Cert Decoder

About Cert Decoder

Cert Decoder is a free online tool for decoding X.509 certificates in PEM format. Users paste certificate content, including the BEGIN/END CERTIFICATE markers, and the tool extracts details such as issuer, subject, validity period, serial number, signature algorithm, Subject Alternative Names, public key information, and certificate extensions. Decoding runs entirely client-side in the browser using JavaScript, and certificate data is not sent to the server.

The page also includes reference material explaining X.509 certificate structure, PEM versus DER encoding, and common certificate uses such as HTTPS/TLS, S/MIME email security, code signing, document signing, and client authentication.

The tool is suited for developers and administrators who need to inspect or verify certificate fields, such as validity dates or SAN entries, when configuring or troubleshooting TLS/SSL setups.

Key features

• Decodes PEM-formatted X.509 certificates directly in the browser
• Extracts issuer, subject, and validity period (not before/not after)
• Shows serial number and signature algorithm
• Reveals Subject Alternative Names (SAN)
• Displays public key information and certificate extensions
• Processes certificate data entirely client-side without sending it to a server

Frequently asked questions