What the certificate decoder is for
The certificate decoder reads an SSL/TLS certificate or a certificate signing request (CSR) in PEM form, the text that starts with -----BEGIN CERTIFICATE-----, and shows what is inside: who it was issued to and by whom, when it is valid, which domain names it covers, the key type and size, the signature algorithm, key usage and its SHA-256 and SHA-1 fingerprints. Paste a whole chain to read every certificate in it. Use it to check a CSR before you send it to a certificate authority, or to see which certificate a server file really holds. Everything is decoded in your browser.
Frequently asked questions
Is it safe to paste my certificate here?
Yes. Certificates and CSRs are public by design, and this page decodes them in your browser without sending them anywhere. Never paste a private key into any website.
How do I check that a CSR has the right domain names?
Paste the CSR and look at the Names (SAN) line. Every domain the certificate should cover, including www, needs to be there; certificate authorities ignore names that are only in the common name.
Why does my certificate show as self-signed?
Its issuer and subject are the same, so no certificate authority vouches for it. Browsers warn about self-signed certificates on public websites; they are fine for testing and internal systems.