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New release: BitTorrent protocol version 2

Monday, September 7th, 2020 by arvid

libtorrent-2.0 has just been released with a few major new features. One of them is support for BitTorrent v2.

Most of the specification work of BEP 52 was done by the8472. The libtorrent support for bittorrent v2 was mostly implemented by Steven Siloti. BiglyBT also has an implementation of BitTorrent v2 to be released in the near future.

What is BitTorrent v2?

BitTorrent v2 kick-started with an effort to transition away from SHA-1 as the hash function for pieces, shortly after google announced having produced a collision. Given a new hash function would not be backwards compatible, a few other changes were proposed as well, while we were taking the compatibility hit anyway. This post describes the new features of the BitTorrent v2 protocol.


The hash function for piece data was changed to SHA-256. One consequence of this is that hashes are 32 bytes instead of 20 bytes. In BitTorrent v2, the info-dictionary is also computed by SHA-256, which poses a compatibility challenge with the DHT and trackers, which have protocols that expect 20 byte hashes. To handle this, DHT- and tracker announces and lookups for v2 torrents use the SHA-256 info-hash truncated to 20 bytes.

This was one of the original rationales for creating a v2 protocol to begin with. It means that fundamentally a v2 torrent will be identified by a different hash than a v1 torrent, which would always create a separate swarm, even when sharing the same files. More on this later, under backwards compatibility.

hash trees

In BitTorrent v1, pieces are hashed and the resulting hashes are included in the .torrent file/metadata (in the info-dictionary). In most cases, the piece hashes is the bulk of the size of .torrent files. To keep the .torrent file size within reason for large files, the piece size can be increased, meaning each hash represents a larger port...

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