A certificate authority, or CA, is an organisation that signs certificates for websites. Browsers and operating systems trust a list of them.
A CA checks that the applicant controls the domain, usually by asking for a file on the site or a record in DNS. If the check passes, it signs the certificate.
Browsers and operating systems ship with a list of trusted CAs. A certificate signed by one of them is accepted without anyone setting up anything on the visitor's side. If a CA loses that trust, its certificates are no longer accepted.
Most CAs do not sign website certificates with their main key. They use intermediate certificates, which form a chain back to a trusted root.
The basic check proves control of the domain. Some certificates also involve checks of the organisation, which are shown in the certificate details.
SSL Domain Detective shows which certificates your domains use and when each one ends. Open SSL Domain Detective
SSL Domain Detective is now available on the App Store.
Last reviewed: 2026-11-02
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