Certificate & CSR Decoder
Paste a PEM certificate, a chain, or a signing request and read every field that matters: who it is for, who signed it, when it expires, what key it carries and what it is allowed to do. The parsing happens in this page. Your certificate is not uploaded.
This reads a certificate you already have. The app fetches the live one from any host or IP on the network you are on, shows the whole chain, the cipher and protocol negotiated, and warns before it expires. Get RECON for iPhone — one-time purchase, no account, nothing leaves the device.
What the decoder shows
An X.509 certificate binds a public key to a name and is signed by an issuer. The subject is who it is for, the issuer is who vouches for it, and the validity window is when that promise holds. Since 2017 browsers ignore the Common Name and match the hostname against the Subject Alternative Name list only, so a certificate with the right CN and no SAN entry is rejected by every modern client. The decoder lists SANs first for that reason.
Reading a CSR
A certificate signing request is what you send to a certificate authority. It contains the subject, the public key and the extensions you are asking for, signed with the matching private key to prove you hold it. Decoding one before you submit it catches the mistakes that cost a reissue: a typo in the domain, a missing SAN for the www variant, a 1024-bit key that the CA will refuse, or an organisation name that does not match your registration.
Fields worth checking
Key type and size: RSA below 2048 bits and signatures using SHA-1 or MD5 are flagged in red; no public CA has issued either for years, and their presence means an old internal CA or a device certificate that should be replaced. Basic Constraints says whether the certificate can sign others. Extended Key Usage says what it may be used for, and a server certificate without TLS server authentication will fail in Chrome. The Authority Information Access URLs are where clients fetch the issuer and check revocation. Signed Certificate Timestamps show the certificate was logged for Certificate Transparency, which public CAs must do.
Fingerprints
The SHA-256 fingerprint is a hash of the DER encoding and identifies exactly this certificate, which is what certificate pinning, SSH-style trust prompts and many audit tools compare. SHA-1 is shown because older tooling still prints it, not because it is a safe identifier.
Chains
Paste a full chain and each certificate is decoded in order. The leaf should be first, followed by intermediates, ending before the root: servers should not send the root, and a chain that does is a common misconfiguration that wastes bytes on every handshake. Each intermediate's subject should match the next certificate's issuer.