Bencode

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Bencode (pronounced "Bee-Encode") is the encoding used by the peer-to-peer file sharing system BitTorrent for storing and transmitting loosely structured data.

It supports four different types of values:

Bencoding is most commonly used in .torrent files. These metadata files are simply bencoded dictionaries.

While less efficient than a pure binary encoding, bencoding is simple and (because numbers are encoded in decimal notation) is unaffected by endianness, which is important for a cross platform application like BitTorrent. It is also fairly flexible, as long as applications ignore unexpected dictionary keys, so that new ones can be added without creating incompatibilities.

[edit] Encoding algorithm

Bencode uses ASCII characters as delimiters and digits.

An integer is encoded as i<number in base 10 notation>e. Note that negative values are allowed by prefixing the number with a minus sign, but leading zeros are not allowed (although the number zero is still represented as "0"). The number 42 would thus be encoded as "i42e".
A byte string (a sequence of bytes, not necessarily characters) is encoded as <length>:<contents>. (This is similar to netstrings, but without the final comma.) The length is encoded in base 10, like integers, but must be non-negative (zero is allowed); the contents are just the bytes that make up the string. The string "spam" would be encoded as "4:spam". The specification does not deal with encoding of characters outside the ASCII set; to mitigate this, some BitTorrent applications explicitly communicate the encoding (most commonly UTF-8) in various non-standard ways.
A list of values is encoded as l<contents>e . The contents consist of the bencoded elements of the list, in order, concatenated. A list consisting of the string "spam" and the number 42 would be encoded as: "l4:spami42ee"; note the absence of separators between elements.
A dictionary is encoded as d<contents>e. The elements of the dictionary are again encoded and concatenated, in such a way that each value immediately succeeds the key associated with it. All keys must be byte strings and must appear in lexicographical order. A dictionary that associates the values 42 and "spam" with the keys "foo" and "bar", respectively, would be encoded as follows: "d3:bar4:spam3:fooi42ee". (This might be easier to read by inserting some spaces: "d 3:bar 4:spam 3:foo i42e e".)

There are no restrictions on what kind of values may be stored in lists and dictionaries; they may (and usually do) contain other lists and dictionaries. This allows for arbitrarily complex data structures to be encoded; it's one of the advantages of using bencoding.

[edit] Trivia

For each possible (complex) value, there is only a single valid bencoding; ie. there is a bijection between values and their encodings. This has the advantage that applications may compare bencoded values by comparing their encoded forms, eliminating the need to decode the values.
Many encodings can be decoded manually, but since the bencoded values often contain binary data, and may become quite complex, it is generally not considered a human-readable encoding.
Bencoding serves similar purposes as markup languages like XML and JSON, allowing complex yet loosely structured data to be stored in a platform independent way.