Bencoding

Bencoding is a simple binary-safe data serialization format created for the BitTorrent protocol. It encodes four data types — byte strings, integers, lists and dictionaries — using a compact, easy-to-parse syntax that is suitable for network transmission and small file metadata.

A byte string is encoded as <length>:<data>, where length is a base-10 ASCII number describing the number of bytes; for example, 4:spam represents the four-byte string "spam". Integers are encoded as i<integer>e using ASCII digits and an optional leading minus sign, for example i42e. Lists are encoded as l<items>e and can contain any bencoded values; e.g., l4:spami42ee represents a list containing "spam" and 42. Dictionaries are encoded as d<pairs>e, with each key encoded as a byte string and followed by its value; keys must be sorted lexicographically by their raw byte values. For example, d3:cow3:moo4:spam4:eggse encodes a dictionary with keys "cow" and "spam".

Bencoding is widely used in .torrent files and in BitTorrent peer exchanges for representing metadata and protocol messages. Its simplicity enables straightforward implementations in many programming languages. Limitations include minimal type richness and potential risks when parsing untrusted input (large allocations, integer overflow); robust parsers validate sizes and enforce resource limits. The format was introduced by Bram Cohen as part of the original BitTorrent specification.