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An optimistic fair exchange protocol based on committed signature

A verifiable committed signature is constructed based on Fischlin's signature scheme and is proved of its security in the standard complexity model. And then, an optimistic fair exchange protocol with punishment function is designed based on the committed signature. It will punish the misbehavior of either party in order to ensure the fairness of the exchange. Through the analysis, not only are some properties of the exchange, such as fairness, timeliness, abuse-free etc, ensured better, but also because of the introduction of punishment idea, both parties' behavior is confined better, which makes the protocol fairer, simpler and more efficient.