International Association for Cryptologic Research

International Association
for Cryptologic Research


Paper: Usable Optimistic Fair Exchange

Alptekin Küpçü
Anna Lysyanskaya
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Abstract: Fairly exchanging digital content is an everyday problem. It has been shown that fair exchange cannot be done without a trusted third party (called the Arbiter). Yet, even with a trusted party, it is still non-trivial to come up with an efficient solution, especially one that can be used in a p2p file sharing system with a high volume of data exchanged. 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. 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) [14] without sacrificing performance.
  title={Usable Optimistic Fair Exchange},
  booktitle={IACR Eprint archive},
  keywords={cryptographic protocols / fair exchange, barter, electronic commerce and payment, anonymity},
  note={under submission 14298 received 6 Oct 2008, last revised 23 Feb 2009},
  author={Alptekin Küpçü and Anna Lysyanskaya},