Checking a certificate against a CRL is done by the Messaging Server after the S/MIME applet sends it a request to do so. Rather than download a CRL to memory each time a certificate is checked, Messaging Server downloads a copy of the CRL to disk and uses that copy for certificate checking. Every CRL has a next-update field which specifies the date after which a newer CRL version should be used. The next-update date can be viewed as an expiration date or time limit for using the CRL. A CRL that is past it’s next-update date is considered old or stale and triggers Messaging Server to download the latest version of the CRL the next time a certificate is checked.
Every time the S/MIME applet requests that a certificate be checked against a CRL, the Messaging Server does the following:
Compares the current date to the next-update date of the CRL.
If the CRL is stale, the Messaging Server downloads the latest version of the CRL to replace the stale CRL on disk and checking proceeds. However, if a newer CRL cannot be found or cannot be downloaded, the value of crlusepastnextupdate in the smime.conf file is used to determine what to do.
If crlusepastnextupdate is set to 0, the stale CRL is not used and the certificate in question has an ambiguous status. The S/MIME applet uses the value of revocationunknown in smime.conf to determine what to do next:
If revocationunknown is set to ok, the certificate is treated as valid and the private or public key is used to sign or encrypt a message.
If revocationunknown is set to revoked, the certificate is treated as invalid, the private or public key is not used to sign or encrypt a message, and a pop-up error message alerts the mail user that the key cannot be used.
If crlusepastnextupdate is set to 1, the S/MIME applet continues to use the stale CRL which causes no interruption of processing within Communications Express Mail, however a message is written to the Messaging Server log file to alert you to the situation.
This sequence of events continues to occur as certificates are checked against the CRL. As long as the Messaging Server can download a newer version of the CRL in a timely manner, and depending on the settings in the smime.conf file, mail processing proceeds without interruption. Check the Messaging Server log periodically for repeated messages that indicate a stale CRL is in use. If a newer CRL cannot be downloaded, you need to investigate why it is inaccessible.