.OTF file signature
A OpenType font file begins with the bytes 4F 54 54 4F (OTTO in ASCII) at the start of the file. The signature is the same whatever the file is named, which is why a renamed or extensionless OTF file is still recognisable.
Signature
| Offset | Hex | ASCII |
|---|---|---|
| 0 | 4F 54 54 4FCFF outlines. An .otf with TrueType outlines starts 00 01 00 00 instead. | OTTO |
MIME type: font/otf · usual extensions: .otf
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.otf
file file.otfWindows (PowerShell):
Format-Hex -Path file.otf -Count 32If the bytes do not match
A file named .otf whose first bytes are something else is not a OTF 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 OTF guide explains what a genuine file should contain.