In the BitTorrent P2Pfile distribution protoco, the seed breaks the file into blocks, and the peers redistribute the blocks to each other. Without any protection, an attacker can easily wreak havoc in a torrent by masquerading as a benevolent peer and sending bogus blocks to a small subset of peers in the torrent.
These unsuspecting peers then redistribute the bogus blocks to other peers, which in turn redistribute the bogus blocks to even more peers.
Thus, it is critical for BitTorrent to have a mechanism that allows a peer to verify the integrity of a block, so that it doesn't redistribute bogus blocks.
Assume that when a peer joins a torrent, it initially gets a .torrent file from a fully trusted source. Describe a simple scheme that allows peers to verify the integrity of blocks.