Optimality in Asynchronous Contract Signing Protocols

摘要

Garay et alter 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, 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 state that "there is no asynchronous optimistic contract signing scheme with three messages in the optimistic case". However, it seems that Ferrer et alter invalidate the previous theorem with a counterexample, presenting an asynchronous protocol with only three messages. In this paper, we clarify this apparent contradiction.