Lower Bounds Implementing Mediators in Asynchronous Systems

TL;DR

This paper establishes tight bounds on the number of players needed to implement mediators in asynchronous systems, showing that below a certain threshold, such implementation is impossible, and relates this to secure computation limitations.

Contribution

It proves the tightness of the known bound for implementing mediators without a trusted third party in asynchronous systems and introduces a reduction from a weaker secure computation notion.

Findings

Bound n > 4(k+t) is necessary for implementation

Below this bound, certain robust equilibria cannot be implemented without a mediator

Provides an alternative proof for the lower bound in asynchronous secure computation

Abstract

Abraham, Dolev, Geffner, and Halpern proved that, in asynchronous systems, a $(k,t)$-robust equilibrium for $n$ players and a trusted mediator can be implemented without the mediator as long as $n > 4(k+t)$, where an equilibrium is $(k,t)$-robust if, roughly speaking, no coalition of $t$ players can decrease the payoff of any of the other players, and no coalition of $k$ players can increase their payoff by deviating. We prove that this bound is tight, in the sense that if $n \le 4(k+t)$ there exist $(k,t)$-robust equilibria with a mediator that cannot be implemented by the players alone. Even though implementing $(k,t)$-robust mediators seems closely related to implementing asynchronous multiparty $(k+t)$-secure computation \cite{BCG93}, to the best of our knowledge there is no known straightforward reduction from one problem to another. Nevertheless, we show that there is a non-trivial reduction from a slightly weaker notion of $(k+t)$-secure computation, which we call $(k+t)$-strict secure computation, to implementing $(k,t)$-robust mediators. We prove the desired lower bound by showing that there are functions on $n$ variables that cannot be $(k+t)$-strictly securely computed if $n \le 4(k+t)$. This also provides a simple alternative proof for the well-known lower bound of $4t+1$ on asynchronous secure computation in the presence of up to $t$ malicious agents.