SSL Certificate Decoder

SSL Certificate Decoder - Parse PEM Certificates Online

Decode and inspect PEM-encoded X.509 SSL/TLS certificates directly in your browser. Our decoder parses the ASN.1 DER structure to extract certificate details.

Paste a PEM-encoded X.509 certificate into the text area—this includes the lines starting with "-----BEGIN CERTIFICATE-----" and ending with "-----END CERTIFICATE-----". Click decode, and the tool parses the certificate's ASN.1 DER structure to extract all fields: issuer, subject, validity dates (not before/not after), serial number, signature algorithm, public key algorithm and size, subject alternative names (SANs), and key usage extensions. The information is displayed in a human-readable format with labeled fields. Use this to inspect certificates from servers, certificate files, or chain bundles without installing OpenSSL or other command-line tools.

System administrators verify certificate details before installing them on web servers to catch configuration errors. Security engineers audit certificate chains to identify weak signature algorithms or short key lengths. Developers debug SSL/TLS issues by examining certificate properties like SANs and expiration dates. DevOps teams validate that new certificates match the intended domains and organizational details. Certificate authorities use decoders to verify certificate generation before issuance. Compliance auditors check that certificates meet organizational standards for key size and validity period.

Always verify the certificate's validity period—certificates issued for more than 398 days won't be trusted by modern browsers. Check that Subject Alternative Names (SANs) include all domains the certificate will serve, including www and non-www variants. Ensure the signature algorithm is SHA-256 or stronger—SHA-1 is deprecated and will cause browser warnings. RSA keys should be at least 2048 bits; 4096 bits provides better future-proofing. Look for critical extensions like Extended Key Usage (EKU) that may restrict the certificate's use cases. For internal testing, self-signed certificates work, but production always requires a trusted CA.

All processing happens directly in your browser. Your files never leave your device — no server uploads, no cloud storage, no data retention. The tool works offline once loaded, requires no registration, and is completely free with no usage limits.