What is an SSL certificate?

An SSL/TLS certificate is a digital document that proves a website's identity and enables encrypted HTTPS connections. It is issued by a Certificate Authority (CA) and contains the domain name, the issuing CA, the certificate's validity period, and the public key used to establish the encrypted connection.

How do I check if a website's SSL certificate is valid?

Enter the domain name (e.g. example.com) in the field above and click Check. The tool connects to the server, retrieves the certificate, and shows you the issuer, expiry date, Subject Alternative Names, and the protocol and cipher in use. A valid certificate will have an expiry date in the future and a trusted CA in the issuer chain.

What does SSL certificate expiry mean?

Every SSL certificate has a validity period — typically 90 days (Let's Encrypt) or up to 398 days (commercial CAs). After expiry, browsers display a warning and refuse to establish a secure connection. Monitoring expiry dates is important because an expired certificate will break HTTPS for all visitors. Most hosting providers and CAs send renewal reminders, but automated renewal (e.g. via Certbot or ACME) is best practice.

What are Subject Alternative Names (SANs)?

Subject Alternative Names (SANs) are the list of hostnames a single certificate is valid for. A certificate for example.com typically also covers www.example.com. Wildcard certificates cover all subdomains (*.example.com). Multi-domain certificates (UCC) can cover dozens of unrelated domains. Checking SANs tells you exactly which hostnames the certificate protects.

What is the difference between SSL and TLS?

SSL (Secure Sockets Layer) is the older protocol — SSL 2.0 and 3.0 are both deprecated and insecure. TLS (Transport Layer Security) is the modern replacement; TLS 1.2 and TLS 1.3 are the current standards. Despite this, 'SSL certificate' remains the common term for the certificate itself, even though the underlying protocol is TLS. All modern browsers require at least TLS 1.2.

Why does my browser show a certificate warning?

Certificate warnings appear when: the certificate has expired, the domain in the browser doesn't match the certificate's SANs (hostname mismatch), the certificate was issued by an untrusted or self-signed CA, or the certificate has been revoked. You can diagnose the exact cause by running an SSL check on the domain above — the tool shows which of these conditions apply.