New Optimistic Fair Exchange Protocol Based on Short Signature

In CIS2008, Gao et al. suggested an abuse-free optimistic fair exchange protocol based on the short signature. Their scheme is very efficient in terms of communication. In this paper, we point out that their scheme has two weaknesses. These weaknesses may lead to an unfair situation that Bob may obtain Alice´s signature, but Alice can not obtain Bob´s signature. In order to overcome these weaknesses, we propose a new secure abuse-free optimistic fair exchange protocol. In the new protocol, both parties can contact the trusted third party and settle the argument before the deadline. Moreover, we give a formal proof of fairness in the strand space model.