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

Iftach Haitner, Yonatan Karidi-Heller
2020 arXiv   pre-print
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
more » ... arty majority protocol is resilient to O(√(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 for many-turn protocols was left completely open. In this work we close the above gap, proving that no n-party protocol (of any round complexity) is resilient to ω(√(n)) (adaptive) corruptions.
arXiv:2005.01565v2 fatcat:tx5d4lcvrncvdg5wfsudw2yfwi