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Ping Example

This tutorial demonstrates how to send a cross-chain message using Oasis OPL.

You'll learn how to:

  • Deploy a Host contract
  • Deploy a Enclave contract
  • Send a cross-chain message

We recommend using Remix for an easy-to-follow experience. The only prerequisite is a set-up Metamask account.

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If you're new to Remix, follow our basic guide for using Remix here.

Overview Ping​

In this example, you'll deploy a host contract on BSC Testnet and a enclave contract on Sapphire Testnet. You'll then send a ping from the host contract to the enclave contract, facilitated by the OPL SDK. The enclave contract will receive the ping and emits an event with the message which was received.

Contract Setup​

  1. Open Remix and create a new file called Ping.sol
  2. Paste the following Ping host contract into it:
Ping.sol Contract
Ping.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import {Host, Result} from "@oasisprotocol/sapphire-contracts/contracts/OPL.sol";

contract Ping is Host {
    event MessageReceived(bytes message);

    constructor(address pong) Host(pong) {
        registerEndpoint("pongMessage", _pongMessage);
    }

    function startPing (bytes calldata _message) external payable {
        postMessage("ping", abi.encode(_message));
    }

    function _pongMessage(bytes calldata _args) internal returns (Result) {
        (bytes memory message) = abi.decode((_args), (bytes));
        emit MessageReceived(message);
        return Result.Success;
    }
}
  1. Create a new file called Pong.sol
  2. Paste the following Pong enclave contract into it:
Pong.sol Contract
Pong.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import {Enclave, Result, autoswitch} from "@oasisprotocol/sapphire-contracts/contracts/OPL.sol";

contract Pong is Enclave {
    event MessageReceived(bytes message);

    constructor(uint nonce, bytes32 chain) Enclave(computeAddress(msg.sender, nonce), autoswitch(chain)) {
        registerEndpoint("ping", _pingMessage);
    }

    function _pingMessage(bytes calldata _args) internal returns (Result) {
        (bytes memory message) = abi.decode((_args), (bytes));
        emit MessageReceived(message);
        return Result.Success;
    }

    function computeAddress(address _origin, uint _nonce) public pure returns (address) {
        if (_nonce == 0x00) {
            return address(uint160(uint256(keccak256(abi.encodePacked(
                bytes1(0xd6), bytes1(0x94), _origin, bytes1(0x80)
            )))));
        }
        if (_nonce <= 0x7f) {
            return address(uint160(uint256(keccak256(abi.encodePacked(
                bytes1(0xd6), bytes1(0x94), _origin, bytes1(uint8(_nonce))
            )))));
        }
        if (_nonce <= 0xff) {
            return address(uint160(uint256(keccak256(abi.encodePacked(
                bytes1(0xd7), bytes1(0x94), _origin, bytes1(0x81), uint8(_nonce)
            )))));
        }
        if (_nonce <= 0xffff) {
            return address(uint160(uint256(keccak256(abi.encodePacked(
                bytes1(0xd8), bytes1(0x94), _origin, bytes1(0x82), uint16(_nonce)
            )))));
        }
        if (_nonce <= 0xffffff) {
            return address(uint160(uint256(keccak256(abi.encodePacked(
                bytes1(0xd9), bytes1(0x94), _origin, bytes1(0x83), uint24(_nonce)
            )))));
        }
        return address(uint160(uint256(keccak256(abi.encodePacked(
            bytes1(0xda), bytes1(0x94), _origin, bytes1(0x84), uint32(_nonce)
        )))));
    }
}

Key points​

  • Host: OPL wrapper for outside contract.
  • Enclave: OPL wrapper for the contract on Sapphire side.
  • registerEndpoint: Registers endpoints in an OPL managed map.
  • postMessage: Call registered endpoints.
  • autoswitch: Finds correct MessageBus address via chain name.

Compiling the Contract​

For compatibility with Sapphire, compile the contract using compiler version 0.8.24 and evm version paris (under advanced configuration).

Deploying the Contract​

Deploy the Ping contract on BSC Testnet and the Pong.sol contract on Sapphire Testnet.

Deploying Pong.sol on Sapphire Testnet​

You'll deploy the contract on Sapphire Testnet first to avoid switching chains back and forth.

  1. Obtain TEST tokens for Sapphire Testnet from the Oasis faucet.
  2. Get next nonce of your account from BSC Testnet
    1. If you didn't do anything on BSC Testnet yet this will 0.
    2. Else you need to get your last nonce, e.g. by checking your account address on bscscan and and inspect the details of your latest transaction, and then add 1.
  3. In Metamask, switch to the Sapphire Testnet network and select Injected Provider - MetaMask as the environment in Remix.
  4. Select the Pong.sol contract.
  5. Fill in the deployment parameters:
  • nonce: 0 (or next nonce as written above)
  • chain: 0x6273630000000000000000000000000000000000000000000000000000000000 (bytes encoded "bsc")
  1. Deploy the contract on Sapphire Testnet
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Copy the address of the deployed contract, you'll need it for the next step as Remix will remove the contract from the UI if you change the chain.

Deploying Ping.sol on BSC Testnet​

  1. Obtain BNB test token for BSC Testnet from the BNB faucet or their discord.
  2. In MetaMask, switch to the BSC Testnet network and select Injected Provider - MetaMask as the environment in Remix.
  3. Select the Ping.sol contract.
  4. Fill in the contract addess you just have deployed on Sapphire Testnet.
  5. Deploy the contract on BSC Testnet.

Executing Ping​

Now that you've deployed the contacts, you can send the ping message cross-chain.

You'll need the following parameter for startPing:

  • _message: The encoded message. e.g. "Hello from BSC" - 0x48656c6c6f2066726f6d20425343000000000000000000000000000000000000.

Additionally you'll have to pay a fee which you send as value. For sending the ping 0.001 tBNB (1000000 gwei) will be enough.

Finally execute the function startPing.

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For the Sapphire Testnet an executor is running to relay the messages every few mintues. If you deploy on mainnet please refer to the Executor chapter.

Checking execution​

To see if you successfully send a ping message cross-chain you can watch for new transactions at the MessageBus address from Celer or your deployed contract on Sapphire Testnet.