Example: Message Proof

Here is a simple proof-of-concept message bridge built on top of ZK slot value proof. For a full-fledged cross-chain message bridge, please refer to the previous section. 
​Source code on GitHub.
The source chain sender contract has a map to store the hash of all messages, with a counter nonce as the map key. Proof of a sent message is equivalent to the proof of a map storage value.
// source chain contract
contract MsgSender {
    uint64 public nonce; // slot 0
    
    // slot 1, map of (nonce -> msgHash)
    // each msgHash is stored in slot keccak256(abi.encode(nonce, 1))
    mapping(uint64 => bytes32) public sent;
    
    event MsgSent(uint64 nonce, address sender, bytes msg);
​
    function sendMsg(bytes calldata _msg) external {
        bytes32 msgHash = keccak256(abi.encodePacked(msg.sender, _msg));
        sent[nonce] = msgHash;
        emit MsgSent(nonce++, msg.sender, _msg);
    }
}
The destination chain receiver contract connects to the Brevis slot value verifier contract to validate that a given <nonce, sender, message> tuple can match the map storage slot value in the source chain sender contract.
// destination chain contract
contract MsgReceiver {
    uint64 public senderChainId;
    bytes32 public senderContractHash;
    ISlotValueVerifier public slotVerifier;
    event MsgReceived(uint64 nonce, address sender, bytes msg);
​
    constructor(
        ISlotValueVerifier _verifier,
        uint64 _senderChainId,
        address _senderContract
    ) {
        slotVerifier = _verifier;
        senderChainId = _senderChainId;
        senderContractHash = keccak256(abi.encodePacked(_senderContract));
    }
​
    function recvMsg(
        uint64 _nonce,
        address _sender,
        bytes calldata _msg,
        bytes calldata _proofData,
        bytes calldata _blkVerifyInfo
    ) external {
        // compute expected slot and msg hash, sender map slot is 1
        bytes32 slotKeyHash = keccak256(abi.encode(keccak256(abi.encode(_nonce, 1))));
        bytes32 msgHash = keccak256(abi.encodePacked(_sender, _msg));
        // retrieve zk verified slot info
        ISlotValueVerifier.SlotInfo memory slotInfo = slotVerifier.verifySlotValue(
            senderChainId,
            _proofData,
            _blkVerifyInfo
        );
        // compare expected and verified values
        require(slotInfo.slotKeyHash == slotKeyHash, "slot key not match");
        require(slotInfo.slotValue == msgHash, "slot value not match");
        require(slotInfo.addrHash == senderContractHash, "sender contract not match");
        emit MsgReceived(_nonce, _sender, _msg);
    }
}
Last modified 4mo ago