Fraud Proof System

    function validateProof(FraudProof calldata proof) internal view {
        // Check batch exists
        require(batches[proof.batchIndex].exists, "Batch not found");
        
        // Check TX index in range
        require(proof.txIndex < batches[proof.batchIndex].txCount, "Invalid TX index");
        
        // Check challenge period active
        require(
            block.timestamp < batches[proof.batchIndex].timestamp + CHALLENGE_PERIOD,
            "Challenge period expired"
        );
        
        // Check merkle proofs valid
        require(verifyMerkleProofs(proof), "Invalid merkle proofs");
    }

    function executeTransactionOnChain(
        bytes32 preStateRoot,
        Transaction calldata tx,
        bytes32[] calldata merkleProofs
    ) internal returns (bytes32 postStateRoot) {
        // Load account states from merkle proofs
        Account memory sender = loadAccount(tx.from, preStateRoot, merkleProofs);
        Account memory recipient = loadAccount(tx.to, preStateRoot, merkleProofs);
        
        // Validate transaction
        require(sender.nonce == tx.nonce, "Invalid nonce");
        require(sender.balance >= tx.value, "Insufficient balance");
        require(ecrecover(tx.hash, tx.signature) == tx.from, "Invalid signature");
        
        // Execute state changes
        sender.balance -= tx.value;
        sender.nonce += 1;
        recipient.balance += tx.value;
        
        // If contract call, execute code
        if (recipient.code.length > 0) {
            bytes memory result = executeEVMCode(recipient.code, tx.data);
            // Apply storage changes
        }
        
        // Compute new state root
        postStateRoot = computeNewRoot(sender, recipient, merkleProofs);
    }

    function verifyFraud(FraudProof calldata proof) external {
        // Execute transaction
        bytes32 computedPostState = executeTransactionOnChain(
            proof.preStateRoot,
            proof.transaction,
            proof.merkleProofs
        );
        
        // Compare with claimed state
        if (computedPostState != proof.claimedPostState) {
            // FRAUD PROVEN!
            
            // Revert batch
            delete batches[proof.batchIndex];
            
            // Slash sequencer
            uint256 slashAmount = sequencerStake / 2;
            sequencerStake -= slashAmount;
            
            // Reward challenger
            payable(msg.sender).transfer(slashAmount);
            
            emit FraudProven(proof.batchIndex, msg.sender, slashAmount);
        } else {
            // Fraud proof invalid
            revert("Fraud proof invalid");
        }
    }

    Round 1: Challenger claims batch invalid
    Round 2: Sequencer provides execution trace
    Round 3: Binary search to narrow down invalid step
    Round 4: Execute single EVM opcode on-chain
    
    Example:
    Batch has 10,000 transactions
    
    Without interactive proofs:
    - Execute all 10,000 on L1 (expensive!)
    
    With interactive proofs:
    - Binary search: log2(10,000) ≈ 14 rounds
    - Execute 1 opcode on L1 (very cheap!)
    
    Gas savings: 10,000x

    uint256 constant CHALLENGE_BOND = 1 ether;
    
    function submitFraudProof(FraudProof calldata proof) external payable {
        require(msg.value >= CHALLENGE_BOND, "Insufficient bond");
        
        // Verify proof
        if (verifyFraud(proof)) {
            // Fraud proven - return bond + reward
            payable(msg.sender).transfer(msg.value + sequencerSlash);
        } else {
            // Invalid proof - lose bond
            // Bond goes to sequencer
        }
    }

    Sequencer posts batches
    → Verifiers monitor silently
    → No fraud → No proofs submitted
    → Efficient operation

    Fraud detected
    → Proof generated
    → Submitted to L1
    → Gas cost incurred