Analysis and Improvement of Micali's Fair Contract Signing Protocol

In PODC 2003, Micali presented a fair electronic exchange protocol for contract signing with an invisible trusted party [17]. The protocol was filed as a US patent No 5666420 in 1997 [16]. In the protocol, two mutually distrusted parties exchange their commitments to a contract in a fair way such that either each of them can obtain the other’s commitment, or neither of them does. The protocol is optimistic in the sense that the trusted party need not be involved in the protocol unless a dispute occurs. In this paper, we show that Micali’s protocol cannot achieve the claimed fairness. In resolving a dispute, the trusted party may face a dilemma situation that no matter what it does, one of the exchanging parties can succeed in cheating. In other words, there is always a party who can get the other’s commitment without the other party obtaining his. We further propose a revised version of contract signing protocol that preserves fairness while remaining optimistic.