Public Key Infrastructure

A Public Key Infrastructure (PKI) is a system that helps users and computers verify whom they are communicating with electronically. It uses digital certificates to bind an entity's identity to its public key, supported by trusted issuers, verification procedures and certificate-management rules. The entity retains the corresponding private key and uses it to demonstrate control of that identity.

What a digital certificate establishes

A digital certificate is an electronic credential signed by a Certifying Authority (CA). Depending on the certificate type, it identifies a person, organisation or internet service and associates that identity with a public key.

• A certificate contains information such as the subject's identity, public key, issuer, serial number and validity period.
• Before issuing a certificate, the CA performs identity or domain-control checks appropriate to the certificate type.
• The CA's digital signature enables a verifier to detect changes to the signed certificate and verify its issuer; the certificate does not contain the subject's private key.

How authentication works on the internet

When a browser connects to an HTTPS website, the server presents its certificate during the Transport Layer Security (TLS) handshake. Authentication requires both a trustworthy certificate and evidence that the server controls the corresponding private key.

• The browser checks that the certificate covers the requested domain name and is within its validity period.
• It verifies certificate signatures along a chain of trust, usually through intermediate CAs to a root CA trusted by the browser or operating system.
• The server proves possession of the corresponding private key through the handshake's cryptographic operations.
• Successful verification helps the browser authenticate the server; TLS also establishes keys to protect the connection. Client authentication can similarly use client certificates.

Trust maintenance and security limits

PKI depends on secure private keys, reliable certificate issuance and continuing certificate management. Revocation allows a certificate to be invalidated before expiry, for example after private-key compromise.

• Certificate Revocation Lists and the Online Certificate Status Protocol (OCSP) provide mechanisms for checking revocation status.
• A valid website certificate does not establish that its content or business is honest; a malicious website can obtain a certificate for a domain it controls.
• Compromised private keys or incorrectly issued certificates can undermine authentication, making key protection and issuer oversight essential.