Two-round Multi-Party Contract Signing

Abstract

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. 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, timeliness and abuse-freeness are presented in the strand space model, and formal analysis shows all these properties are satisfied in this two-round protocol.