An SSL certificate is a digital certificate that proves a site's identity and enables HTTPS on its domain.
An SSL certificate is a small data file installed on your web server that does two jobs. It proves to visitors' browsers that the site really belongs to the domain in the address bar, and it holds the public key used to set up encryption between the browser and the server. With a valid certificate in place, your site loads over HTTPS and browsers treat it as secure.
Strictly speaking, modern certificates are used with TLS, the successor to SSL, but the name "SSL certificate" stuck. For anyone buying or installing one, they're the same thing.
How an SSL certificate works
- A visitor's browser connects to your site and asks for its certificate.
- The server sends the certificate, which is digitally signed by a certificate authority (CA) the browser already trusts.
- The browser checks the signature, the domain name and the expiry date.
- If all is well, browser and server agree on session keys, and everything after that is encrypted.
This handshake takes a fraction of a second and happens at the start of each new connection.
Types of SSL certificates
By validation level:
- Domain validated (DV): proves you control the domain. Issued in minutes, often free, and right for most sites.
- Organisation validated (OV): the CA also checks that your organisation exists.
- Extended validation (EV): the strictest checks. Browsers no longer show EV differently in the address bar, so its visible benefit is small.
By coverage:
- Single domain: one name, usually with and without www
- Wildcard: a domain and all its first-level subdomains
- Multi-domain (SAN): several different domains on one certificate
Why an SSL certificate matters for your website
Without one, Chrome and other browsers label your pages "Not secure", which puts visitors off and hurts trust on checkout and contact forms. HTTPS is also a light ranking signal for Google, and many modern browser features only work over HTTPS. Free certificates from authorities like Let's Encrypt mean there's no reason to go without. AblePage issues and renews free SSL certificates automatically on every plan.
Common SSL certificate problems
- Expired certificate: browsers show a full-page warning. Automatic renewal prevents this.
- Name mismatch: the certificate doesn't cover the exact hostname, such as www or a subdomain.
- Mixed content: the page loads over HTTPS but pulls images or scripts over HTTP, so the browser no longer shows it as fully secure.
- Missing intermediate certificate: some devices can't build the chain of trust back to the CA.
After installing a certificate, add a redirect from HTTP to HTTPS so every visitor uses the secure version of your site.