An Almost-Optimally Fair Three-Party Coin-Flipping Protocol

Abstract

In a multiparty fair coin-flipping protocol, the parties output a common (close to) unbiased bit, even when some corrupted parties try to bias the output. Cleve [in Proceedings of the 20th Annual ACM Symposium on Theory of Computing (STOC), ACM, New York, 1986, pp. 364--369] has shown that in the case of dishonest majority (i.e., at least half of the parties can be corrupted), in any $m$-round coin-flipping protocol the corrupted parties can bias the honest parties' common output bit by $\Omega(\frac 1{m})$. For more than two decades the best known coin-flipping protocols against dishonest majority had bias $\Theta(\frac {\ell}{\sqrt{m}})$, where $\ell$ is the number of corrupted parties. This was changed by a recent breakthrough result of Moran, Naor, and Segev [in Theory of Cryptography, Lecture Notes in Comput. Sci. 5444, Springer, Berlin, 2009, pp. 1--18], who constructed an $m$-round, two-party coin-flipping protocol with optimal bias $\Theta(\frac 1 m)$. In a subsequent work, Beimel, Omri, and Orlov [in Proceedings of the 22nd Annual ACM Symposium on Theory of Computing (STOC), ACM, New York, 1990, pp. 503--513] extended this result to the multiparty case in which less than $\frac23$ of the parties can be corrupted. Still, for the case of $\frac23$ (or more) corrupted parties, the best known protocol had bias $\Theta(\frac {\ell}{\sqrt{m}})$. In particular, this was the state of affairs for the natural three-party case. We take a step toward eliminating the above gap, presenting an $m$-round, three-party coin-flipping protocol, with bias $\frac{O(\log^3 m)}m$. Our approach (which we also apply to the two-party case) does not follow the “threshold round" paradigm used in the work of Moran, Naor, and Segev and Beimel, Omri, and Orlov but rather is a variation of the majority protocol of Cleve used to obtain the aforementioned $\Theta(\frac {\ell}{\sqrt{m}})$-bias protocol.