Verifiable Encryption for P2P Block Exchange

Free-riding is an important problem in Peer-to-Peer (P2P) file-sharing networks. When peers refuse to contribute upload bandwidth, the whole network can collapse. A relatively new free-riding vulnerability in BitTorrent is the Large View Exploit, in which a peer connects to as many other peers as possible to increase the chance to get free data. This exploit can not be thwarted by tit-for-tat-like mechanisms which have traditionally been used to ban free-riding. Several approaches have been proposed to combat the Large View Exploit in fully decentralized systems, most of which rely on encryption. However, the use of regular encryption makes it impossible to verify the correctness of received data. In this paper we propose a novel encryption method which does allow verification of the plaintext data without decryption, at the expense of encryption strength. We show that a colluding peer still has to send data that is at least 40% of the size of the original data to allow decryption.