Fraud Proof

A fraud proof is a challenge demonstrating that a batch of transactions posted by a rollup operator was executed incorrectly. Optimistic rollups assume batches are valid and publish them immediately; during a challenge window, commonly around a week, any watcher can submit a fraud proof, and if it succeeds the bad batch is reverted and the proposer's bond is slashed. Security therefore rests on economics and liveness rather than pure mathematics: at least one honest party must be watching, must have the data needed to reconstruct the state, and must be able to get its challenge on-chain in time. That window is also why withdrawing directly from an optimistic rollup takes days unless you pay a third-party liquidity provider.