A certification authority (CA) accepts a certificate request, verifies the requester's information according to the policy of the CA, and then uses its private key to apply its digital signature to the certificate. The CA then issues the certificate to the subject of the certificate for use as a security credential within a public key infrastructure (PKI). A CA is also responsible for revoking certificates and publishing a certificate revocation list (CRL).

A CA can be an outside entity, such as VeriSign, or it can be a CA that you create for use by your organization by installing Active Directory Certificate Services (AD CS). Each CA can have distinct proof-of-identity requirements for certificate requesters, such as a domain account, employee badge, driver's license, notarized request, or physical address. Identification checks such as this often warrant an onsite CA, so that organizations can validate their own employees or members.

Microsoft enterprise CAs use a person's user account credentials as proof of identity. In other words, if you are logged on to a domain and request a certificate from an enterprise CA, the CA can authenticate your identity based on your account in Active Directory Domain Services (AD DS).

Every CA also has a certificate to confirm its own identity, issued by another trusted CA or, in the case of root CAs, issued by itself. It is important to remember that anyone can create a CA. Therefore, a user or administrator must decide whether to trust that CA and, by extension, the policies and procedures that the CA has in place for confirming the identity of the entities that are issued certificates by that CA.

Root and subordinate CAs

A root CA is meant to be the most trusted type of CA in an organization's PKI. If the root CA is compromised or issues a certificate to an unauthorized entity, then any certificate-based security in your organization becomes vulnerable. Therefore, both the physical security and the certificate issuance policy of a root CA are normally more rigorous than those for subordinate CAs. While root CAs can be used to issue certificates to end users for such tasks as sending secure e-mail, in most organizations they will only be used to issue certificates to other CAs, called subordinate CAs.

A subordinate CA is a CA that has been issued a certificate by another CA in your organization. Typically, a subordinate CA will issue certificates for specific uses, such as secure e-mail, Web-based authentication, or smart card authentication. Subordinate CAs can also issue certificates to other CAs that are more subordinate. Together, a root CA, the subordinate CAs that have been certified by the root, and subordinate CAs that have been certified by other subordinate CAs form a certification hierarchy.

For more information about certification hierarchies, see Public Key Infrastructures.

Enterprise and stand-alone CAs

This version of AD CS supports the installation of stand-alone CAs and enterprise CAs. For information about the operational characteristics of enterprise CAs and stand-alone CAs, see Enterprise Certification Authorities and Stand-Alone Certification Authorities.