Question

How does an Optimistic Rollup determine the validity of a state transition if a sequencer submits an invalid block batch to the main chain?

Accepted Answer

Optimistic Rollups operate on the principle of fraud proofs to ensure state validity. When a sequencer submits a batch of transactions to the main chain, the system assumes by default that all transactions are valid, which is why it is called optimistic. To protect against invalid state transitions, the network enforces a challenge period, which is a set duration of time during which any participant, known as a verifier, can monitor the submission. If a verifier detects that the sequencer submitted an invalid state root, such as one resulting from fraudulent transactions or incorrect computation, the verifier initiates a fraud proof. A fraud proof is a cryptographic challenge that forces the execution of the specific disputed transaction on the main chain. During this process, the main chain smart contract re-executes the transaction using the underlying data submitted by the sequencer. If the re-executed result does not match the state root submitted by the sequencer, the system identifies the batch as invalid. The main chain then reverts the incorrect state, slashes a portion of the sequencer&#x27;s staked collateral as a penalty, and rewards the verifier for identifying the fraud. This mechanism ensures that as long as at least one honest participant monitors the network and submits a fraud proof within the challenge period, the integrity of the state is maintained.

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How does an Optimistic Rollup determine the validity of a state transition if a sequencer submits an invalid block batch to the main chain?

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