TLS certificate

The subject, historically a distinguished name and in practice now superseded by the subject alternative name extension, which lists every hostname the certificate covers.

The public key, and the algorithm it uses. RSA and elliptic curve keys are both common.

The validity period, with a not-before and a not-after date. Publicly trusted certificates are now limited to a little over a year, and the limit has been progressively shortened.

The issuer, and the signature that links this certificate to the issuing authority's own certificate.

Extensions: key usage constraints, the addresses of revocation services, and Certificate Transparency proofs that the certificate was logged publicly.

Domain validation proves only that the requester controls the domain, usually by publishing a token in DNS or at a URL. It is automated, free from several authorities, and is what the great majority of the web uses.

Organisation validation adds checks on the legal entity behind the domain.

Extended validation adds a more extensive vetting process. Browsers no longer display it differently, so its practical benefit to visitors is now essentially nil.

Cryptographically the three are identical. The encryption is the same; only the claim about identity differs.

An expired certificate produces a full-page browser interstitial. It is not a degradation; the site is effectively offline for ordinary visitors.

Short lifetimes limit the damage a compromised key can do, because revocation on the web has never worked reliably.

Short lifetimes are only workable with automated renewal, which is why the ACME protocol and its clients became standard infrastructure.

The failure to watch for is not the expiry itself but the renewal automation silently breaking — a CAA record added, a validation path blocked, a cron job that stopped. The certificate then expires on schedule with nobody watching.

`openssl s_client -connect example.com:443 -servername example.com </dev/null | openssl x509 -noout -dates -subject -ext subjectAltName` prints the validity window and the covered names.

In a browser, the padlock leads to the certificate viewer showing the same fields with the chain.

Check every hostname in use. A certificate covering the apex but not `www`, or covering the site but not an API subdomain, breaks for exactly the visitors who reach those names.

Confirm the chain is complete. A server that sends its leaf certificate without the intermediate works in browsers that have cached the intermediate and fails in clients that have not.