Decode an X.509 certificate from PEM or DER into a readable security report, inspect every common extension and ASN.1 node, test hostname coverage, verify the certificate against a supplied issuer, and export machine-readable JSON — entirely in your browser.
Paste the issuer certificate. For a self-issued certificate, you can use the current certificate itself. Verification proves the signature against the supplied key; it does not establish OS/browser trust.
The decoder follows the X.509/PKIX structure in RFC 5280 and recognizes common certificate extensions including Basic Constraints, Key Usage, Extended Key Usage, Subject Alternative Name, key identifiers, certificate policies, AIA, CRL distribution points, Name Constraints and TLS Feature.
DNS/IP matching prioritizes Subject Alternative Name. Wildcards are constrained to the entire left-most DNS label. Common Name is shown only as a legacy fallback when no DNS SAN is present.
Supported RSA PKCS#1, ECDSA and Ed25519 certificate signatures can be verified with Web Crypto where the browser supports the algorithm. Unsupported algorithms are reported rather than guessed.
The tree is parsed with strict DER length handling, nesting/node limits, byte offsets, tag names and value previews. This is useful for debugging malformed or unexpected certificate encodings.
A cryptographically valid certificate can still be untrusted, revoked, policy-inappropriate, name-mismatched or expired. This local decoder does not claim to replace a browser or OS path validator.
The certificate never leaves your device. No remote library, analytics endpoint or certificate lookup service is used.