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Difficulties in designing multi-party contract signing (MPCS) protocols include providing concise and efficient protocols and at the same time keeping the protocols fair and abuse-free. This paper proposed an optimistic MPCS protocol. The number of rounds is reduced to two and the message transmission number to O(n2), which is the best known so far. Formal definitions of fairness and abuse-freeness are presented in the strand space model, and formal analysis shows that fairness and abuse-freeness are satisfied in the protocol.