Abstract

Fair exchange protocols play an important role in application areas such as e-commerce 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 report presents a family of fair exchange protocols for two participants which make use of the presence of a trusted third party, under a variety of assumptions concerning participant misbehaviour, message delays and node reliability. The development is systematic, beginning with the strongest set of the assumptions and gradually weakening the assumptions to the weakest set. The resulting protocol family exposes the impact of a given set of assumptions on solving the problem of fair exchange. Specifically, it highlights the relationships that exist between fairness and assumptions on the nature of participant misbehaviour, communication delays and node crashes. The report also shows that the restrictions assumed on a dishonest participant’s misbehaviour can be realized through the use of smartcards and smartcard-based protocols.

Keywords

Fair Exchange, Security, Trusted Third Party (TTP), Smartcards, Crash tolerance, Distributed Systems.