Bencode Explained

Bencode (pronounced like Bee-encode) is the encoding used by the peer-to-peer file sharing system BitTorrent for storing and transmitting loosely structured data.^[1]

It supports four different types of values:

• byte strings,
• integers,
• lists, and
• dictionaries (associative arrays).

Bencoding is most commonly used in torrent files, and as such is part of the BitTorrent specification. These metadata files are simply bencoded dictionaries.

Bencoding is simple and (because numbers are encoded as text 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.

Encoding algorithm

Bencode uses ASCII characters as delimiters and digits.

• An integer is encoded as iASCII>e. Leading zeros are not allowed (although the number zero is still represented as "0"). Negative values are encoded by prefixing the number with a hyphen-minus. The number 42 would thus be encoded as, 0 as ; and -42 as . Negative zero is not permitted.
• A byte string (a sequence of bytes, not necessarily characters) is encoded as :. 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 . 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. This is identical to how netstrings work, except that netstrings additionally append a comma suffix after the byte sequence.
• A list of values is encoded as le . 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: . Note the absence of separators between elements, and the first character is the letter 'l', not digit '1'.
• A dictionary is encoded as de. The elements of the dictionary are encoded with each key immediately followed by its value. 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 (in other words,

Notes and References

1. https://www.bittorrent.org/beps/bep_0003.html The BitTorrent Protocol Specification