About The PEM / X.509 Certificate Decoder

PEM / X.509 Certificate Decoder extracts readable certificate metadata from PEM blocks or DER base64 input. It helps inspect subject, issuer, serial number, validity dates, SAN entries, key details, algorithms, and fingerprints.

Decoding runs locally in your browser. It shows certificate contents for inspection, but it does not prove that a certificate chain is trusted by a browser or operating system.

How to Decode X.509 Certificates Online

1. Paste a PEM certificate block, certificate chain, or compatible DER base64 input.
2. Choose readable text for manual review or JSON for structured output.
3. Click Decode Certificate and review the parsed certificate details.

Choosing Options Correctly

Use readable text when you are troubleshooting by eye. Use JSON when you want structured fields for notes, scripts, or comparison with another parser.

Common Use Cases

• Checking certificate subject, issuer, and expiration dates.
• Reviewing SAN entries for hostname coverage.
• Inspecting pasted certificate chains during TLS debugging.

Quick FAQ

Does decoding mean the certificate is trusted?
No. Decoding only shows fields. Trust requires chain validation, hostname checks, expiry checks, revocation policy, and trusted roots.

Why did decoding fail?
The PEM may be missing headers, contain extra text, be encrypted, use an unsupported format, or have broken Base64.

Can I paste certificate chains?
Yes if the tool supports multiple PEM blocks, but review each certificate in the chain separately.

What should I check first?
Look at subject, issuer, SANs, validity dates, key usage, extended key usage, and whether the public key type matches your needs.