has been compromised, the amount of time it takes for the key to appear in the CRL could be an issue. Another issue with CRL’s is that they are distributed with a specific lifetime, and often cached on intermediate systems. If a system cannot reach the CA, it will attempt to use the existing, cached CRL, and it may find that it is out of date.

Although many CA’s still make use of the CRL method, the newer method is the OCSP⁴⁰⁶ (Online Certificate Status Protocol). OCSP allows for on-line checking of certificate validity, by sending a request to a web site containing information on valid certificates. Thus, it tends to use more up-to-date data than the CRL approach uses. The drawback is that the client must be on-line to the network containing the CA, in order to contact the site furnishing validity data via OCSP. This may not be an issue in corporate environments using an internal CA, since network connectivity is assumed.

4.5.5  Revocation 1 4.5.5.1  Status Checking