Skip to main content

zkBridge Cross-Chain Messaging

You will need the SendApi interface and ReceiveApi interface.

Send Chain​

To send a message, call the zkBridge send() function. here is an simple example

// perform a zkBridge send() in a solidity smart contract function
// the msg.value is the "fee" that zkBridge needs to pay for the cross chain message

//TODO handle your business
//.......
bytes memory payload =abi.encode(
    tokenAddress,   // Address of the token
    tokenChain,     // Chain ID of the token
    amount,         // amount of the token
    to,             // Address of the recipient.
    toChain         // Chain ID of the recipient
)

uint64 nonce = IZKBridge(zkBridgeAddress).send{value:msg.value}(dstChainId, dstAddress, payload);

Estimating Message Fees​

You will note in the topmost example we call send() with value: msg.value this is because send() requires a bit of native gas token so the relayer can complete the message delivery on the destination chain. If you don't set this value you might get error when calling zkBridge.send()

function estimateFees(uint16 dstChainId) external view override returns (uint256 fee) {
    return IZKBridge(zkBridgeAddress).estimateFee(dstChainId);
}

Receive Chain​

Destination contracts must implement zkReceive() to handle incoming messages

The code snippet explains how the message will be received. To receive a message, your User Application contract must implement the IZKBridgeReceiver interface and override the zkReceive function

pragma solidity ^0.8.0;

contract Demo is IZKBridgeReceiver {
    // @notice ZKBridge endpoint will invoke this function to deliver the message on the destination
    // @param srcChainId - the source endpoint identifier
    // @param srcAddress - the source sending contract address from the source chain
    // @param nonce - the ordered message nonce
    // @param message - the signed payload is the UA bytes has encoded to be sent
    function zkReceive(uint16 srcChainId, address srcAddress, uint64 nonce, bytes calldata payload) external override {
        (
            address tokenAddress,
            uint16 tokenChain,
            uint256 amount,
            address to,
            uint16 toChain
        ) = abi.decode(payload, (address, uint16, uint256, address, uint16));
         //TODO handle your business
    }
}

Contract Addresses

Testnet​

  • Sepolia
    • chainId:119
    • Address:0xa8a4547Be2eCe6Dde2Dd91b4A5adFe4A043b21C7
  • Binance Smart Chain
    • chainId:103
    • Address:0xa8a4547Be2eCe6Dde2Dd91b4A5adFe4A043b21C7
  • EXPchain
    • chainId:131
    • Address:0xa8a4547Be2eCe6Dde2Dd91b4A5adFe4A043b21C7

EVM (Solidity) Interfaces

IZKBridge.sol​

// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

interface IZKBridge {

  // @notice send a zkBridge message to the specified address at a zkBridge endpoint.
  // @param dstChainId - the destination chain identifier
  // @param dstAddress - the address on destination chain
  // @param payload - a custom bytes payload to send to the destination contract
  function send(uint16 dstChainId, address dstAddress, bytes memory payload) external payable returns (uint64 nonce);

  // @notice gets a quote in source native gas, for the amount that send() requires to pay for message delivery
  // @param dstChainId - the destination chain identifier
  function estimateFee(uint16 dstChainId) external view returns (uint256 fee);
}

IZKBridgeReceiver.sol​

// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

interface IZKBridgeReceiver {
  // @notice zkBridge endpoint will invoke this function to deliver the message on the destination
  // @param srcChainId - the source endpoint identifier
  // @param srcAddress - the source sending contract address from the source chain
  // @param nonce - the ordered message nonce
  // @param payload - a custom bytes payload from send chain
  function zkReceive(uint16 srcChainId, address srcAddress, uint64 nonce, bytes calldata payload) external;
}

Index