What this tool reads
Paste a certificate in PEM format — the text block starting with -----BEGIN CERTIFICATE----- that you get when exporting a certificate from a browser, server, or certificate authority — and this tool decodes it into readable fields: who it was issued to, who issued it, when it is valid, and which domains it covers. Nothing is guessed; every field comes directly from the certificate's own structured data.
Worked example
Decoding a real, publicly published root certificate (DigiCert's Global Root CA) gives:
- Subject: C=US, O=DigiCert Inc, OU=www.digicert.com, CN=DigiCert Global Root CA
- Valid from 11/10/2006 to 11/10/2031
- Serial number: 083be056904246b1a1756ac95991c74a
- Public key: RSA 2048-bit
- Signature algorithm: SHA1withRSA
Certificate chains
Servers often present more than one certificate: the site's own certificate (the leaf) plus one or more intermediate certificates that link it back to a trusted root. Paste all of them concatenated together and this tool decodes each one, clearly labeling the leaf certificate separately from the intermediates, in the order you pasted them.
What this cannot tell you
Decoding a certificate only reads what is written inside it — it does not check whether the certificate is currently trusted by browsers, whether it has been revoked, or whether it actually matches the domain you think it belongs to. Those checks require contacting the certificate authority or the live server, which this tool does not do.
Privacy
A certificate is public information by design: whenever your browser connects to an HTTPS site, the site hands over this exact certificate to anyone who asks. It never contains a private key, so pasting one here does not expose anything sensitive.