Economic Censorship Games in Fraud Proofs

TL;DR

This paper models economic censorship in optimistic rollup fraud proofs as game-theoretic interactions, analyzing how challenge period length impacts security against bribery-based censorship attacks.

Contribution

It introduces three game-theoretic models of economic censorship in fraud proofs and derives the challenge period length needed for security.

Findings

Longer challenge periods increase resistance to bribery attacks.

Economic censorship can be mitigated by adjusting challenge periods based on attacker's budget.

Game models quantify the trade-off between security and protocol efficiency.

Abstract

Optimistic rollups rely on fraud proofs -- interactive protocols executed on Ethereum to resolve conflicting claims about the rollup's state -- to scale Ethereum securely. To mitigate against potential censorship of protocol moves, fraud proofs grant participants a significant time window, known as the challenge period, to ensure their moves are processed on chain. Major optimistic rollups today set this period at roughly one week, mainly to guard against strong censorship that undermines Ethereum's own crypto-economic security. However, other forms of censorship are possible, and their implication on optimistic rollup security is not well understood. This paper considers economic censorship attacks, where an attacker censors the defender's transactions by bribing block proposers. At each step, the attacker can either censor the defender -- depleting the defender's time allowance at the cost of the bribe -- or allow the current transaction through while conserving funds for future censorship. We analyze three game theoretic models of these dynamics and determine the challenge period length required to ensure the defender's success, as a function of the number of required protocol moves and the players' available budgets.