TLS Certificate Decoder & Expiry Checker

How it works

This tool ships its own minimal ASN.1/DER parser rather than calling a certificate library — it base64-decodes the PEM block, walks the resulting bytes as nested ASN.1 tags to find the issuer and subject name fields (matching the OID for Common Name), reads the Subject Alternative Name extension by its own OID, and decodes the notBefore/notAfter fields from their UTCTime or GeneralizedTime encoding into real dates.

FAQ

Does this validate the certificate chain or check if it's trusted?

No — it only decodes the fields printed inside the certificate itself, such as issuer, subject, serial, and expiry; it doesn't verify a signature or check whether the issuing CA is trusted.

Why does it need my certificate's PEM text instead of just a domain name?

Because it parses the ASN.1/DER structure locally with a small hand-written parser, byte by byte, in your browser — it never connects to the domain or a certificate authority to fetch anything.