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On the Complexity of Fair Coin Flipping

  • Iftach HaitnerEmail author
  • Nikolaos Makriyannis
  • Eran Omri
Conference paper
Part of the Lecture Notes in Computer Science book series (LNCS, volume 11239)

Abstract

A two-party coin-flipping protocol is \(\varepsilon \)-fair if no efficient adversary can bias the output of the honest party (who always outputs a bit, even if the other party aborts) by more than \(\varepsilon \). Cleve [STOC ’86] showed that r-round o(1 / r)-fair coin-flipping protocols do not exist. Awerbuch et al. [Manuscript ’85] constructed a \(\varTheta (1/\sqrt{r})\)-fair coin-flipping protocol, assuming the existence of one-way functions. Moran et al. [Journal of Cryptology ’16] constructed an r-round coin-flipping protocol that is \(\varTheta (1/r)\)-fair (thus matching the aforementioned lower bound of Cleve [STOC ’86]), assuming the existence of oblivious transfer.

The above gives rise to the intriguing question of whether oblivious transfer, or more generally “public-key primitives”, is required for an \(o(1/\sqrt{r})\)-fair coin flipping. This question was partially answered by Dachman-Soled et al. [TCC ’11] and Dachman-Soled et al. [TCC ’14], who showed that restricted types of fully black-box reductions cannot establish \(o(1/\sqrt{r})\)-fair coin-flipping protocols from one-way functions. In particular, for constant-round coin-flipping protocols, [10] yields that black-box techniques from one-way functions can only guarantee fairness of order \(1/\sqrt{r}\).

We make progress towards answering the above question by showing that, for any constant Open image in new window , the existence of an \(1/(c\cdot \sqrt{r})\)-fair, r-round coin-flipping protocol implies the existence of an infinitely-often key-agreement protocol, where c denotes some universal constant (independent of r). Our reduction is non black-box and makes a novel use of the recent dichotomy for two-party protocols of Haitner et al. [FOCS ’18] to facilitate a two-party variant of the attack of Beimel et al. [FOCS ’18] on multi-party coin-flipping protocols.

Keywords

Coin-flipping Fairness Key-agreement 

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Copyright information

© International Association for Cryptologic Research 2018

Authors and Affiliations

  • Iftach Haitner
    • 1
    Email author
  • Nikolaos Makriyannis
    • 1
  • Eran Omri
    • 2
  1. 1.School of Computer ScienceTel Aviv UniversityTel AvivIsrael
  2. 2.Department of Computer ScienceAriel UniversityArielIsrael

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