.CAT file signature
A Windows Catalog file file begins with the bytes 30 82 at the start of the file. The signature is the same whatever the file is named, which is why a renamed or extensionless CAT file is still recognisable.
Signature
| Offset | Hex | ASCII |
|---|---|---|
| 0 | 30 82DER-encoded PKCS#7; 30 82 is an ASN.1 SEQUENCE with a two-byte length. |
MIME type: application/vnd.ms-pki.seccat · usual extensions: .cat
How to check a file's signature
Without installing anything: drop the file on our detector. It reads the bytes in your browser and shows the hex dump alongside the verdict; nothing is uploaded.
macOS and Linux (Terminal):
xxd -l 32 file.cat
file file.catWindows (PowerShell):
Format-Hex -Path file.cat -Count 32If the bytes do not match
A file named .cat whose first bytes are something else is not a CAT file. The common explanations are a wrong or deliberately misleading extension, a download that saved an HTML error page instead of the file, or a container such as ZIP wrapping the real content. The detector reports what the bytes say; the CAT guide explains what a genuine file should contain.