What is an SSL/TLS Certificate?

An SSL/TLS certificate is a digital file that verifies a website's identity and enables an encrypted connection. It's the thing that puts the "S" in HTTPS. These certificates come from trusted Certificate Authorities (CAs) and contain information about who owns the site and when the certificate expires.

You'll often encounter certificates in PEM format — a Base64-encoded text block that starts with -----BEGIN CERTIFICATE-----. It's the most widely shared format you'll run into.

Why Do Developers Need to Decode a Certificate?

• When a user reports a security warning in their browser, the fastest way to diagnose it is to paste the certificate here and check: is it expired? Is it issued to the wrong domain? Is the chain incomplete? You get the answer in seconds.
• Before installing a new certificate on a server, it's worth decoding it first just to confirm the Subject (domain name), Issuer, and validity dates are all what you expect. There's nothing worse than deploying a cert that's already wrong.
• The Subject Alternative Names (SANs) field is where you'll find all the domains and subdomains the certificate actually covers — not just the primary one. Worth checking if you're not sure whether a wildcard includes a particular subdomain.
• For security audits, you can quickly check the signature algorithm and public key size to confirm they still meet current standards. Old SHA-1 certs and 1024-bit keys are still out there.

How This Tool Helps

Paste the PEM-formatted certificate text (or upload the .crt or .pem file directly) and the tool breaks it down into a readable format — no command-line tools needed.

All parsing happens entirely in your browser using the @peculiar/x509 library and the native Web Crypto API. No certificate data is ever sent to a server.