Optimistic Rollups: Fraud Proofs & Withdrawal Windows

What you'll learn

• The Optimistic Assumption
• Arbitrum BoLD: Permissionless Fraud Proofs
• Optimism Cannon: The MIPS Fault-Proof VM
• The 7-Day Window: Why That Number?
• The Superchain and the Base Phenomenon

Key terms

Optimistic rollup

A rollup that accepts proposed state roots provisionally and allows challenges within a fixed window (typically 7 days). Security depends on at least one honest verifier being online during the window.

Fraud proof

A cryptographic proof that an L2 state transition was computed incorrectly. In Arbitrum BoLD and Optimism Cannon, fraud proofs are verified onchain through interactive bisection plus a final single-instruction execution.

Interactive bisection

The dispute protocol where two parties recursively narrow down to a single execution step where they disagree, allowing the L1 contract to verify only that one step rather than the entire batch.

7-day challenge window

The standard fraud-proof window across major optimistic rollups. Calibrated to survive sustained L1 censorship while keeping withdrawal UX bearable. Has become a Schelling point even though shorter windows might be feasible.

Superchain

The federation of OP Stack rollups (Optimism, Base, Mode, Zora, Frax, Worldchain, Lisk, Soneium, Ink, and many more) sharing a fault-proof system, sequencer protocol, and cross-chain messaging.

Base

Coinbase's OP Stack L2, launched July 13, 2023. The largest L2 by monthly active users since shortly after launch, driven by Coinbase's onboarding funnel and consumer positioning.

L2OutputOracle (legacy)

Optimism's pre-Cannon bridge contract that simply trusted Output Proposers to submit correct state roots, with no onchain fraud proof. A Stage 0 design now superseded by the DisputeGameFactory + Cannon system.

DisputeGameFactory

The Optimism L1 contract that instantiates fault-proof disputes. Pairs with Cannon to provide permissionless fraud proofs on the OP Stack.