Bencoding

Bencoding is the wire protocol the peer-to-peer file sharing system BitTorrent uses to exchange control information among nodes.

Bencoded messages are nested dictionaries and lists (as in Python), which can contain strings and integers. Extensibility is supported by ignoring unexpected dictionary keys, so additional optional ones can be added later.

Bencoding is done as follows:

Strings are length-prefixed base ten followed by a colon and the string. For example 4:spam corresponds to 'spam'. Integers are represented by an 'i' followed by the number in base 10 followed by an 'e'. For example i3e corresponds to 3 and i-3e corresponds to -3. Integers have no size limitation. i-0e is invalid. All encodings with a leading zero, such as i03e, are invalid, other than i0e, which of course corresponds to 0. Lists are encoded as an 'l' followed by their elements (also bencoded) followed by an 'e'. For example l4:spam4:eggse corresponds to ['spam', 'eggs']. Dictionaries are encoded as a 'd' followed by a list of alternating keys and their corresponding values followed by an 'e'. For example, d3:cow3:moo4:spam4:eggse corresponds to {'cow': 'moo', 'spam': 'eggs'} and d4:spaml1:a1:bee corresponds to {'spam': ['a', 'b']} . Keys must be strings and appear in sorted order (sorted as raw strings, not alphanumerics).