RFC 2459        Internet X.509 Public Key Infrastructure    January 1999


   However, the ISO/IEC/ITU and ANSI X9 standard extensions are very
   broad in their applicability.  In order to develop interoperable
   implementations of X.509 v3 systems for Internet use, it is necessary
   to specify a profile for use of the X.509 v3 extensions tailored for
   the Internet.  It is one goal of this document to specify a profile
   for Internet WWW, electronic mail, and IPsec applications.
   Environments with additional requirements may build on this profile
   or may replace it.

3.2  Certification Paths and Trust

   A user of a security service requiring knowledge of a public key
   generally needs to obtain and validate a certificate containing the
   required public key. If the public-key user does not already hold an
   assured copy of the public key of the CA that signed the certificate,
   the CA's name, and related information (such as the validity period
   or name constraints), then it might need an additional certificate to
   obtain that public key.  In general, a chain of multiple certificates
   may be needed, comprising a certificate of the public key owner (the
   end entity) signed by one CA, and zero or more additional
   certificates of CAs signed by other CAs.  Such chains, called
   certification paths, are required because a public key user is only
   initialized with a limited number of assured CA public keys.

   There are different ways in which CAs might be configured in order
   for public key users to be able to find certification paths.  For
   PEM, RFC 1422 defined a rigid hierarchical structure of CAs.  There
   are three types of PEM certification authority:

      (a)  Internet Policy Registration Authority (IPRA):  This
      authority, operated under the auspices of the Internet Society,
      acts as the root of the PEM certification hierarchy at level 1.
      It issues certificates only for the next level of authorities,
      PCAs.  All certification paths start with the IPRA.

      (b)  Policy Certification Authorities (PCAs):  PCAs are at level 2
      of the hierarchy, each PCA being certified by the IPRA.  A PCA
      shall establish and publish a statement of its policy with respect
      to certifying users or subordinate certification authorities.
      Distinct PCAs aim to satisfy different user needs. For example,
      one PCA (an organizational PCA) might support the general
      electronic mail needs of commercial organizations, and another PCA
      (a high-assurance PCA) might have a more stringent policy designed
      for satisfying legally binding digital signature requirements.








 
RFC 2459        Internet X.509 Public Key Infrastructure    January 1999


      (c)  Certification Authorities (CAs):  CAs are at level 3 of the
      hierarchy and can also be at lower levels. Those at level 3 are
      certified by PCAs.  CAs represent, for example, particular
      organizations, particular organizational units (e.g., departments,
      groups, sections), or particular geographical areas.

   RFC 1422 furthermore has a name subordination rule which requires
   that a CA can only issue certificates for entities whose names are
   subordinate (in the X.500 naming tree) to the name of the CA itself.
   The trust associated with a PEM certification path is implied by the
   PCA name. The name subordination rule ensures that CAs below the PCA
   are sensibly constrained as to the set of subordinate entities they
   can certify (e.g., a CA for an organization can only certify entities
   in that organization's name tree). Certificate user systems are able
   to mechanically check that the name subordination rule has been
   followed.

   The RFC 1422 uses the X.509 v1 certificate formats. The limitations
   of X.509 v1 required imposition of several structural restrictions to
   clearly associate policy information or restrict the utility of
   certificates.  These restrictions included:

      (a) a pure top-down hierarchy, with all certification paths
      starting from IPRA;

      (b) a naming subordination rule restricting the names of a CA's
      subjects; and

      (c) use of the PCA concept, which requires knowledge of individual
      PCAs to be built into certificate chain verification logic.
      Knowledge of individual PCAs was required to determine if a chain
      could be accepted.

   With X.509 v3, most of the requirements addressed by RFC 1422 can be