An Optimally Fair Coin Toss

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An Optimally Fair Coin Toss
We address one of the foundational problems in cryptography: the bias of coin-flipping protocols. Coin-flipping protocols allow mutually distrustful parties to generate a common unbiased random bit, guaranteeing that even if one of the parties is malicious, it cannot significantly bias the output of the honest party. A classical result by Cleve [STOC '86] showed that for any twoparty r-round coin-flipping protocol there exists an efficient adversary that can bias the output of the honest party by (1/r). However, the best previously known protocol only guarantees O(1/ r) bias, and the question of whether Cleve's bound is tight has remained open for more than twenty years. In this paper we establish the optimal trade-off between the round complexity and the bias of two-party coin-flipping protocols. Under standard assumptions, we show that Cleve's lower bound is tight: we construct an r-round protocol with bias O(1/r). Department of Computer Science and Applied Mathemat...
Tal Moran, Moni Naor, Gil Segev
Added 25 Nov 2009
Updated 25 Nov 2009
Type Conference
Year 2009
Where TCC
Authors Tal Moran, Moni Naor, Gil Segev
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