A fraud proof is evidence used by a blockchain verification system to challenge an incorrect claim about a computation or state transition. Optimistic rollups use such challenges to help reject invalid results submitted to a base chain. The term fault proof is also used: the mechanism establishes an error, not necessarily deliberate fraud by a person.
From a claim to a dispute
A proposer submits a commitment to a rollup’s resulting state. Verifiers can reconstruct execution from available data and dispute an incorrect claim within the permitted period. Some systems use an interactive process that narrows the disagreement to a small computation the base-chain contract can check.
A successful challenge prevents the disputed result from being accepted under the protocol’s rules. Bonds and penalties depend on the implementation. A proof showing that a withdrawal was included in the rollup is a separate object from a fraud proof disputing a state claim.
Security depends on the surrounding system
This differs from a validity proof, which demonstrates correct computation for acceptance rather than relying on an opportunity to challenge an incorrect result. Optimistic designs need available data and capable challengers able to respond in time.
Challenge periods, participation permissions, upgrade powers and emergency controls vary. A protocol advertising fraud proofs does not automatically eliminate censorship, contract bugs or administrator dependencies, nor guarantee immediate withdrawals.