We provide an efficient optimistic fair exchange mechanism for bartering digital files, where receiving a payment in return to a file (buying) is also considered fair. The exchange is optimistic, removing the need for the Arbiter's involvement unless a dispute occurs. While the previous solutions employ costly cryptographic primitives for every file or block exchanged, our protocol employs them only once per peer, therefore achieving O(n) efficiency improvement when n blocks are exchanged between two peers. The rest of our protocol uses very efficient cryptography, making it perfectly suitable for a p2p file sharing system where tens of peers exchange thousands of blocks and they do not know beforehand which ones they will end up exchanging. Therefore, our system yields to one-two orders of magnitude improvement in terms of both computation and communication (40 seconds vs. 42 minutes, 1.6MB vs. 200MB). Thus, for the first time, a provably secure (and privacy respecting when payments are made using e-cash) fair exchange protocol is being used in real bartering applications (e.g., BitTorrent) without sacrificing performance.
Category / Keywords: cryptographic protocols / fair exchange, barter, peer-to-peer file sharing, BitTorrent. Publication Info: full version of the CT-RSA 2010 paper with the same title Date: received 6 Oct 2008, last revised 26 Jan 2010 Contact author: kupcu at cs brown edu Available format(s): PDF | BibTeX Citation Note: updated performance numbers Version: 20100126:181700 (All versions of this report) Short URL: ia.cr/2008/431 Discussion forum: Show discussion | Start new discussion