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X.509 Certificate Decoder

Decode X.509 certificates to inspect validity and SANs

Runs in your browserDebugging03 — Encoding & Crypto

What is X.509 Certificate Decoder?

This SSL certificate decoder parses a PEM-encoded X.509 certificate and displays the subject, issuer, serial number, signature algorithm, validity period with days remaining, and the full list of subject alternative names. Parsing happens locally in the browser and the certificate is never uploaded.

How to use X.509 Certificate Decoder

  1. 1Paste the certificate, starting with the -----BEGIN CERTIFICATE----- line.
  2. 2Click "Decode" to see every field broken out.
  3. 3Focus on the remaining validity and on whether the SAN list actually covers the domain you plan to serve.

How do I do this in code?

Use the tool above for one-off work; for anything you repeat, move it into a script or your project.

# Inspect a local certificate
openssl x509 -in cert.pem -noout -text

# Fetch a live certificate and check its validity dates
echo | openssl s_client -servername example.com -connect example.com:443 2>/dev/null \
  | openssl x509 -noout -dates -subject

Frequently asked questions

My browser says the certificate is invalid, so why does everything look fine here?+

This tool only parses the fields inside the certificate itself; it performs no chain validation. Your browser additionally checks that the chain is complete and trusted, that the hostname matches a SAN entry, that the certificate has not been revoked, and that the signature algorithm and key strength are still acceptable. A missing intermediate certificate is by far the most common cause.

What is the difference between CN and SAN?+

CN (Common Name) is the legacy field, and modern browsers ignore it entirely — hostname matching is decided solely by the SAN (Subject Alternative Name) list. When requesting a certificate, make sure every hostname you intend to serve is present in the SAN.

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