Cross-Chain Bridge Security: Preventing Multi-Billion Dollar Exploits

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// Trust-minimized light client bridge
pragma solidity ^0.8.19;

import "./MerklePatricia.sol";
import "./RLPDecoder.sol";

contract LightClientBridge {
    using RLPDecoder for bytes;
    using RLPDecoder for RLPDecoder.RLPItem;
    
    struct BlockHeader {
        bytes32 parentHash;
        bytes32 stateRoot;
        bytes32 transactionsRoot;
        bytes32 receiptsRoot;
        uint256 blockNumber;
        uint256 timestamp;
        bytes32 blockHash;
        bool finalized;
    }
    
    struct RelayerSet {
        address[] relayers;
        uint256 threshold;
        uint256 epoch;
        mapping(address => bool) isRelayer;
    }
    
    mapping(uint256 => BlockHeader) public headers;
    mapping(bytes32 => bool) public processedTransactions;
    
    RelayerSet public currentRelayerSet;
    uint256 public latestBlockNumber;
    uint256 public challengePeriod = 7 days;
    
    event HeaderSubmitted(uint256 indexed blockNumber, bytes32 blockHash);
    event TransactionProven(bytes32 indexed txHash, address recipient, uint256 amount);
    event RelayerSetUpdated(address[] newRelayers, uint256 newThreshold);
    
    modifier onlyRelayer() {
        require(currentRelayerSet.isRelayer[msg.sender], "Not authorized relayer");
        _;
    }
    
    function submitHeader(
        bytes memory _headerRLP,
        bytes[] memory _signatures
    ) external onlyRelayer {
        // Decode header
        BlockHeader memory header = _decodeHeader(_headerRLP);
        
        // Verify header signatures
        require(
            _verifyRelayerSignatures(_headerRLP, _signatures),
            "Invalid signatures"
        );
        
        // Verify header chain consistency
        require(
            header.parentHash == headers[header.blockNumber - 1].blockHash,
            "Invalid parent hash"
        );
        
        // Store header
        headers[header.blockNumber] = header;
        latestBlockNumber = header.blockNumber;
        
        emit HeaderSubmitted(header.blockNumber, header.blockHash);
    }
    
    function proveTransaction(
        bytes memory _txData,
        bytes memory _receipt,
        bytes memory _proof,
        uint256 _blockNumber,
        uint256 _txIndex
    ) external {
        BlockHeader memory header = headers[_blockNumber];
        require(header.blockHash != bytes32(0), "Header not found");
        require(
            block.timestamp >= header.timestamp + challengePeriod,
            "Challenge period not passed"
        );
        
        // Verify transaction inclusion in block
        bytes32 txHash = keccak256(_txData);
        require(
            MerklePatricia.verify(
                _proof,
                header.transactionsRoot,
                abi.encodePacked(_txIndex),
                _txData
            ),
            "Invalid transaction proof"
        );
        
        // Verify receipt inclusion
        require(
            MerklePatricia.verify(
                _proof,
                header.receiptsRoot,
                abi.encodePacked(_txIndex),
                _receipt
            ),
            "Invalid receipt proof"
        );
        
        // Parse and execute cross-chain transaction
        _executeCrossChainTx(txHash, _txData, _receipt);
    }
    
    function _decodeHeader(bytes memory _headerRLP) internal pure returns (BlockHeader memory) {
        RLPDecoder.RLPItem[] memory headerItems = _headerRLP.toRLPItem().toList();
        
        return BlockHeader({
            parentHash: bytes32(headerItems[0].toUint()),
            stateRoot: bytes32(headerItems[3].toUint()),
            transactionsRoot: bytes32(headerItems[4].toUint()),
            receiptsRoot: bytes32(headerItems[5].toUint()),
            blockNumber: headerItems[8].toUint(),
            timestamp: headerItems[11].toUint(),
            blockHash: keccak256(_headerRLP),
            finalized: false
        });
    }
    
    function _verifyRelayerSignatures(
        bytes memory _data,
        bytes[] memory _signatures
    ) internal view returns (bool) {
        require(
            _signatures.length >= currentRelayerSet.threshold,
            "Insufficient signatures"
        );
        
        bytes32 messageHash = keccak256(_data);
        address[] memory signers = new address[](_signatures.length);
        
        // Recover signers
        for (uint i = 0; i < _signatures.length; i++) {
            signers[i] = _recoverSigner(messageHash, _signatures[i]);
            require(currentRelayerSet.isRelayer[signers[i]], "Invalid signer");
            
            // Check for duplicate signers
            for (uint j = 0; j < i; j++) {
                require(signers[i] != signers[j], "Duplicate signer");
            }
        }
        
        return true;
    }
    
    function _executeCrossChainTx(
        bytes32 _txHash,
        bytes memory _txData,
        bytes memory _receipt
    ) internal {
        require(!processedTransactions[_txHash], "Already processed");
        processedTransactions[_txHash] = true;
        
        // Parse transaction data for bridge operations
        (address recipient, uint256 amount, address token) = _parseBridgeTx(_txData);
        
        // Mint or release tokens
        if (token == address(0)) {
            // Native ETH transfer
            payable(recipient).transfer(amount);
        } else {
            // ERC20 token transfer
            IMintableERC20(token).mint(recipient, amount);
        }
        
        emit TransactionProven(_txHash, recipient, amount);
    }
    
    function _parseBridgeTx(bytes memory _txData) internal pure returns (
        address recipient,
        uint256 amount,
        address token
    ) {
        // Simplified parsing - real implementation would decode transaction input
        // This would extract the bridge operation details from the transaction
        return (address(0), 0, address(0));
    }
    
    function _recoverSigner(bytes32 _messageHash, bytes memory _signature) 
        internal 
        pure 
        returns (address) 
    {
        bytes32 ethSignedMessageHash = keccak256(abi.encodePacked(
            "\x19Ethereum Signed Message:\n32",
            _messageHash
        ));
        
        (bytes32 r, bytes32 s, uint8 v) = _splitSignature(_signature);
        return ecrecover(ethSignedMessageHash, v, r, s);
    }
    
    function _splitSignature(bytes memory _signature) internal pure returns (
        bytes32 r,
        bytes32 s,
        uint8 v
    ) {
        require(_signature.length == 65, "Invalid signature length");
        
        assembly {
            r := mload(add(_signature, 32))
            s := mload(add(_signature, 64))
            v := byte(0, mload(add(_signature, 96)))
        }
    }
    
    function updateRelayerSet(
        address[] memory _newRelayers,
        uint256 _newThreshold,
        bytes[] memory _signatures
    ) external {
        require(_newThreshold > 0 && _newThreshold <= _newRelayers.length, "Invalid threshold");
        
        // Verify current relayer set approved the update
        bytes32 updateHash = keccak256(abi.encodePacked(_newRelayers, _newThreshold));
        require(
            _verifyRelayerSignatures(abi.encodePacked(updateHash), _signatures),
            "Invalid update signatures"
        );
        
        // Update relayer set
        for (uint i = 0; i < currentRelayerSet.relayers.length; i++) {
            currentRelayerSet.isRelayer[currentRelayerSet.relayers[i]] = false;
        }
        
        currentRelayerSet.relayers = _newRelayers;
        currentRelayerSet.threshold = _newThreshold;
        currentRelayerSet.epoch++;
        
        for (uint i = 0; i < _newRelayers.length; i++) {
            currentRelayerSet.isRelayer[_newRelayers[i]] = true;
        }
        
        emit RelayerSetUpdated(_newRelayers, _newThreshold);
    }
}

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// ZK-SNARK bridge circuit for cross-chain state verification
pragma circom 2.0.0;

include "circomlib/circuits/poseidon.circom";
include "circomlib/circuits/merkletree.circom";
include "circomlib/circuits/eddsa.circom";

template CrossChainStateProof(levels) {
    // Public inputs
    signal input sourceChainId;
    signal input targetChainId;
    signal input blockNumber;
    signal input stateRoot;
    signal input recipient;
    signal input amount;
    
    // Private inputs
    signal private input accountAddress;
    signal private input accountBalance;
    signal private input accountNonce;
    signal private input merkleProof[levels];
    signal private input merkleIndices[levels];
    
    // Output
    signal output valid;
    
    // Components
    component merkleTree = MerkleTreeInclusionProof(levels);
    component stateHash = Poseidon(4);
    component balanceCheck = GreaterEqualThan(64);
    
    // Verify account state is included in state root
    stateHash.inputs[0] <== accountAddress;
    stateHash.inputs[1] <== accountBalance;
    stateHash.inputs[2] <== accountNonce;
    stateHash.inputs[3] <== sourceChainId;
    
    merkleTree.leaf <== stateHash.out;
    merkleTree.root <== stateRoot;
    
    for (var i = 0; i < levels; i++) {
        merkleTree.pathElements[i] <== merkleProof[i];
        merkleTree.pathIndices[i] <== merkleIndices[i];
    }
    
    // Verify sufficient balance for transfer
    balanceCheck.in[0] <== accountBalance;
    balanceCheck.in[1] <== amount;
    
    // All checks must pass
    valid <== merkleTree.valid * balanceCheck.out;
}

component main = CrossChainStateProof(32);

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// ZK-proof bridge verifier contract
pragma solidity ^0.8.19;

import "./verifier.sol"; // Generated from circom circuit
import "./PlonkVerifier.sol";

contract ZKBridge {
    using SafeMath for uint256;
    
    struct ChainState {
        bytes32 stateRoot;
        uint256 blockNumber;
        uint256 timestamp;
        bool verified;
    }
    
    struct CrossChainTransfer {
        uint256 sourceChain;
        uint256 targetChain;
        address sender;
        address recipient;
        uint256 amount;
        address token;
        uint256 nonce;
        bool processed;
    }
    
    mapping(uint256 => ChainState) public chainStates;
    mapping(bytes32 => CrossChainTransfer) public transfers;
    mapping(bytes32 => bool) public processedProofs;
    
    PlonkVerifier public immutable verifier;
    
    event StateUpdated(uint256 indexed chainId, bytes32 stateRoot, uint256 blockNumber);
    event TransferInitiated(bytes32 indexed transferId, uint256 sourceChain, uint256 targetChain);
    event TransferCompleted(bytes32 indexed transferId, address recipient, uint256 amount);
    
    constructor(PlonkVerifier _verifier) {
        verifier = _verifier;
    }
    
    function updateChainState(
        uint256 _chainId,
        bytes32 _stateRoot,
        uint256 _blockNumber,
        uint256[24] memory _proof,
        uint256[] memory _publicInputs
    ) external {
        // Verify ZK proof of state transition
        require(
            verifier.verifyProof(_proof, _publicInputs),
            "Invalid state proof"
        );
        
        // Verify public inputs match claimed state
        require(_publicInputs[0] == _chainId, "Chain ID mismatch");
        require(_publicInputs[1] == uint256(_stateRoot), "State root mismatch");
        require(_publicInputs[2] == _blockNumber, "Block number mismatch");
        
        // Ensure monotonic block progression
        require(
            _blockNumber > chainStates[_chainId].blockNumber,
            "Block number must increase"
        );
        
        chainStates[_chainId] = ChainState({
            stateRoot: _stateRoot,
            blockNumber: _blockNumber,
            timestamp: block.timestamp,
            verified: true
        });
        
        emit StateUpdated(_chainId, _stateRoot, _blockNumber);
    }
    
    function initiateCrossChainTransfer(
        uint256 _targetChain,
        address _recipient,
        uint256 _amount,
        address _token
    ) external payable {
        require(_amount > 0, "Invalid amount");
        require(_recipient != address(0), "Invalid recipient");
        
        bytes32 transferId = keccak256(abi.encodePacked(
            block.chainid,
            _targetChain,
            msg.sender,
            _recipient,
            _amount,
            _token,
            block.timestamp
        ));
        
        transfers[transferId] = CrossChainTransfer({
            sourceChain: block.chainid,
            targetChain: _targetChain,
            sender: msg.sender,
            recipient: _recipient,
            amount: _amount,
            token: _token,
            nonce: block.timestamp,
            processed: false
        });
        
        // Lock tokens on source chain
        if (_token == address(0)) {
            require(msg.value == _amount, "ETH amount mismatch");
        } else {
            IERC20(_token).transferFrom(msg.sender, address(this), _amount);
        }
        
        emit TransferInitiated(transferId, block.chainid, _targetChain);
    }
    
    function completeCrossChainTransfer(
        bytes32 _transferId,
        uint256[24] memory _proof,
        uint256[] memory _publicInputs
    ) external {
        require(!processedProofs[_transferId], "Already processed");
        
        // Verify ZK proof of transfer inclusion
        require(
            verifier.verifyProof(_proof, _publicInputs),
            "Invalid transfer proof"
        );
        
        // Decode public inputs
        uint256 sourceChain = _publicInputs[0];
        address recipient = address(uint160(_publicInputs[1]));
        uint256 amount = _publicInputs[2];
        address token = address(uint160(_publicInputs[3]));
        
        // Verify chain state is up to date
        ChainState memory sourceState = chainStates[sourceChain];
        require(sourceState.verified, "Source chain state not verified");
        
        processedProofs[_transferId] = true;
        
        // Release tokens on target chain
        if (token == address(0)) {
            payable(recipient).transfer(amount);
        } else {
            // Mint tokens or release from vault
            _releaseTokens(token, recipient, amount);
        }
        
        emit TransferCompleted(_transferId, recipient, amount);
    }
    
    function _releaseTokens(address _token, address _recipient, uint256 _amount) internal {
        // Try to mint if possible, otherwise transfer from vault
        try IMintableERC20(_token).mint(_recipient, _amount) {
            // Minting successful
        } catch {
            // Fall back to vault transfer
            IERC20(_token).transfer(_recipient, _amount);
        }
    }
    
    function emergencyWithdraw(address _token, uint256 _amount) external {
        require(msg.sender == owner(), "Only owner");
        
        if (_token == address(0)) {
            payable(msg.sender).transfer(_amount);
        } else {
            IERC20(_token).transfer(msg.sender, _amount);
        }
    }
}

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// Threshold signature bridge with slashing
pragma solidity ^0.8.19;

contract ThresholdSignatureBridge {
    struct Validator {
        address validator;
        uint256 stake;
        bool active;
        uint256 slashCount;
        uint256 joinedEpoch;
    }
    
    struct Proposal {
        bytes32 txHash;
        address recipient;
        uint256 amount;
        address token;
        uint256 sourceChain;
        uint256 requiredSignatures;
        uint256 currentSignatures;
        mapping(address => bool) hasSigned;
        bool executed;
        uint256 deadline;
    }
    
    Validator[] public validators;
    mapping(address => uint256) public validatorIndex;
    mapping(bytes32 => Proposal) public proposals;
    
    uint256 public totalStake;
    uint256 public minimumStake = 100 ether;
    uint256 public slashAmount = 10 ether;
    uint256 public threshold = 67; // 67% threshold
    uint256 public currentEpoch;
    
    event ValidatorAdded(address indexed validator, uint256 stake);
    event ValidatorSlashed(address indexed validator, uint256 amount);
    event ProposalCreated(bytes32 indexed proposalId, address recipient, uint256 amount);
    event ProposalExecuted(bytes32 indexed proposalId);
    
    modifier onlyValidator() {
        require(_isActiveValidator(msg.sender), "Not active validator");
        _;
    }
    
    function addValidator(address _validator) external payable {
        require(msg.value >= minimumStake, "Insufficient stake");
        require(!_isValidator(_validator), "Already validator");
        
        validators.push(Validator({
            validator: _validator,
            stake: msg.value,
            active: true,
            slashCount: 0,
            joinedEpoch: currentEpoch
        }));
        
        validatorIndex[_validator] = validators.length - 1;
        totalStake += msg.value;
        
        emit ValidatorAdded(_validator, msg.value);
    }
    
    function proposeTransfer(
        bytes32 _txHash,
        address _recipient,
        uint256 _amount,
        address _token,
        uint256 _sourceChain,
        bytes memory _signature
    ) external onlyValidator {
        require(proposals[_txHash].recipient == address(0), "Proposal exists");
        
        // Verify signature of transaction data
        bytes32 messageHash = keccak256(abi.encodePacked(
            _txHash, _recipient, _amount, _token, _sourceChain
        ));
        
        require(
            _verifySignature(messageHash, _signature, msg.sender),
            "Invalid signature"
        );
        
        uint256 requiredSigs = _calculateRequiredSignatures();
        
        Proposal storage proposal = proposals[_txHash];
        proposal.txHash = _txHash;
        proposal.recipient = _recipient;
        proposal.amount = _amount;
        proposal.token = _token;
        proposal.sourceChain = _sourceChain;
        proposal.requiredSignatures = requiredSigs;
        proposal.currentSignatures = 1;
        proposal.hasSigned[msg.sender] = true;
        proposal.executed = false;
        proposal.deadline = block.timestamp + 24 hours;
        
        emit ProposalCreated(_txHash, _recipient, _amount);
        
        // Execute immediately if threshold met
        if (proposal.currentSignatures >= requiredSigs) {
            _executeProposal(_txHash);
        }
    }
    
    function signProposal(
        bytes32 _txHash,
        bytes memory _signature
    ) external onlyValidator {
        Proposal storage proposal = proposals[_txHash];
        
        require(proposal.recipient != address(0), "Proposal doesn't exist");
        require(!proposal.executed, "Already executed");
        require(!proposal.hasSigned[msg.sender], "Already signed");
        require(block.timestamp <= proposal.deadline, "Proposal expired");
        
        // Verify signature
        bytes32 messageHash = keccak256(abi.encodePacked(
            _txHash, proposal.recipient, proposal.amount, 
            proposal.token, proposal.sourceChain
        ));
        
        require(
            _verifySignature(messageHash, _signature, msg.sender),
            "Invalid signature"
        );
        
        proposal.hasSigned[msg.sender] = true;
        proposal.currentSignatures++;
        
        // Execute if threshold reached
        if (proposal.currentSignatures >= proposal.requiredSignatures) {
            _executeProposal(_txHash);
        }
    }
    
    function _executeProposal(bytes32 _txHash) internal {
        Proposal storage proposal = proposals[_txHash];
        
        require(!proposal.executed, "Already executed");
        require(
            proposal.currentSignatures >= proposal.requiredSignatures,
            "Insufficient signatures"
        );
        
        proposal.executed = true;
        
        // Transfer tokens
        if (proposal.token == address(0)) {
            payable(proposal.recipient).transfer(proposal.amount);
        } else {
            IERC20(proposal.token).transfer(proposal.recipient, proposal.amount);
        }
        
        emit ProposalExecuted(_txHash);
    }
    
    function slashValidator(
        address _validator,
        bytes32 _evidence,
        bytes[] memory _signatures
    ) external {
        require(_isActiveValidator(_validator), "Not active validator");
        
        // Verify evidence with sufficient validator signatures
        bytes32 evidenceHash = keccak256(abi.encodePacked(_evidence, _validator));
        uint256 validSignatures = 0;
        
        for (uint i = 0; i < _signatures.length; i++) {
            address signer = _recoverSigner(evidenceHash, _signatures[i]);
            if (_isActiveValidator(signer) && signer != _validator) {
                validSignatures++;
            }
        }
        
        require(
            validSignatures >= _calculateRequiredSignatures(),
            "Insufficient evidence"
        );
        
        // Slash validator
        uint256 validatorIdx = validatorIndex[_validator];
        Validator storage validator = validators[validatorIdx];
        
        uint256 slashAmountActual = validator.stake < slashAmount ? 
            validator.stake : slashAmount;
        
        validator.stake -= slashAmountActual;
        validator.slashCount++;
        totalStake -= slashAmountActual;
        
        // Deactivate if stake too low
        if (validator.stake < minimumStake) {
            validator.active = false;
        }
        
        emit ValidatorSlashed(_validator, slashAmountActual);
    }
    
    function _calculateRequiredSignatures() internal view returns (uint256) {
        uint256 activeValidators = _getActiveValidatorCount();
        return (activeValidators * threshold + 99) / 100; // Ceiling division
    }
    
    function _getActiveValidatorCount() internal view returns (uint256) {
        uint256 count = 0;
        for (uint i = 0; i < validators.length; i++) {
            if (validators[i].active) {
                count++;
            }
        }
        return count;
    }
    
    function _isValidator(address _address) internal view returns (bool) {
        if (validators.length == 0) return false;
        return validators[validatorIndex[_address]].validator == _address;
    }
    
    function _isActiveValidator(address _address) internal view returns (bool) {
        if (!_isValidator(_address)) return false;
        return validators[validatorIndex[_address]].active;
    }
    
    function _verifySignature(
        bytes32 _messageHash,
        bytes memory _signature,
        address _expectedSigner
    ) internal pure returns (bool) {
        address signer = _recoverSigner(_messageHash, _signature);
        return signer == _expectedSigner;
    }
    
    function _recoverSigner(bytes32 _messageHash, bytes memory _signature)
        internal
        pure
        returns (address)
    {
        bytes32 ethSignedMessageHash = keccak256(abi.encodePacked(
            "\x19Ethereum Signed Message:\n32",
            _messageHash
        ));
        
        (bytes32 r, bytes32 s, uint8 v) = _splitSignature(_signature);
        return ecrecover(ethSignedMessageHash, v, r, s);
    }
    
    function _splitSignature(bytes memory _signature) internal pure returns (
        bytes32 r,
        bytes32 s,
        uint8 v
    ) {
        require(_signature.length == 65, "Invalid signature length");
        
        assembly {
            r := mload(add(_signature, 32))
            s := mload(add(_signature, 64))
            v := byte(0, mload(add(_signature, 96)))
        }
    }
}

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// Optimistic bridge with fraud proof system
pragma solidity ^0.8.19;

contract OptimisticBridge {
    struct CrossChainClaim {
        bytes32 txHash;
        uint256 sourceChain;
        address recipient;
        uint256 amount;
        address token;
        uint256 claimTime;
        uint256 challengeDeadline;
        address claimant;
        bool executed;
        bool challenged;
    }
    
    struct Challenge {
        bytes32 claimId;
        address challenger;
        uint256 deposit;
        bytes evidence;
        bool resolved;
        bool challengeSuccessful;
    }
    
    mapping(bytes32 => CrossChainClaim) public claims;
    mapping(bytes32 => Challenge) public challenges;
    mapping(address => uint256) public relayerBonds;
    
    uint256 public constant CHALLENGE_PERIOD = 7 days;
    uint256 public constant CHALLENGE_DEPOSIT = 1 ether;
    uint256 public constant RELAYER_BOND = 10 ether;
    
    event ClaimSubmitted(bytes32 indexed claimId, address claimant);
    event ClaimChallenged(bytes32 indexed claimId, address challenger);
    event ChallengeResolved(bytes32 indexed claimId, bool successful);
    event ClaimExecuted(bytes32 indexed claimId, address recipient, uint256 amount);
    
    modifier onlyBondedRelayer() {
        require(relayerBonds[msg.sender] >= RELAYER_BOND, "Insufficient bond");
        _;
    }
    
    function bondRelayer() external payable {
        require(msg.value >= RELAYER_BOND, "Insufficient bond amount");
        relayerBonds[msg.sender] += msg.value;
    }
    
    function submitClaim(
        bytes32 _txHash,
        uint256 _sourceChain,
        address _recipient,
        uint256 _amount,
        address _token,
        bytes memory _proof
    ) external onlyBondedRelayer {
        bytes32 claimId = keccak256(abi.encodePacked(
            _txHash, _sourceChain, _recipient, _amount, _token
        ));
        
        require(claims[claimId].claimant == address(0), "Claim already exists");
        
        // Basic validation of proof format
        require(_proof.length > 0, "Invalid proof");
        
        claims[claimId] = CrossChainClaim({
            txHash: _txHash,
            sourceChain: _sourceChain,
            recipient: _recipient,
            amount: _amount,
            token: _token,
            claimTime: block.timestamp,
            challengeDeadline: block.timestamp + CHALLENGE_PERIOD,
            claimant: msg.sender,
            executed: false,
            challenged: false
        });
        
        emit ClaimSubmitted(claimId, msg.sender);
    }
    
    function challengeClaim(
        bytes32 _claimId,
        bytes memory _evidence
    ) external payable {
        require(msg.value >= CHALLENGE_DEPOSIT, "Insufficient deposit");
        
        CrossChainClaim storage claim = claims[_claimId];
        require(claim.claimant != address(0), "Claim doesn't exist");
        require(!claim.executed, "Claim already executed");
        require(!claim.challenged, "Claim already challenged");
        require(block.timestamp <= claim.challengeDeadline, "Challenge period expired");
        
        bytes32 challengeId = keccak256(abi.encodePacked(_claimId, msg.sender));
        
        challenges[challengeId] = Challenge({
            claimId: _claimId,
            challenger: msg.sender,
            deposit: msg.value,
            evidence: _evidence,
            resolved: false,
            challengeSuccessful: false
        });
        
        claim.challenged = true;
        
        emit ClaimChallenged(_claimId, msg.sender);
    }
    
    function resolveChallenge(
        bytes32 _challengeId,
        bool _challengeSuccessful,
        bytes memory _proof
    ) external {
        Challenge storage challenge = challenges[_challengeId];
        require(!challenge.resolved, "Challenge already resolved");
        
        CrossChainClaim storage claim = claims[challenge.claimId];
        
        // Verify the resolution proof (simplified)
        bool proofValid = _verifyResolutionProof(
            challenge.claimId,
            _challengeSuccessful,
            _proof
        );
        
        require(proofValid, "Invalid resolution proof");
        
        challenge.resolved = true;
        challenge.challengeSuccessful = _challengeSuccessful;
        
        if (_challengeSuccessful) {
            // Challenge successful - slash claimant, reward challenger
            _slashRelayer(claim.claimant);
            payable(challenge.challenger).transfer(challenge.deposit * 2);
        } else {
            // Challenge failed - reward claimant, slash challenger
            payable(claim.claimant).transfer(challenge.deposit);
        }
        
        emit ChallengeResolved(challenge.claimId, _challengeSuccessful);
    }
    
    function executeClaim(bytes32 _claimId) external {
        CrossChainClaim storage claim = claims[_claimId];
        
        require(claim.claimant != address(0), "Claim doesn't exist");
        require(!claim.executed, "Already executed");
        require(
            block.timestamp > claim.challengeDeadline || !claim.challenged,
            "Challenge period active"
        );
        
        // If challenged, ensure challenge was resolved in claimant's favor
        if (claim.challenged) {
            bytes32 challengeId = _findChallengeId(_claimId);
            Challenge storage challenge = challenges[challengeId];
            require(challenge.resolved, "Challenge not resolved");
            require(!challenge.challengeSuccessful, "Challenge was successful");
        }
        
        claim.executed = true;
        
        // Execute the cross-chain transfer
        if (claim.token == address(0)) {
            payable(claim.recipient).transfer(claim.amount);
        } else {
            IERC20(claim.token).transfer(claim.recipient, claim.amount);
        }
        
        emit ClaimExecuted(_claimId, claim.recipient, claim.amount);
    }
    
    function _verifyResolutionProof(
        bytes32 _claimId,
        bool _challengeSuccessful,
        bytes memory _proof
    ) internal view returns (bool) {
        // Simplified proof verification
        // Real implementation would verify fraud proofs against source chain state
        return _proof.length > 0;
    }
    
    function _slashRelayer(address _relayer) internal {
        uint256 slashAmount = relayerBonds[_relayer] / 2;
        relayerBonds[_relayer] -= slashAmount;
        
        // Send slashed amount to insurance fund
        // payable(insuranceFund).transfer(slashAmount);
    }
    
    function _findChallengeId(bytes32 _claimId) internal view returns (bytes32) {
        // Simplified challenge lookup
        // Real implementation would maintain proper mappings
        return keccak256(abi.encodePacked(_claimId, "challenge"));
    }
    
    function withdrawBond() external {
        require(relayerBonds[msg.sender] > 0, "No bond to withdraw");
        
        uint256 bondAmount = relayerBonds[msg.sender];
        relayerBonds[msg.sender] = 0;
        
        payable(msg.sender).transfer(bondAmount);
    }
}

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// Comprehensive bridge monitoring system
const ethers = require('ethers');
const WebSocket = require('ws');

class BridgeSecurityMonitor {
    constructor(bridgeContract, providers) {
        this.bridgeContract = bridgeContract;
        this.providers = providers; // Multiple RPC providers for redundancy
        this.alerts = [];
        this.thresholds = {
            largeTransfer: ethers.utils.parseEther('1000'), // 1000 ETH
            validatorFailureRate: 0.1, // 10%
            unusualVolumeMultiplier: 5,
            maxTransfersPerBlock: 100
        };
        
        this.metrics = {
            hourlyVolume: 0,
            dailyVolume: 0,
            validatorUptime: new Map(),
            recentTransfers: [],
            failedValidations: 0
        };
    }
    
    async startMonitoring() {
        // Monitor all bridge events
        this.bridgeContract.on('*', (event) => {
            this.handleBridgeEvent(event);
        });
        
        // Monitor validator behavior
        setInterval(() => this.checkValidatorHealth(), 60000); // Every minute
        
        // Monitor transaction volume patterns
        setInterval(() => this.analyzeVolumePatterns(), 300000); // Every 5 minutes
        
        // Check for consensus failures
        setInterval(() => this.checkConsensusHealth(), 30000); // Every 30 seconds
        
        console.log('Bridge security monitoring started');
    }
    
    async handleBridgeEvent(event) {
        const eventName = event.event;
        const args = event.args;
        
        switch (eventName) {
            case 'TransferInitiated':
                await this.analyzeTransfer(args);
                break;
                
            case 'ValidatorSlashed':
                await this.handleValidatorSlash(args);
                break;
                
            case 'ClaimChallenged':
                await this.handleChallenge(args);
                break;
                
            case 'EmergencyPause':
                await this.handleEmergencyPause(args);
                break;
        }
    }
    
    async analyzeTransfer(transferArgs) {
        const { recipient, amount, token } = transferArgs;
        
        // Check for unusually large transfers
        if (amount.gt(this.thresholds.largeTransfer)) {
            await this.createAlert('LARGE_TRANSFER', {
                amount: ethers.utils.formatEther(amount),
                recipient,
                token,
                severity: 'HIGH'
            });
        }
        
        // Track volume metrics
        this.updateVolumeMetrics(amount);
        
        // Check for rapid-fire transfers (potential exploit)
        this.checkTransferVelocity(transferArgs);
        
        // Analyze recipient patterns
        await this.analyzeRecipientBehavior(recipient, amount);
    }
    
    async checkValidatorHealth() {
        const validators = await this.bridgeContract.getValidators();
        
        for (const validator of validators) {
            const uptime = await this.calculateValidatorUptime(validator);
            this.metrics.validatorUptime.set(validator, uptime);
            
            if (uptime < (1 - this.thresholds.validatorFailureRate)) {
                await this.createAlert('VALIDATOR_DEGRADED', {
                    validator,
                    uptime: uptime * 100,
                    severity: 'MEDIUM'
                });
            }
        }
    }
    
    async analyzeVolumePatterns() {
        const currentHourlyVolume = this.metrics.hourlyVolume;
        const historicalAverage = await this.getHistoricalVolumeAverage();
        
        if (currentHourlyVolume > historicalAverage * this.thresholds.unusualVolumeMultiplier) {
            await this.createAlert('UNUSUAL_VOLUME', {
                currentVolume: ethers.utils.formatEther(currentHourlyVolume),
                averageVolume: ethers.utils.formatEther(historicalAverage),
                multiplier: currentHourlyVolume / historicalAverage,
                severity: 'HIGH'
            });
        }
        
        // Reset hourly metrics
        this.metrics.hourlyVolume = 0;
    }
    
    async checkConsensusHealth() {
        const latestBlocks = await Promise.all(
            this.providers.map(provider => provider.getBlockNumber())
        );
        
        const maxBlock = Math.max(...latestBlocks);
        const minBlock = Math.min(...latestBlocks);
        
        // Check for significant block height divergence
        if (maxBlock - minBlock > 5) {
            await this.createAlert('CONSENSUS_DIVERGENCE', {
                maxBlock,
                minBlock,
                divergence: maxBlock - minBlock,
                severity: 'CRITICAL'
            });
        }
        
        // Check validator response times
        await this.checkValidatorResponseTimes();
    }
    
    async checkValidatorResponseTimes() {
        const validators = await this.bridgeContract.getValidators();
        const startTime = Date.now();
        
        const responses = await Promise.allSettled(
            validators.map(async (validator) => {
                // Ping validator endpoint
                const response = await fetch(`${validator.endpoint}/health`);
                return {
                    validator: validator.address,
                    responseTime: Date.now() - startTime,
                    status: response.status
                };
            })
        );
        
        const slowValidators = responses
            .filter(r => r.status === 'fulfilled')
            .map(r => r.value)
            .filter(r => r.responseTime > 5000); // 5 second threshold
        
        if (slowValidators.length > validators.length * 0.3) {
            await this.createAlert('VALIDATOR_PERFORMANCE_DEGRADED', {
                slowValidators: slowValidators.length,
                totalValidators: validators.length,
                severity: 'MEDIUM'
            });
        }
    }
    
    checkTransferVelocity(transferArgs) {
        const currentBlock = transferArgs.blockNumber;
        const recentTransfers = this.metrics.recentTransfers.filter(
            t => currentBlock - t.blockNumber <= 5 // Last 5 blocks
        );
        
        if (recentTransfers.length > this.thresholds.maxTransfersPerBlock * 5) {
            this.createAlert('HIGH_TRANSFER_VELOCITY', {
                transfersInWindow: recentTransfers.length,
                threshold: this.thresholds.maxTransfersPerBlock * 5,
                severity: 'HIGH'
            });
        }
        
        // Add to recent transfers
        this.metrics.recentTransfers.push({
            ...transferArgs,
            timestamp: Date.now()
        });
        
        // Clean old transfers
        this.metrics.recentTransfers = this.metrics.recentTransfers.filter(
            t => Date.now() - t.timestamp < 300000 // Keep last 5 minutes
        );
    }
    
    async analyzeRecipientBehavior(recipient, amount) {
        // Check if recipient is a known exchange or centralized entity
        const recipientInfo = await this.getRecipientInfo(recipient);
        
        if (recipientInfo.isExchange && amount.gt(this.thresholds.largeTransfer)) {
            await this.createAlert('LARGE_EXCHANGE_TRANSFER', {
                exchange: recipientInfo.name,
                amount: ethers.utils.formatEther(amount),
                recipient,
                severity: 'MEDIUM'
            });
        }
        
        // Check for new addresses receiving large amounts
        if (!recipientInfo.isKnown && amount.gt(this.thresholds.largeTransfer.div(10))) {
            await this.createAlert('NEW_ADDRESS_LARGE_TRANSFER', {
                recipient,
                amount: ethers.utils.formatEther(amount),
                severity: 'MEDIUM'
            });
        }
    }
    
    async createAlert(type, data) {
        const alert = {
            id: `${Date.now()}-${Math.random()}`,
            type,
            timestamp: new Date(),
            data,
            acknowledged: false
        };
        
        this.alerts.push(alert);
        
        // Send notifications based on severity
        await this.sendNotification(alert);
        
        // Auto-pause bridge for critical alerts
        if (data.severity === 'CRITICAL') {
            await this.considerEmergencyPause(alert);
        }
    }
    
    async sendNotification(alert) {
        // Send to various notification channels
        const message = this.formatAlertMessage(alert);
        
        // Slack notification
        await this.sendSlackAlert(message, alert.data.severity);
        
        // Discord notification
        await this.sendDiscordAlert(message, alert.data.severity);
        
        // Email for high severity
        if (['HIGH', 'CRITICAL'].includes(alert.data.severity)) {
            await this.sendEmailAlert(alert);
        }
        
        // SMS for critical
        if (alert.data.severity === 'CRITICAL') {
            await this.sendSMSAlert(alert);
        }
    }
    
    async considerEmergencyPause(alert) {
        // Automatic pause conditions
        const autoPauseConditions = [
            'CONSENSUS_DIVERGENCE',
            'VALIDATOR_MAJORITY_OFFLINE',
            'POTENTIAL_EXPLOIT_DETECTED'
        ];
        
        if (autoPauseConditions.includes(alert.type)) {
            console.log(`CRITICAL ALERT: ${alert.type} - Initiating emergency pause`);
            
            try {
                const pauseTx = await this.bridgeContract.emergencyPause();
                await pauseTx.wait();
                
                await this.createAlert('EMERGENCY_PAUSE_ACTIVATED', {
                    trigger: alert.type,
                    txHash: pauseTx.hash,
                    severity: 'CRITICAL'
                });
            } catch (error) {
                console.error('Failed to execute emergency pause:', error);
                
                await this.createAlert('EMERGENCY_PAUSE_FAILED', {
                    trigger: alert.type,
                    error: error.message,
                    severity: 'CRITICAL'
                });
            }
        }
    }
    
    formatAlertMessage(alert) {
        return `🚨 Bridge Alert: ${alert.type}\n` +
               `Severity: ${alert.data.severity}\n` +
               `Time: ${alert.timestamp.toISOString()}\n` +
               `Details: ${JSON.stringify(alert.data, null, 2)}`;
    }
    
    // Additional helper methods would be implemented here...
    updateVolumeMetrics(amount) {
        this.metrics.hourlyVolume += amount;
        this.metrics.dailyVolume += amount;
    }
    
    async getHistoricalVolumeAverage() {
        // Query historical data to calculate baseline
        return ethers.utils.parseEther('500'); // Placeholder
    }
    
    async calculateValidatorUptime(validator) {
        // Calculate uptime based on successful responses
        return 0.95; // Placeholder
    }
    
    async getRecipientInfo(address) {
        // Query address database for known entities
        return {
            isKnown: false,
            isExchange: false,
            name: null
        };
    }
}

// Usage
const provider = new ethers.providers.JsonRpcProvider('https://mainnet.infura.io/v3/YOUR_KEY');
const bridgeContract = new ethers.Contract(BRIDGE_ADDRESS, BRIDGE_ABI, provider);

const monitor = new BridgeSecurityMonitor(bridgeContract, [provider]);
monitor.startMonitoring();