Metainfo files are bencoded dictionaries with the following keys:

announce
The URL of the tracker.
info

This maps to a dictionary, with keys described below.

The name key maps to a UTF-8 encoded string which is the suggested name to save the file (or directory) as. It is purely advisory.

piece length maps to the number of bytes in each piece the file is split into. For the purposes of transfer, files are split into fixed-size pieces which are all the same length except for possibly the last one which may be truncated. piece length is almost always a power of two, most commonly 2 18 = 256 K (BitTorrent prior to version 3.2 uses 2 20 = 1 M as default).

pieces maps to a string whose length is a multiple of 20. It is to be subdivided into strings of length 20, each of which is the SHA1 hash of the piece at the corresponding index.

There is also a key length or a key files, but not both or neither. If length is present then the download represents a single file, otherwise it represents a set of files which go in a directory structure.

In the single file case, length maps to the length of the file in bytes.

For the purposes of the other keys, the multi-file case is treated as only having a single file by concatenating the files in the order they appear in the files list. The files list is the value files maps to, and is a list of dictionaries containing the following keys:

length - The length of the file, in bytes.

path - A list of UTF-8 encoded strings corresponding to subdirectory names, the last of which is the actual file name (a zero length list is an error case).

In the single file case, the name key is the name of a file, in the muliple file case, it's the name of a directory.

All strings in a .torrent file that contains text must be UTF-8 encoded.