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. - BitTorrent v2 / HN

see also

• Build your own BitTorrent

hash trees / per-file hash trees