Constructing fair-exchange protocols for e-commerce via distributed computation of RSA signatures

Applications such as e-commerce payment protocols, elec-tronic contract signing, and certi¯ed e-mail delivery require that fair exchange be assured. A fair-exchange protocol al-lows two parties to exchange items in a fair way so that either each party gets the other's item, or neither party does. We describe a novel method of constructing very ef-¯cient fair-exchange protocols by distributing the computa-tion of RSA signatures. Speci¯cally, we employ multisig-natures based on the RSA-signature scheme. To date, the vast majority of fair-exchange protocols require the use of zero-knowledge proofs, which is the most computationally intensive part of the exchange protocol. Using the intrinsic features of our multisignature model, we construct protocols that require no zero-knowledge proofs in the exchange proto-col. Use of zero-knowledge proofs is needed only in the pro-tocol setup phase|this is a one-time cost. Furthermore, our scheme uses multisignatures that are compatible with the underlying standard (single-signer) signature scheme, which makes it possible to readily integrate the fair-exchange fea-ture with existing e-commerce systems