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arXiv · 2005.01565

A Tight Lower Bound on Adaptively Secure Full-Information Coin Flip

Iftach Haitner·Yonatan Karidi-Heller
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Abstract

In a distributed coin-flipping protocol, Blum [ACM Transactions on Computer Systems '83], the parties try to output a common (close to) uniform bit, even when some adversarially chosen parties try to bias the common output. In an adaptively secure full-information coin flip, Ben-Or and Linial [FOCS '85], the parties communicate over a broadcast channel, and a computationally unbounded adversary can choose which parties to corrupt along the protocol execution. Ben-Or and Linial proved that the $n$-party majority protocol is resilient to $O(\sqrt{n})$ corruptions (ignoring poly-logarithmic factors), and conjectured this is a tight upper bound for any $n$-party protocol (of any round complexity). Their conjecture was proved to be correct for single-turn (each party sends a single message) single-bit (a message is one bit) protocols Lichtenstein, Linial and Saks [Combinatorica '89], symmetric protocols Goldwasser, Tauman Kalai and Park [ICALP '15], and recently for (arbitrary message length) single-turn protocols Tauman Kalai, Komargodski and Raz [DISC '18]. Yet, the question of many-turn protocols was left entirely open. In this work, we close the above gap, proving that no $n$-party protocol (of any round complexity) is resilient to $ω(\sqrt{n})$ (adaptive) corruptions.

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Iftach Haitner, Yonatan Karidi-Heller. 2024-10-27. A Tight Lower Bound on Adaptively Secure Full-Information Coin Flip. https://arxiv.org/abs/2005.01565

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