Public Key Infrastructure (PKI)

A PKI is built around components and procedures for managing public and private key pairs. A certificate authority (CA) issues certificates, and any certificate it issues is trusted by all entities that trust the CA. A certificate is a digital document, signed by a CA, that proves who owns a public key. A registration authority (RA) verifies the identity of an entity before its certificate is issued, and a validation authority lets others check that a certificate has not been revoked.

An entity registers by sending a certificate signing request that contains its public key and a signature made with its private key. The RA checks the entity's identity and approves the request, and the CA delivers the signed certificate. The public key can be shared widely, while the private key proves ownership of the identity and must be kept secret.

CAs are arranged in a hierarchy. A root CA at the top issues intermediate CA certificates, and intermediates issue certificates to end entities, usually in two or three tiers. Separating CAs limits the damage from a compromised CA. Certificates whose private key is compromised or whose device is decommissioned are revoked, and the revocation is published through certificate revocation lists (CRLs) or the Online Certificate Status Protocol (OCSP).

A public PKI authenticates entities on public services such as internet-facing websites, while a private PKI serves one organization, for example for VPNs, internal web applications, IoT, and end user device authentication. Because compromise of a PKI can give access to a wide range of data and services, it is an attractive target for attackers. The algorithms PKI systems use to sign data are not resistant to attack by a quantum computer and will need to be replaced.