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X.509 Certificate Decoder

Free online X.509 certificate decoder. Parse PEM certificates to view subject, issuer, validity, SAN, fingerprints, and extensions. No sign-up required.

What Is an X.509 Certificate?

X.509 is the standard format for public key certificates used in SSL/TLS, S/MIME, and other security protocols. An X.509 certificate binds a public key to an identity (domain name, organization) and is signed by a Certificate Authority (CA). Certificates contain information about the subject, issuer, validity period, public key, signature algorithm, and extensions like Subject Alternative Names (SAN).

How to Use This Certificate Decoder

  1. Paste a PEM-encoded certificate (starting with -----BEGIN CERTIFICATE-----) into the input area.
  2. Click Decode Certificate to parse and display the certificate details.
  3. Review the subject, issuer, validity dates, and signature algorithm.
  4. Check Subject Alternative Names (SAN) for all covered domains.
  5. Copy SHA-256 or SHA-1 fingerprints for certificate pinning or verification.

Common Use Cases

  • Debug SSL/TLS Issues — Quickly inspect server certificates to diagnose SSL handshake failures or certificate mismatch errors.
  • Verify Certificate Details — Confirm the subject, issuer, and validity of certificates before deployment.
  • Certificate Pinning — Extract SHA-256 fingerprints for HTTP Public Key Pinning (HPKP) or mobile app certificate pinning.