TLS Certificate Decoder

TLS certificates bind a public key to an identity (domain, organization). They're used for HTTPS to prove a server's identity. Certificates contain the subject (who it's for), issuer (who signed it), validity dates, and the public key. They're signed by a Certificate Authority (CA) or self-signed.

Certificates have a Not Before and Not After date. Browsers reject expired certificates. Most CAs issue certs valid for 90 days or 1 year. Check expiration regularly and renew before expiry. This tool shows days until expiration and whether the cert is already expired.

DER is the binary encoding of an X.509 certificate. PEM is base64-encoded DER with header/footer lines. PEM is common in configs and tools; DER is used in some protocols. This tool accepts PEM and decodes it to extract key fields.