Cryptology ePrint Archive: Report 2007/182

Optimistic Fair Exchange in a Multi-user Setting

Yevgeniy Dodis and Pil Joong Lee and Dae Hyun Yum

Abstract: This paper addresses the security of optimistic fair exchange in a multi-user setting. While the security of public key encryption and public key signature schemes in a single-user setting guarantees the security in a multi-user setting, we show that the situation is different in the optimistic fair exchange. First, we show how to break, in the multi-user setting, an optimistic fair exchange scheme provably secure in the single-user setting. This example separates the security of optimistic fair exchange between the single-user setting and the multi-user setting. We then define the formal security model of optimistic fair exchange in the multi-user setting, which is the first complete security model of optimistic fair exchange in the multi-user setting. We prove the existence of a generic construction meeting our multi-user security based on one-way functions in the random oracle model and trapdoor one-way permutations in the standard model. Finally, we revisit two well-known methodologies of optimistic fair exchange, which are based on the verifiably encrypted signature and the sequential two-party multisignature, respectively. Our result shows that these paradigms remain valid in the multi-user setting.

Category / Keywords: cryptographic protocols / fair exchange

Publication Info: A preliminary version appeared in PKC 2007.

Date: received 15 May 2007

Contact author: dhyum at postech ac kr

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Version: 20070520:132411 (All versions of this report)

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