X.509 Certificate Decoder — Free, No Upload, Browser-Based

Decode any PEM X.509 certificate instantly in your browser — no upload, no server, no account. Paste a certificate and see the subject, issuer, serial number, validity window, Subject Alternative Names, key usage flags, and all extensions decoded from raw DER. Because everything runs locally via a hand-written ASN.1 parser, your certificates never leave your device — ideal for inspecting internal CA certificates, client certs, or anything you would not want to paste into an online tool.

如何使用此工具

1. Open the tool and paste a full PEM certificate — including the -----BEGIN CERTIFICATE----- and -----END CERTIFICATE----- header and footer lines.
2. The decoder parses the DER structure immediately and displays subject, issuer, validity dates, Subject Alternative Names, and any extensions without sending data anywhere.
3. Review the decoded fields. Expiry status is highlighted (valid / expiring soon / expired), and SAN names can be copied in one click.