Certificate authority (CA)

Definition

A certificate authority (CA) is an organization trusted to issue and sign digital certificates.

How it works

Before issuing a certificate for HTTPS, a CA checks that whoever is asking really controls the domain. Browsers and operating systems ship with a list of root authorities they trust, so when your server presents its certificate, a visitor's browser can trace the signature chain back to a name it recognizes.

Example: Let's Encrypt is a certificate authority that issues domain-validated certificates through an automated process.

Often confused: The CA signs and issues the certificate; installing it on the server and keeping it renewed is your job — or your host's.

Related terms

• Certificate signing request (CSR)
• Let's Encrypt
• SSL certificate
• Transport Layer Security (TLS)

Learn more from Bitfoo

• Review SSL certificate options

Sources

• IETF RFC 5280: Internet X.509 Public Key Infrastructure
• Let's Encrypt: How it works

Last reviewed 2026-07-19.