addressed using certificate extensions, without a need to restrict
   the CA structures used.  In particular, the certificate extensions
   relating to certificate policies obviate the need for PCAs and the
   constraint extensions obviate the need for the name subordination
   rule.  As a result, this document supports a more flexible
   architecture, including:

      (a) Certification paths may start with a public key of a CA in a
      user's own domain, or with the public key of the top of a
      hierarchy.  Starting with the public key of a CA in a user's own
      domain has certain advantages.  In some environments, the local
      domain is the most trusted.






 
RFC 2459        Internet X.509 Public Key Infrastructure    January 1999


      (b)  Name constraints may be imposed through explicit inclusion of
      a name constraints extension in a certificate, but are not
      required.

      (c)  Policy extensions and policy mappings replace the PCA
      concept, which permits a greater degree of automation.  The
      application can determine if the certification path is acceptable
      based on the contents of the certificates instead of a priori
      knowledge of PCAs. This permits automation of certificate chain
      processing.

3.3  Revocation

   When a certificate is issued, it is expected to be in use for its
   entire validity period.  However, various circumstances may cause a
   certificate to become invalid prior to the expiration of the validity
   period. Such circumstances include change of name, change of
   association between subject and CA (e.g., an employee terminates
   employment with an organization), and compromise or suspected
   compromise of the corresponding private key.  Under such
   circumstances, the CA needs to revoke the certificate.

   X.509 defines one method of certificate revocation.  This method
   involves each CA periodically issuing a signed data structure called
   a certificate revocation list (CRL).  A CRL is a time stamped list
   identifying revoked certificates which is signed by a CA and made
   freely available in a public repository.  Each revoked certificate is
   identified in a CRL by its certificate serial number. When a
   certificate-using system uses a certificate (e.g., for verifying a
   remote user's digital signature), that system not only checks the
   certificate signature and validity but also acquires a suitably-
   recent CRL and checks that the certificate serial number is not on
   that CRL.  The meaning of "suitably-recent" may vary with local
   policy, but it usually means the most recently-issued CRL.  A CA
   issues a new CRL on a regular periodic basis (e.g., hourly, daily, or
   weekly).  An entry is added to the CRL as part of the next update
   following notification of revocation. An entry may be removed from
   the CRL after appearing on one regularly scheduled CRL issued beyond
   the revoked certificate's validity period.

   An advantage of this revocation method is that CRLs may be
   distributed by exactly the same means as certificates themselves,
   namely, via untrusted communications and server systems.

   One limitation of the CRL revocation method, using untrusted
   communications and servers, is that the time granularity of
   revocation is limited to the CRL issue period.  For example, if a
   revocation is reported now, that revocation will not be reliably




 
RFC 2459        Internet X.509 Public Key Infrastructure    January 1999


   notified to certificate-using systems until the next periodic CRL is
   issued -- this may be up to one hour, one day, or one week depending
   on the frequency that the CA issues CRLs.

   As with the X.509 v3 certificate format, in order to facilitate
   interoperable implementations from multiple vendors, the X.509 v2 CRL
   format needs to be profiled for Internet use.  It is one goal of this
   document to specify that profile.  However, this profile does not
   require CAs to issue CRLs. Message formats and protocols supporting
   on-line revocation notification may be defined in other PKIX
   specifications.  On-line methods of revocation notification may be
   applicable in some environments as an alternative to the X.509 CRL.
   On-line revocation checking may significantly reduce the latency
   between a revocation report and the distribution of the information
   to relying parties.  Once the CA accepts the report as authentic and
   valid, any query to the on-line service will correctly reflect the
   certificate validation impacts of the revocation.  However, these
   methods impose new security requirements; the certificate validator
   shall trust the on-line validation service while the repository does
   not need to be trusted.

3.4  Operational Protocols