PKI architecture
A PKI system consists of PKI entities, CAs, RAs and a certificate/CRL repository, as shown in Figure 82.
Figure 81: PKI architecture
PKI entity—An end user using PKI certificates. The PKI entity can be an operator, an organization, a device like a router or a switch, or a process running on a computer. PKI entities use SCEP to communicate with the CA or RA.
CA—Certification authority that grants and manages certificates. A CA issues certificates, defines the certificate validity periods, and revokes certificates by publishing CRLs.
RA—Registration authority, which offloads the CA by processing certificate enrollment requests. The RA accepts certificate requests, verifies user identity, and determines whether to ask the CA to issue certificates.
The RA is optional in a PKI system. In cases when there is security concern over exposing the CA to direct network access, it is advisable to delegate some of the tasks to an RA. Then, the CA can concentrate on its primary tasks of signing certificates and CRLs.
Certificate/CRL repository—A certificate distribution point that stores certificates and CRLs, and distributes these certificates and CRLs to PKI entities. It also provides the query function. A PKI repository can be a directory server using the LDAP or HTTP protocol, of which LDAP is commonly used.