X.509 Certificate Decoder
Decode PEM certificates into readable fields.
What is Cert Decoder?
An X.509 certificate decoder that parses a PEM certificate and extracts the subject, issuer, validity dates, subject alternative names, public key algorithm, and fingerprint.
How it works
The certificate is parsed with pkijs/asn1js, and the relevant fields are surfaced as a readable report.
- Enter your input. Type or paste your input into the field above — the tool responds instantly.
- Review the result. Output updates live as you type. Everything runs entirely in your browser.
- Copy or download. Use the Copy button to grab the result, or Download to save it as a file.
Examples
Inspecting an SSL certificate
Paste a PEM certificate and decode its subject, issuer, validity dates, and subject alternative names.
Common mistakes
Truncated PEM blocks
PEM includes the -----BEGIN/END----- lines — paste the full block, not just the base64 in the middle.
Frequently asked questions
Does it validate the certificate?
It decodes and checks internal consistency and dates; full chain-of-trust validation requires the CA chain.
What is a SAN and why does it matter?
Subject Alternative Names list the domains a certificate covers — browsers use SANs, not the CN, for hostname matching.