Certificate authority

Each operating system and browser ships a trust store: a list of root certificates it will accept as the top of a chain. Mozilla, Apple, Microsoft and Google each maintain their own, with substantial overlap.

Roots are kept offline. Authorities issue from intermediate certificates signed by the root, which limits the damage if an intermediate key is compromised — the intermediate can be revoked without invalidating the root.

Inclusion in a trust store is not automatic. It requires an audit against the CA/Browser Forum baseline requirements and a review by each store operator.

A client with an out-of-date trust store rejects certificates from authorities added since. This is why very old devices fail on sites that work everywhere else.

For domain validation, control of the name: a token published in a DNS TXT record, a file served at a defined URL, or a response to an address at the domain. The ACME protocol automates exactly this.

It does not verify that the requester is the rightful owner in any legal sense, only that they currently control the name.

Before issuing, the authority is required to check the domain's CAA records and refuse if it is not listed.

Every certificate issued must be submitted to public Certificate Transparency logs, which is what makes mis-issuance detectable after the fact.

Authorities have been distrusted before, for mis-issuance, for poor validation practices, and for failing their audits. The process is public and usually gradual, with a deadline after which certificates from that authority stop being accepted.

The consequence for site operators is a forced migration on someone else's schedule, which is one argument for automated issuance: switching authority is a configuration change rather than a project.

Revocation of an individual certificate remains weakly enforced. Browsers use short-lived lists of high-priority revocations rather than checking every certificate, which is part of why certificate lifetimes were shortened instead.

OCSP stapling lets a server present a recent signed statement that its certificate is not revoked, avoiding a separate client lookup.

Which authority issued your certificate, visible in the certificate's issuer field, and whether it matches what your CAA record permits.

Whether renewal is automated and monitored. An automated renewal nobody watches fails silently until the expiry.

Whether the chain your server sends includes the intermediate. Omitting it is the single most common certificate misconfiguration.

Certificate Transparency logs for your domain, which reveal certificates issued for your names — including any you did not request.