Inbound Messages

interface IOutbox {
    event MessageSent(
        uint256 indexed nonce,
        uint64 indexed destinationChainId,
        address indexed target,
        address sender,
        bytes payload
    );

    function send(uint64 destinationChainId, address target, bytes calldata payload)
        external
        returns (uint256 nonce);

    function messages(uint256 nonce) external view returns (bytes32);
    function nextNonce() external view returns (uint256);
}

messages[nonce] = keccak256(abi.encode(
    sourceChainId,
    destinationChainId,
    nonce,
    msg.sender,
    target,
    payload
))

slot = keccak256(abi.encode(nonce, uint256(0)))

interface IForeignInbox {
    error CodeHashMismatch();
    error InvalidOutboxCodeHash();

    event MessageExecuted(
        uint256 indexed nonce,
        address indexed target,
        address sender
    );

    function execute(
        bytes32 checkpointBlockHash,
        uint256 nonce,
        address sender,
        address target,
        bytes calldata payload,
        bytes[] calldata accountProof,
        bytes[] calldata storageProof
    ) external;

    function foreignSender() external view returns (address);
    function consumed(uint256 nonce) external view returns (bool);
    function lightClient() external view returns (address);
    function foreignOutbox() external view returns (address);
    function foreignOutboxCodeHash() external view returns (bytes32);
    function foreignChainId() external view returns (uint256);
}

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "eth_getProof",
  "params": [
    "0xFOREIGN_OUTBOX",
    ["0xMESSAGE_STORAGE_SLOT"],
    "0xCHECKPOINT_BLOCK_NUMBER"
  ]
}

contract ForeignReceiver {
    address immutable inbox;

    constructor(address inbox_) {
        inbox = inbox_;
    }

    function receiveMessage(bytes calldata data) external {
        require(msg.sender == inbox, "only ForeignInbox");
        address sender = IForeignInbox(msg.sender).foreignSender();
        // Authorize `sender` and process `data`.
    }
}