To deliver interchain messages, the relayer calls Mailbox.process(). This function takes as parameters the message to deliver, as well as arbitrary metadata that can be specified by the relayer. The Mailbox will pass the message and metadata to the recipient’s Interchain Security Module (ISM) for verification. If the ISM successfully verifies the message, the Mailbox delivers the message to the recipient by calling recipient.handle().
See Message.sol for more details on Hyperlane message encoding

​
Handle

This function is called by the Mailbox contract when a message is received.
To ensure only valid interchain messages are accepted, it is important to restrict access control to the Mailbox address.

​
Solidity

function handle(
    uint32 _origin,
    bytes32 _sender,
    bytes calldata _messageBody
) external;
Parameters
  • origin: Domain of origin chain
  • sender: Address of sender on origin chain as bytes32
  • messageBody: Raw bytes content of message body
Sender addresses are left-padded to bytes32 for compatibility with virtual machines that are addressed differently. The following utility is provided in the TypeCasts library for convenience.
function bytes32ToAddress(bytes32 _bytes32) internal pure returns (address) {
    return address(uint160(uint256(_bytes32)));
}

​
Access Control

The handle function should be restricted to the Mailbox address if the contract should only accept calls from interchain messages. The following utility is provided in the MailboxClient library for convenience. 
modifier onlyMailbox() {
    require(
        msg.sender == address(mailbox),
        "MailboxClient: sender not mailbox"
    );
    _;
}

​
Examples

function handle(
    uint32 _origin,
    bytes32 _sender,
    bytes calldata _messageBody
) external override onlyMailbox {
    lastSender = _sender;
    lastData = _messageBody;
    emit ReceivedMessage(_origin, _sender, _messageBody);
}

​
Verify

When a message is received, the Mailbox will verify security with an Interchain Security Module before calling the message recipient’s handle.

​
Default Security

To query the default ISM address, you can call the defaultIsm function.

​
Modular Security

To leverage Hyperlane’s modular security, message recipients can specify a custom Interchain Security Module to verify anything about incoming messages. The Mailbox will defer to this ISM when specified.