Blocky saysOptimistic rollups assume batches are valid and allow a challenge window to dispute fraud.
Optimistic rollups (Arbitrum, Optimism) post transaction batches to L1 and assume they're valid — anyone can submit a fraud proof during a challenge window (typically ~7 days) to revert a bad batch. This is cheap and EVM-compatible, but withdrawals to L1 wait out the challenge period (mitigated by liquidity providers). "Innocent until proven fraudulent."
Power-ups you unlock
Assume validity; fraud proofs can dispute
~7-day challenge window
Cheap and EVM-compatible
Withdrawals wait out the window
Double-Spend Dan attacks — common mistakes
Forgetting the withdrawal delay
Assuming instant L1 finality from the L2
Confusing fraud proofs with validity proofs
Boss battleExplain the challenge window and why withdrawals are delayed.
Example code
<!doctype html><html><head><meta charset="utf-8"></head>
<body style="background:#06040d;color:#e6e0ff;font-family:monospace;padding:20px"><pre>post batch → assume valid
~7 days: anyone can submit a fraud proof
→ withdrawal waits out the window</pre></body></html>