Fraud Proof
Evidence submitted to prove an optimistic rollup transaction was invalid, clawing back incorrect state.
A fraud proof is the enforcement mechanism of optimistic rollups: during the challenge window, a watcher who spots an invalid transaction submits a fraud proof to the base chain, which verifies it and reverts the bad state (penalizing whoever posted it). The system is “optimistic” precisely because it trusts batches unless someone proves fraud.
The security assumption is that at least one honest, capable party is watching and economically motivated to challenge — the “one honest verifier” model. This is why the challenge window exists and why withdrawals are delayed. It contrasts with zk-rollups’ validity proofs, which prove correctness upfront rather than catching fraud after the fact. Whether fraud proofs are actually live and permissionless is a key maturity test for any optimistic rollup.