Blockchain
Fraud proof
An optimistic rollup's mechanism for challenging invalid state transitions — the 'challenge window' that anchors L2 security.
A fraud proof is the mechanism optimistic rollups use to keep their state honest. The rollup posts transaction batches to Ethereum L1 optimistically (assumed valid); anyone can challenge a batch during a challenge window (typically seven days on Optimism-family chains) by submitting a fraud proof that shows the batch's state transition was incorrect. A valid fraud proof forces the rollup to revert.
OP Stack chains (Optimism, Base) shipped working fraud proofs in 2024. Before then, they were "training wheels" optimistic rollups with multisig-controlled upgrade paths; the security model depended on trusted operators. Post-fraud-proofs, the security model is genuinely decentralized: anyone with L1 access can challenge fraudulent state.
For LearnCoin anchoring, this matters because it's what gives Base credibility as a settlement layer. A credential Merkle root committed to Base will stay committed — because changing it would require submitting a fraudulent L1 state claim that any challenger could invalidate.