Two-round Multi-Party Contract Signing

A multi-party contract signing (MPCS) protocol allows a set of parties to exchange messages with each other to reach a state in which either each of them has a pre-agreed contract text signed by all the others, or none of them does. A good MPCS protocol should satisfy the properties of fairness, timeliness and abuse-freeness, and at the same time, be concise and efficient. Typical MPCS protocols designed for asynchronous networks need O(n²) rounds running, such as the protocol proposed by Garay and Mackenzie (GM). Mukhamedov and Ryan (MR) improved the GM protocol and reduced the round to ¿n/2¿ +1. This paper reviewed some existing MPCS protocols, and proposed an optimistic two-round MPCS protocol, with the number of rounds reduced and fixed to two and the message transmission number to O(n²). Our protocol, like the GM protocol and the MR protocol, provide abuse-freeness based on private contract signatures. Formal definitions of fairness and abuse-freeness are presented in the strand space model, and formal analysis shows all these properties are satisfied in this two-round protocol.