Efficiency of Optimistic Fair Exchange Using Trusted Devices

Efficiency of asynchronous optimistic fair exchange using trusted devices is studied. It is shown that three messages in the optimistic subprotocol are sufficient and necessary for exchanging idempotent items. When exchanging nonidempotent items, however, three messages in the optimistic subprotocol are sufficient only under the assumption that trusted devices have unbounded storage capacity. This assumption is often not satisfiable in practice. It is then proved that exchanging nonidempotent items using trusted devices with a bounded storage capacity requires exactly four messages in the optimistic subprotocol.