in the operation of some cryptosystems, usually public key infrastructures (pkis), a certificate revocation list (crl) is a list of certificates (or more specifically, a list of serial numbers for certificates) that have been revoked, and therefore, entities presenting those (revoked) certificates should no longer be trusted. ==revocation states== there are two different states of revocation defined in rfc 3280: * revoked: a certificate is irreversibly revoked if, for example, it is discovered that the certificate authority (ca) had improperly issued a certificate, or if a private-key is thought to have been compromised. certificates may also be revoked for failure of the identified entity to adhere to policy requirements, such as publication of false documents, mis-representation of software behavior, or violation of any other policy specified by the ca operator or its customer. the most common reason for revocation is the user no longer being in sole possession of the private key (e.g., the token containing the private key has been lost or stolen). * hold: this reversible status can be used to note the temporary invalidity of the certificate (e.g., if the user is unsure if the private key has been lost). if, in this example, the private key was found and nobody had access to it, the status could be reinstated, and the certificate is valid again, thus removing the certificate from future crls. ==reasons for revocation== reasons to revoke a certificate according to rfc 5280 p69 are: * unspecified (0) * keycompromise (1) * cacompromise (2) * affiliationchanged (3) * superseded (4) * cessationofoperation (5) * certificatehold (6) * removefromcrl (8) * privilegewithdrawn (9) * aacompromise (10) obs.: value 7 is not used. ==publishing revocation lists== a crl is generated and published periodically, often at a defined interval. a crl can also be published immediately after a certificate has been revoked. the crl is always issued by the ca which issues the corresponding certificates. all crls have a lifetime during which they are valid; this timeframe is often 24 hours or less. during a crl's validity period, it may be consulted by a pki-enabled application to verify a certificate prior to use. to prevent spoofing or denial-of-service attacks, crls usually carry a digital signature associated with the ca by which they are published. to validate a specific crl prior to relying on it, the certificate of its corresponding ca is needed, which can usually be found in a public directory (e.g., preinstalled in web browsers). the certificates for which a crl should be maintained are often x.509/public key certificates, as this format is commonly used by pki schemes. ==revocation vs. expiration== expiration dates are not a substitute for a crl. while all expired certificates are considered invalid, not all unexpired certificat
Key_management Public-key_cryptography