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SSL Certificate Decoder

Paste a PEM-encoded SSL/TLS certificate and decode it entirely in your browser. Extracts subject, issuer, validity dates, SANs, public key type and size, signature algorithm, and serial number — no upload, no server.

Disclaimer: Free tool provided “as is” by MonitorGiant. No warranty or liability for any data loss, security issues, or infrastructure problems arising from use of this tool. Results are for informational purposes only. · A Free Tool by MonitorGiant

What is SSL Certificate Decoder?

An SSL/TLS certificate is an X.509 document encoded in a binary format called DER, commonly wrapped in Base64 as PEM. It contains the domain identity, issuing authority, validity window, cryptographic key details, and the list of hostnames it covers (SANs). This decoder parses the raw DER structure entirely in your browser using a built-in ASN.1 parser — no certificate data leaves your device.

How to use this tool

  1. 1 Paste a PEM certificate into the text area — it should start with -----BEGIN CERTIFICATE----- and end with -----END CERTIFICATE-----. You can get one from your web server, from OpenSSL (openssl s_client -connect domain:443), or from any certificate file.
  2. 2 Click 'Decode Certificate'. The parser reads the binary ASN.1 DER structure directly in your browser.
  3. 3 Review the subject (who the cert was issued to), issuer (the CA that signed it), and validity dates with days-remaining count.
  4. 4 Check the SANs list — these are all the hostnames covered by the certificate, including wildcard entries.
  5. 5 Use 'Load sample' to try the decoder immediately with the ISRG Root X1 (Let's Encrypt root) certificate.

When would you use this?

  • Verifying a certificate before installing it on a server — confirm the CN, SANs, expiry date, and key type without running openssl.
  • Debugging certificate chain issues — quickly check which CA issued a certificate and what domains it covers.
  • Security audits — verify that deployed certificates use the expected key size (RSA-2048+, EC P-256+) and haven't expired.

Related tools

SSL / TLS Security Checker Full live audit of a domain's SSL certificate and security headers. DKIM Key Generator Generate RSA-2048 DKIM key pairs in-browser. JWT Decoder Decode and inspect JSON Web Token headers and claims.

How works

  1. 1

    Paste the PEM certificate

    Copy the certificate text from your server config, a .crt/.pem file, or from openssl s_client output. The decoder accepts the full PEM block including the header and footer lines.

  2. 2

    Decode in-browser

    The parser strips the Base64 wrapper, reads the DER binary directly, and walks the ASN.1 structure to extract every standard field. No data is sent anywhere.

  3. 3

    Review all fields

    Subject, issuer, validity, SANs, public key type, signature algorithm, and serial number are all displayed. The status strip immediately flags expired or expiring-soon certificates.

Certificate parsing happens entirely in your browser using a built-in ASN.1/DER parser. Your certificate data is never transmitted to any server.

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