Constructing Optimistic Fair Exchange Protocols from Committed Signatures

In PODC 2003, Park et al. [32] first introduce a connection between fair exchange and sequential two-party multi-signature scheme and provide a novel method of constructing fair exchange protocol by distributing the computation of RSA signature. This approach avoids the design of verifiable encryption scheme at the expense of having cosigner store a piece of prime signer’s secret key. Dodis and Reyzin [20] showed that the protocol in [32] is totally breakable in the registration phase, then presented a remedy scheme which is provably secure in the random oracle model, by utilizing Boldyreva non-interactive two-party multi-signature scheme [8]. Security in the random oracle model does not imply security in the real world. In this paper, we provide the first two efficient committed signatures which are provably secure in the standard complexity model from strong RSA assumption. Then we construct efficient optimistic fair exchange protocols from those new primitives.