A certificate revocation list (CRL) is a list of certificates
that have been revoked before their scheduled expiration date.
There are several reasons why a certificate might need to be
revoked and placed on a CRL. For instance, the key specified in
the certificate might have been compromised or the user
specified in the certificate may no longer have authority to use
the key. For example, suppose the user name associated with a
key is ``Alice Avery, Vice President, Argo Corp.'' If Alice were
fired, her company would not want her to be able to sign
messages with that key, and therefore the company would place
the certificate on a CRL. When verifying a signature, one
examines the relevant CRL to make sure the signer's certificate
has not been revoked. Whether it is worth the time to perform
this check depends on the importance of the signed document. A
CRL is maintained by a CA, and it provides information about
revoked certificates that were issued by that CA. CRLs only list
current certificates, since expire