X.509 Certificate Decoder
Decode a PEM certificate to see its subject, issuer, validity, SANs and key details.
An X.509 certificate decoder parses the DER structure inside a PEM file to reveal its subject, issuer, serial number, validity dates, signature and public-key algorithms, SHA-256 fingerprint and Subject Alternative Names — the last being what browsers actually match a hostname against.
Runs in your browser- Parsed locally
- The DER structure is decoded in your browser; nothing is transmitted
- Browser rule
- Hostname matching uses Subject Alternative Names, not the Common Name
- Privacy
- Runs entirely in your browser — nothing is uploaded
- Cost
- Free, unlimited, no sign-up
Frequently asked questions
Why does my certificate fail even though the Common Name is right?
How to use the x.509 certificate decoder
- 1Enter or paste your certificate (pem).
- 2Press Run, then download the result when it is ready.
Embed this tool
Put the working x.509 certificate decoder on your own site. It runs in your visitors' browsers exactly as it does here — free, no account, nothing uploaded.
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