What is a .crt file?

About the format

A .crt file is a certificate — functionally identical to a .cer, the extension being a Unix-world habit where .cer is the Windows one. Like .cer files, the encoding varies: PEM (Base64 text between BEGIN/END CERTIFICATE lines, the form web-server documentation usually means) or binary DER. You'll meet .crt files constantly when setting up HTTPS: the certificate your CA issues (yourdomain.crt), the intermediate/CA bundle (ca-bundle.crt), and the combined chain the server should present. The classic mistake this extension is involved in: installing only yourdomain.crt without the intermediate, which works in some browsers (they cache intermediates) and fails everywhere else — curl errors, mobile apps failing, 'unable to verify the first certificate'. The fix is serving the full chain: your certificate followed by the intermediates. A .crt never contains the private key; the pair for a server is .crt + .key, and keeping that .key file secret is what the whole system rests on.

How to open a .crt file

Is a .crt file dangerous?

No — public material. Only trusting/installing one as a root CA carries weight.

Related file types