BitTorrent file signature
Magic number at the start of the file · d8:announce
A .torrent file is bencoded text that begins with a dictionary, so it typically starts d8:announce (a dictionary whose first key is the 8-character word announce). Torrents without a tracker may start d4:info instead.
What this magic number means
A magic number is a short, fixed run of bytes at a known position that tells a program what a file really is, no matter what the filename says. If a file starts with the bytes 64 38 3A 61 6E 6E 6F 75 6E 63 65 (the text d8:announce), it is a BitTorrent file.
What each byte of 64 38 3A 61 means
Here is the signature byte by byte, the way a hex editor shows it: the position in the file, the value in hex and in decimal, and the character that value stands for in ASCII. Bytes with no printable character show a dot.
| Byte offset | Hex | Decimal | ASCII |
|---|---|---|---|
| 0 | 64 | 100 | d |
| 1 | 38 | 56 | 8 |
| 2 | 3A | 58 | : |
| 3 | 61 | 97 | a |
| 4 | 6E | 110 | n |
| 5 | 6E | 110 | n |
| 6 | 6F | 111 | o |
| 7 | 75 | 117 | u |
| 8 | 6E | 110 | n |
| 9 | 63 | 99 | c |
| 10 | 65 | 101 | e |
Formats that use the 64 38 3A 61 signature
These file types in our database carry this signature:
How to check a file's signature
You can read the bytes of any file yourself. They are shown in hex, the same way this page lists them.
Linux & macOS
- xxd -l 16 example.torrent
- hexdump -C -n 16 example.torrent
- file example.torrent
Windows (PowerShell)
- Format-Hex -Path example.torrent -Count 16
Python
- open("example.torrent","rb").read(11).hex()
In your browser
- Drop the file into the WhatFileType identifier, which reads the signature without uploading it.
Frequently asked questions
What is the BitTorrent file signature?
BitTorrent files start with the hex bytes 64 38 3A 61 6E 6E 6F 75 6E 63 65 (d8:announce in ASCII). This magic number identifies the format regardless of the file's name or extension.
How do I check a file's magic number?
Open the file in a hex editor, or run a command such as xxd -l 16 example.torrent on Linux or macOS, or Format-Hex -Path example.torrent -Count 16 in Windows PowerShell, and read the first bytes.
Can a file fake the BitTorrent signature?
Renaming a file does not change its bytes, so the extension can lie but the signature usually cannot. A genuine BitTorrent file has these exact bytes; a file with the wrong bytes is not really BitTorrent, whatever its name says.
Related signatures
Sources
Reuse this signature
This entry is part of the WhatFileType file signature reference, published under a CC BY 4.0 licence with the full provenance for every entry, a copy-paste embed and a JSON export. Credit WhatFileType and the data is yours to republish.