A public certificate authority (CA) is a trusted organization that browsers and operating systems recognize as allowed to issue TLS certificates for public websites. When a CA signs a certificate, it vouches that a domain owner has been checked according to the browser ecosystem’s rules. This trust is what lets HTTPS work at internet scale.
Public CAs matter because they sit at the center of web trust: if one is compromised, misused, or issues a bad certificate, attackers can impersonate sites, intercept traffic, or enable phishing with a padlock icon. Defenders rely on public CAs for automated certificate issuance, revocation, and compliance with browser policies. In modern deployments, public CA infrastructure must also support short-lived certificates, transparency logging, and new cryptographic schemes without breaking TLS compatibility.