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X.509 Certificate Decoder

An offline X.509 parser. Paste any standard SSL/TLS PEM certificate to extract human-readable metadata like the Issuer CA, Expiration Limits, Thumbprints, and Subject Alternative Names (SANs) completely offline.

What is the X.509 Certificate Parser?

An X.509 certificate viewer that decodes a PEM-encoded certificate entirely in your browser, showing its subject, issuer, validity dates, and fingerprints without sending the certificate anywhere.

How it works

Pasting a PEM block loads the node-forge library, which base64-decodes the payload and parses the underlying ASN.1 structure to pull out fields like the subject and issuer distinguished names, the validity window, the serial number, and any Subject Alternative Names. It then hashes the DER-encoded certificate with SHA-1 and SHA-256 to produce the fingerprints shown on screen.

SysAdmin & security use cases

  • Checking why a browser flags a certificate as untrusted by inspecting its issuer chain
  • Confirming exactly which hostnames a wildcard certificate's SANs cover before deploying it
  • Reading the precise expiration date, in UTC, ahead of a renewal deadline
  • Grabbing the SHA-256 fingerprint to pin a certificate in a mobile app or API client