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Certificate Decoder

Decode and inspect X.509 SSL/TLS Certificates (PEM format). Verify expiry dates, Subject Alternative Names (SANs), and issuer details 100% locally in your browser.

Certificate Decoder
Decode and inspect PEM-formatted SSL/TLS certificates locally in your browser.
Local Only
Paste Certificate (PEM format)
Decoded Details

Waiting for Certificate input...

How ZeroData protects your privacy

  • ✓ No Uploads: Processing happens entirely via client-side JavaScript.
  • ✓ No Storage: We do not have a database. We physically cannot save your data.
  • ✓ No Tracking: We don't log what you process or track your inputs.
  • ✓ Verifiable: Check your DevTools Network tab. You will see 0 outbound requests.

Why Parse Certificates Locally?

SSL/TLS certificates often contain sensitive organizational information, internal domain names in their SANs, and are frequently handled alongside their highly sensitive private keys. Uploading your certificates to a random third-party website for decoding is an unnecessary security risk.

Our Certificate Decoder runs entirely within your browser. It uses WebAssembly and JavaScript to parse the ASN.1 structure of the PEM file locally. This guarantees that your certificate's metadata and organizational structure are never transmitted over the internet or logged by a remote server.

Understanding Certificate Fields

When you decode a certificate, you'll see several important fields:

  • Subject: The entity (domain, person, or organization) that the certificate represents. The Common Name (CN) is typically the primary domain.
  • Issuer: The Certificate Authority (CA) that verified the subject and signed the certificate.
  • Validity: The precise dates during which the certificate is considered legally and technically valid.
  • SANs (Subject Alternative Names): A list of all hostnames secured by this certificate. Modern browsers enforce SANs over the legacy Common Name field.

Debugging SSL Errors

A common cause of HTTPS errors like NET::ERR_CERT_COMMON_NAME_INVALID is a mismatch between the domain the user typed and the domains listed in the certificate's SANs. By pasting your certificate here, you can instantly verify which domains are actually covered before you restart your web server.

Common Use Cases

  • Verifying the expiry date of an SSL/TLS certificate before deploying it.
  • Checking which domains and subdomains are covered by a certificate's SANs.
  • Debugging SSL connection errors to see if the certificate matches the private key.
  • Inspecting the issuer and subject details of a self-signed certificate.
  • Extracting the public key size and algorithm (e.g., RSA 2048-bit) from a PEM file.

Frequently Asked Questions

What is an X.509 Certificate?

An X.509 certificate is a digital certificate that uses the X.509 public key infrastructure (PKI) standard to verify that a public key belongs to the user, computer, or service identity contained within the certificate.

Is my certificate data uploaded to a server?

No. This tool parses the PEM-formatted certificate locally in your browser using the node-forge library. Your certificate details never leave your computer.

What is the difference between a CSR and a Certificate?

A Certificate Signing Request (CSR) is a message sent to a Certificate Authority to apply for a digital identity certificate. The Certificate is the final, signed document issued by the Authority that verifies the identity and binds it to the public key.

Why does my certificate say 'Expired'?

Every certificate has a 'Valid From' and 'Valid To' date. If the current date is past the 'Valid To' date, the certificate is considered expired and browsers will show a security warning if it is used for HTTPS.

What are Subject Alternative Names (SANs)?

SANs allow a single SSL certificate to secure multiple hostnames (domains or IP addresses). Modern browsers require the domain you are visiting to be listed in the SANs extension rather than just the Common Name.

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