Optimality in asynchronous contract signing protocols

Abstract

Garay et alter [9] prove that for the multi-party contract signing with n participants, at least n rounds are necessary. To date, the best solution is Baum-Waidner's scheme [6], with t+2 rounds (where t is the number of dishonest signatories, so with t=n-1, the number of rounds is n+1). Here, we propose an optimal solution with exactly n rounds. On the other hand, Pfitzmann et alter [10] state that "there is no asynchronous optimistic contract signing scheme with three messages in the optimistic case". However, it seems that Ferrer et alter [7] invalidate the previous theorem with a counterexample, presenting an asynchronous protocol with only three messages. In this paper, we clarify this apparent contradiction. © Springer-Verlag 2004.