Systematic Development of a Family of Fair Exchange Protocols

Fair exchange protocols play an important role in application areas such as ecommerce where protocol participants require mutual guarantees that a transaction involving exchange of items has taken place in a specific manner. A protocol is fair if no protocol participant can gain any advantage over an honest participant by misbehaving. In addition, such a protocol is fault tolerant if the protocol can ensure that an honest participant does not suffer any loss of fairness despite any failures of the participant’s node. This paper presents a family of fair exchange protocols for two participants under a variety of assumptions concerning participant misbehaviour, node reliability and message delays. While the paper considers, in line with other known protocols, that a dishonest user can control the protocol execution subject to known cryptographic assumptions, it also considers a class of dishonest users whose abilities to abuse are restricted. The restricted-abuse assumption results in a round-optimal (2-round) protocol that eliminates any need for an “after-the-fact” dispute resolution, using traditional symmetric keys and the RSA signatures. The paper also indicates how this assumption can be realised through the use of smart cards. Concerning the node reliability, the development presented here shows how a non-fault tolerant version of a protocol can be made crash-tolerant, thereby highlighting issues that need to be addressed in such transformation. The third dimension concerns the case of the bound on message delays being known or unknown.