Certificate decoder
Paste a certificate (PEM or base64 DER) or drop a file to read its subject, issuer, SANs, validity, serial, signature algorithm and extensions — with a raw JSON view.
Runs locally in your browser — nothing is uploaded
Decoded certificate
—Subject alternative names
Extensions
ASN.1 as JSON
Long values are truncated with a byte count. This is a structural view, not a re-encoded certificate.
Questions
Which certificate file types can I inspect?
PEM text (.pem, .crt, .cer), base64 pasted from anywhere, and raw DER binary files. Converted to the same in-memory structure and displayed identically.
Does decoding use the certificate’s private key?
There is no private key involved. A certificate is public data; this page only parses it locally. Nothing is uploaded.
Why is my EC certificate rejected?
CertKit v1 parses RSA certificates only; EC and Ed25519 objects produce a clear unsupported message. EC support is the first item on the roadmap.
What is the raw JSON view?
The certificate’s complete ASN.1 structure, rendered as JSON, so you can inspect exact fields, OIDs and bit strings without installing anything.
How this runs locally
CertKit is a static site. Parsing and cryptography run inside this browser tab using a vendored copy of node-forge 1.4.0 served from this domain. Open DevTools → Network, run the tool, and you will see no request that contains your file — nothing is uploaded, and nothing is stored.