An almost-optimally fair three-party coin-flipping protocol

Iftach Haitner, Eliad Tsfadia

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review


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 [STOC 1986] has shown that in the case of dishonest majority (i.e., at least half of the parties can be corrupted), in any m-round coinflipping protocol, the corrupted parties can bias the honest parties' common output bit by Ω(1/m). For more than two decades, the best known coin-flipping protocols against dishonest majority had bias ⊖(√ ℓ m), where ℓ is the number of corrupted parties. This was changed by a recent breakthrough result of Moran et al. [TCC 2009], who constructed an m-round, two-party coin-flipping protocol with optimal bias θ(1/m). In a subsequent work, Beimel et al. [Crypto 2010] extended this result to the multiparty case in which less than 2/3 of the parties can be corrupted. Still for the case of 2 3 (or more) corrupted parties, the best known protocol had bias θ(√ ℓ m). In particular, this was the state of affairs for the natural three-party case. We make a step towards eliminating the above gap, presenting an m-round, three-party coin-flipping protocol, with bias O(log2 m) m . Our approach (which we also apply for the two-party case) does not follow the "threshold round" paradigm used in the work of Moran et al. and Beimel et al., but rather is a variation of the majority protocol of Cleve, used to obtain the aforementioned θ(√ℓ m)-bias protocol.

Original languageEnglish
Title of host publicationSTOC 2014 - Proceedings of the 2014 ACM Symposium on Theory of Computing
PublisherAssociation for Computing Machinery
Number of pages9
ISBN (Print)9781450327107
StatePublished - 2014
Event4th Annual ACM Symposium on Theory of Computing, STOC 2014 - New York, NY, United States
Duration: 31 May 20143 Jun 2014

Publication series

NameProceedings of the Annual ACM Symposium on Theory of Computing
ISSN (Print)0737-8017


Conference4th Annual ACM Symposium on Theory of Computing, STOC 2014
Country/TerritoryUnited States
CityNew York, NY


  • Coin-flipping protocols
  • Fair computation
  • Fairness

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