Optimistic Rollup Fraud Proof: Architecture and Challenge

Frequently Asked Questions

01.What is an optimistic rollup fraud proof?

A fraud proof is an on-chain mechanism that lets a verifier challenge an invalid state transition posted by a rollup sequencer. The rollup assumes posted state roots are correct unless a verifier opens a dispute during the challenge window. The disputed computation is bisected over multiple rounds until a single instruction can be re-executed on L1, which determines who was honest. Arbitrum Nitro proves disputed execution against a Geth client compiled to WASM, and Optimism Bedrock uses the Cannon interactive fault prover for the same purpose.

02.Why is the optimistic rollup challenge period seven days?

Seven days is the design margin Optimism and early Arbitrum chose to absorb worst-case censorship attacks on the underlying Ethereum L1. The window must outlast any plausible reorganisation, network partition, or denial-of-service campaign that could block an honest verifier from posting a challenge transaction. Shorter windows weaken security; longer windows worsen user withdrawal latency. Engineering teams treat seven days as the floor for production rollups in 2024.

03.How does Arbitrum Nitro differ from Optimism Bedrock at the proving layer?

Arbitrum Nitro compiles its execution code twice: once to native code for fast L2 execution, once to WAVM, a restricted WebAssembly dialect, for use in disputes. Disputes bisect WAVM execution until a single instruction can be re-run on L1. Optimism Bedrock pairs op-geth at the execution layer with the Cannon interactive fault prover, which uses a MIPS-based virtual machine for the disputed step. Both follow the same multi-round interactive bisection model with different proving substrates.