How SSL/TLS Certificates Work — and Why HTTPS Matters for SEO
An SSL certificate (more precisely, a TLS certificate) does two jobs: it encrypts traffic between the browser and the server, and it proves the server’s identity — that you are really connected to the site you think you are. The second job matters as much as the first; encryption to an impostor protects nothing.
What gets validated on every connection
- The chain of trust: your certificate must be signed, through one or more intermediates, up to a root CA already trusted by the browser. A missing intermediate certificate is the classic mobile-only failure — desktop browsers sometimes paper over the gap via cached intermediates or AIA fetching, while phones show a full-page error.
- Validity dates: an expired certificate produces a red full-page warning for every visitor. Certificates live 90 days (Let’s Encrypt) to about a year (most CAs), so expiry is a recurring event, not a one-time setup.
- Name match: the certificate must cover the exact hostname in the address bar, via CN or — almost always today — SAN entries. Forgetting
wwwor an API subdomain is the most common mismatch.
You can inspect any of this without a browser:
openssl s_client -connect example.com:443 -servername example.com | openssl x509 -noout -dates -subject -ext subjectAltName
The SEO angle
Google has confirmed HTTPS as a ranking signal, and Chrome labels plain-HTTP sites “Not secure” in the address bar — a trust penalty that costs more than any ranking factor. If your site answers on both HTTP and HTTPS, add a one-way 301 redirect from HTTP to HTTPS immediately: serving the same content on both splits signals across duplicate URLs. Follow the redirect with HSTS so browsers skip the insecure hop entirely.
Because certificate state is a moving target — expiry, chain, protocol versions — it deserves periodic external verification rather than a one-time install-and-forget. The WhoDNS SSL check reports the expiry date, issuer, full chain, supported TLS versions and hostname coverage for any site, from the outside, the way a visitor’s browser sees it.
🔗 Related tool: Run a full SSL check on your site