Asynchronous Optimistic Fair Exchange Based on Revocable Items

Abstract

We study the benefits of revocable items (like electronic payments which can be undone by the bank) for the design of efficient fair exchange protocols. We exploit revocability to construct a new optimistic fair exchange protocol that even works with asynchronous communication channels. All previous protocols with comparable properties follow the idea of Asokan’s exchange protocol for two generatable items [Aso98, ASW98]. But compared to that, our protocol is more efficient: We need less messages in the faultless case and our conflict resolution is less complicated. Furthermore, we show that the generatability, which is required by [Aso98, ASW98], is difficult to implement in the context of some electronic payments. Instead, revocability of payments may be much easier to realize. Thus, our new protocol is very well suited for the fair exchange of revocable payments for digital goods.