What is a Fraud Proof?
A fraud proof is cryptographic evidence demonstrating that a state transition claimed by an optimistic rollup sequencer was invalid. Optimistic rollups assume state updates are correct unless challenged. Fraud proofs are the mechanism for detecting and reverting incorrect claims, punishing malicious proposers, and maintaining security. They provide economic security without requiring expensive validity proofs for every batch.
How Fraud Proofs Work
When a sequencer posts a state root to Ethereum, a challenge period begins (typically 7 days on major rollups). During this window, anyone monitoring the rollup can submit a fraud proof if they detect an incorrect state transition. The proof demonstrates that executing the disputed transaction according to the rollup's rules results in a different state than what was claimed.
Interactive fraud proofs (used by Arbitrum) narrow down the dispute through a bisection protocol to a single computational step, minimizing on-chain verification costs. The disputing parties take turns identifying which half of execution they disagree on until reaching a single instruction that can be verified on-chain. Non-interactive fraud proofs submit complete re-execution evidence in a single transaction.
If a fraud proof succeeds, the incorrect state is reverted, and the malicious proposer loses their staked bond (often substantial. Hundreds of thousands of dollars). Honest challengers receive rewards from this bond. The system incentivizes correct operation while severely punishing dishonest claims.
Challenge Period Implications
The 7-day challenge period has significant user experience implications. Withdrawals from optimistic rollups to Ethereum mainnet require waiting this full period to ensure no fraud proofs invalidate the exit's claimed state. This creates natural friction for cross-chain operations and capital efficiency constraints.
Fast bridge services offer immediate withdrawals by taking on the challenge period risk themselves, verifying state independently and fronting users their funds for a fee. This bridges user experience gaps while maintaining security.
Security Model
Fraud proof security requires at least one honest party monitoring the rollup and willing to submit challenges (the "1-of-N" assumption). If no one watches or all watchers collude with the sequencer, invalid states could theoretically finalize. Rollups implement various mechanisms to ensure watcher existence: public verification tools, financial incentives for challenges, and multiple independent monitoring services.