Electronic commerce is a main trend in Internet applications, which should be based on fair exchange. A new fair exchange protocol with off-line semi-trusted third party is proposed. It is also efficient in the sense that it only needs 4 messages in a transaction. In the meantime all messages are encrypted naturally, which makes it appealing over large open networks that have little or no physical security, such as Internet. It is based on publicly verifiable secret sharing (PVSS) theory.%电子商务是Internet应用的发展趋势，它的基础之一是公平的交易协议.提出一种新的解决方案，它所依赖的第三方不必完全可信，且只需离线工作.协议还是高效的，表现为在一般情况下，一次交易只需交互4条信息.同时，交互的信息都自然地被加密，因而特别适合Internet这种几乎没有物理安全性的公共网络.它的理论基础是公开可验证秘密分享原理.
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