Exploring the Openzeppelin CrossChain Functionality

What is the new CrossChain support and how can you use it.

For the first time Openzeppelin Contracts have added CrossChain Support. In particular the following chains are currently supported:

  • Polygon: One of the most popular sidechains right now. We've discussed it previously here.
  • Optimism: A Layer 2 chain based on optimistic rollups. We discussed the tech previously here.
  • Arbitrum: A different Layer 2 chain also based on optimistic rollups. 
  • AMB: The Arbitrary Message Bridge is a general tool one can use to relay any data between two chains.
Son of a bitch Meme

What are the Difficulties with CrossChain?

So what exactly are the things to consider in CrossChain communication?

  • Why even send data to begin with? Well you might have governance-owned contract on Ethereum mainnet for example controlled by a native ERC-20. And you want this contract to be able to change fundamental things, for example upgrade another contract on the child chain. Then you need a way to allow the contract from the mainnet to send data to the child in a secure way.
  • The sender problem: One difficulty of sending cross-chain messages is determining the sender address, because from the contract's perspective msg.sender will actually be a child system address. What exactly msg.sender will be of course depends on the child chain, but it won't be the actual sender from the root chain.
  • The access control problem: Another issue with CrossChain is the potential for an address existing twice, so be aware that a contract with identical address can exist on child and root chain.
  • Signature Double-Use: And if you allow for any signatures in your contract, they might be double used. This is why you should always double check the chain id or better use EIP-712 which handles all the complexities with secure signatures.

How the OZ CrossChain Support works

Openzeppelin has added contracts to support the usage in Polygon, Optimism, Arbitrum and AMB. There is a master interface CrossChainEnabled.sol which all four implementations make use of. And there is a new AccessControlCrossChain.sol to allow for secure access control via roles but with CrossChain support. An overview of it all can be seen here.

Each chain implementation contains a different mechanism to retrieve the original CrossChain sender which roughly looks like this:

function processMessageFromRoot(
    uint256, /* stateId */
    address rootMessageSender,
    bytes calldata data
)
AMB_Bridge(bridge).messageSender()
LibArbitrumL2.crossChainSender(LibArbitrumL2.ARBSYS)
Optimism_Bridge(messenger).xDomainMessageSender()

For the full details, check out the contract code here.

How to use it - Polygon Example

Let's take a deeper dive in how you would actually use this with the example of Polygon.

We'll create contracts on the root and child chain with secure access control.

1. Creating the Root Contract

So first let's create a contract that we will deploy on the root blockchain. In the normal case this would be the Ethereum mainnet. We can inherit from the FxBaseRootTunnel.sol contract and pass check point and root addresses depending on the network:

  • GOERLI_CHECKPOINT_MANAGER = 0x2890bA17EfE978480615e330ecB65333b880928e
  • GOERLI_FX_ROOT = 0x3d1d3E34f7fB6D26245E6640E1c50710eFFf15bA
  • MAINNET_CHECKPOINT_MANAGER = 0x86E4Dc95c7FBdBf52e33D563BbDB00823894C287
  • MAINNET_FX_ROOT = 0xfe5e5D361b2ad62c541bAb87C45a0B9B018389a2

It will give you two internal functions to work with

  1. _processMessageFromChild: Override this to respond to messages sent from the child contract.
  2. _sendMessageToChild: Call this to send a message to the child.
import {FxBaseRootTunnel} from
    "fx-portal/contracts/tunnel/FxBaseRootTunnel.sol";

// see left for full addresses
address constant GOERLI_CP_MANAGER = 0x2890bA17EfE978480615e...;
address constant GOERLI_FX_ROOT = 0x3d1d3E34f7fB6D26245E6640...;

contract PolygonRoot is FxBaseRootTunnel {
    bytes public latestData;

    constructor()
        FxBaseRootTunnel(GOERLI_CP_MANAGER, GOERLI_FX_ROOT) {
    }

    function _processMessageFromChild(
        bytes memory data
    ) internal override {
        latestData = data;
    }

    function sendMessageToChild(bytes memory message) public {
        _sendMessageToChild(message);
    }
}
import {CrossChainEnabledPolygonChild} from
    "oz/contracts/crosschain/polygon/CrossChainEnabledPolygonChild.sol";
import {AccessControlCrossChain} from
    "oz/contracts/access/AccessControlCrossChain.sol";

address constant MUMBAI_FX_CHILD = 0xCf73231F...; // see right

contract PolygonChild is
    CrossChainEnabledPolygonChild,
    AccessControlCrossChain
{
    event MessageSent(bytes message);
    uint256 public myNumber = 12;

    constructor(
        address rootParent
    ) CrossChainEnabledPolygonChild(MUMBAI_FX_CHILD) {
        _grantRole(
            _crossChainRoleAlias(DEFAULT_ADMIN_ROLE),
            rootParent
        );
    }

    function setNumberForParentChain(
        uint256 newNumber
    ) external onlyRole(DEFAULT_ADMIN_ROLE) {
        myNumber = newNumber;
    }

    function _sendMessageToRoot(
        bytes memory message
    ) internal {
        emit MessageSent(message);
    }
}

2. Creating the Child Contract

And then we can create the child contract that we will deploy on the child blockchain. In our case this will be the Polygon network. We can inherit from the Openzeppelin CrossChainEnabledPolygonChild.sol contract and pass FX Portal Child contract depending on the network:

  • MUMBAI_FX_CHILD = 0xCf73231F28B7331BBe3124B907840A94851f9f11
  • MAINNET_FX_CHILD = 0x8397259c983751DAf40400790063935a11afa28a

And now we can also make use of the AccessControlCrossChain.sol from Openzeppelin. Just inherit from it in the contract and we'll get the usual access control functions along with a new _crossChainRoleAlias function.

In our example upon deployment we will pass the previously deployed root contract address here and immediately grant it the admin role, but since this is actually a crosschain communication, it works a little differently:

  1. Of course the onlyRole modifier cannot just check the msg.sender, so instead it uses the CrossChainEnabled.sol interface to determine the actual CrossChain sender.
  2. And to further prevent access from contracts in the child chain with the same address as in the root chain, we need to distinguish between senders from msg.sender directly or from CrossChain. For that we can grant a specific role using _crossChainRoleAlias.

And then let's add a test function setNumberForParentChain which only the CrossChain root is allowed to call.

And just for completeness, if you wanted to send a message back to the root, in Polygon you could do so by emitting the MessageSent event.

3. Get Encoded Data Helper

This is completely optional, but for our testing you could add an extra function like this:

function getEncodedSetNumberData(uint256 newNumber) external pure returns (bytes memory) {
    return abi.encodeWithSelector(PolygonChild.setNumberForParentChain.selector, newNumber);
}

Basically it will return the encoded data if you wanted to call setNumberForParentChain. This data is what you would need to send along in the root contract via sendMessageToChild. Of course in most setups you would implement this just using Web3.js or whatever frontend framework you're using.

4. Testing the CrossChain Transfer on Remix

Okay so now let's actually deploy this to the testnet. We are using

  • the Goerli network as root, the latest Ethereum testnet
  • and Mumbai, the Polygon testnet

But the flow would be identical for mainnet, just using different addresses.

So we can just copy the full code here into Remix:

// SPDX-License-Identifier: MIT
pragma solidity 0.8.13;

import {CrossChainEnabledPolygonChild} from "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/crosschain/polygon/CrossChainEnabledPolygonChild.sol";
import {AccessControlCrossChain} from "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/access/AccessControlCrossChain.sol";

import {FxBaseRootTunnel} from "https://github.com/fx-portal/contracts/blob/main/contracts/tunnel/FxBaseRootTunnel.sol";

address constant MUMBAI_FX_CHILD = 0xCf73231F28B7331BBe3124B907840A94851f9f11;
address constant GOERLI_CHECKPOINT_MANAGER = 0x2890bA17EfE978480615e330ecB65333b880928e;
address constant GOERLI_FX_ROOT = 0x3d1d3E34f7fB6D26245E6640E1c50710eFFf15bA;

address constant MAINNET_FX_CHILD = 0x8397259c983751DAf40400790063935a11afa28a;
address constant MAINNET_CHECKPOINT_MANAGER = 0x86E4Dc95c7FBdBf52e33D563BbDB00823894C287;
address constant MAINNET_FX_ROOT = 0xfe5e5D361b2ad62c541bAb87C45a0B9B018389a2;

contract PolygonChild is CrossChainEnabledPolygonChild, AccessControlCrossChain {
    uint256 public myNumber;

    constructor(address rootParent) CrossChainEnabledPolygonChild(MUMBAI_FX_CHILD) {
        _grantRole(_crossChainRoleAlias(DEFAULT_ADMIN_ROLE), rootParent);
        myNumber = 12;
    }

    function setNumberForParentChain(uint256 newNumber) external onlyRole(DEFAULT_ADMIN_ROLE) {
        myNumber = newNumber;
    }

    function getEncodedSetNumberData(uint256 newNumber) external pure returns (bytes memory) {
        return abi.encodeWithSelector(PolygonChild.setNumberForParentChain.selector, newNumber);
    }
}

contract PolygonRoot is FxBaseRootTunnel {
    bytes public latestData;

    constructor() FxBaseRootTunnel(GOERLI_CHECKPOINT_MANAGER, GOERLI_FX_ROOT) {}

    function _processMessageFromChild(bytes memory data) internal override {
        latestData = data;
    }

    function sendMessageToChild(bytes memory message) public {
        _sendMessageToChild(message);
    }
}

And now you can

  1. Switch MetaMask to Goerli and deploy the PolygonRoot.
  2. Switch MetaMask to Mumbai and copy the address and deploy PolygonChild for the constructor input.
  3. Switch MetaMask to Goerli  and call setFxChildTunnel on PolygonRoot passing the child address.
  4. Now encode the data you want to send. For example to set the number for setNumberForParentChain as 42, the encoded data would be: 0x21148d91000000000000000000000000000000000000000000000000000000000000002a. The easiest way to get this is via getEncodedSetNumberData.
  5. Call sendMessageToChild and pass along the encoded data. This will initiate the CrossChain transfer. It might take a while.
  6. Wait.... meanwhile you can double check the events from the FX_CHILD here or simply the transfers of the zero address here.
Mumbai Zero
Mumbai FX Child

If that looks confusing to you, then that's no surprise. The zero address is a special system address in Polygon which is used to commit the CrossChain transfers. And the transfers also end up in the events list of the child.

In my tests it took anywhere between 2 to 25 minutes until the CrossChain transfer on Polygon was completed.

And that's it!

If you did everything correctly, you can switch back to Mumbai and read the newly set number which should have changed to 42.

Sponge Bob 42

Markus Waas

Solidity Developer

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    Previously we looked at the big picture of Solidity and the create-eth-app which already mentioned TheGraph before. This time we will take a closer look at TheGraph which essentially became part of the standard stack for developing Dapps in the last year. But let's first see how we would do...

  • truffle buidler typescript

    Adding Typescript to Truffle and Buidler

    How to use TypeChain to utilize the powers of Typescript in your project

    Unlike compiled languages, you pretty much have no safeguards when running JavaScript code. You'll only notice errors during runtime and you won't get autocompletion during coding. With Typescript you can get proper typechecking as long as the used library exports its types. Most Ethereum...

  • Balance Rope

    Integrating Balancer in your contracts

    What is Balancer and how to use it

    What is Balancer? Balancer is very similar to Uniswap. If you're not familiar with Uniswap or Balancer yet, they are fully decentralized protocols for automated liquidity provision on Ethereum. An easier-to-understand description would be that they are decentralized exchanges (DEX) relying on...

  • mousetrap

    Navigating the pitfalls of securely interacting with ERC20 tokens

    Figuring out how to securely interact might be harder than you think

    You would think calling a few functions on an ERC-20 token is the simplest thing to do, right? Unfortunately I have some bad news, it's not. There are several things to consider and some errors are still pretty common. Let's start with the easy ones. Let's take a very common token: ... Now to...

  • Aave

    Why you should automatically generate interests from user funds

    How to integrate Aave and similar systems in your contracts

    If you're writing contracts that use, hold or manage user funds, you might want to consider using those funds for generating free extra income. What's the catch? That's right, it's basically free money and leaving funds unused in a contract is wasting a lot of potential. The way these...

  • Matic Logo

    How to use Polygon (Matic) in your Dapp

    Deploying and onboarding users to  Polygon  to avoid the high gas costs

    Gas costs are exploding again, ETH2.0 is still too far away and people are now looking at layer 2 solutions. Here's a good overview of existing layer 2 projects: https://github.com/Awesome-Layer-2/awesome-layer-2. Today we will take a closer look at Polygon (previously known as Matic) as a...

  • Migrating from Truffle to Buidler

    And why you should probably keep both.

    Why Buidler? Proper debugging is a pain with Truffle. Events are way too difficult to use as logging and they don't even work for reverted transactions (when you would need them most). Buidler gives you a console.log for your contracts which is a game changer. And you'll also get stack traces...

  • Factory

    Contract factories and clones

    How to deploy contracts within contracts as easily and gas-efficient as possible

    The factory design pattern is a pretty common pattern used in programming. The idea is simple, instead of creating objects directly, you have an object (the factory) that creates objects for you. In the case of Solidity, an object is a smart contract and so a factory will deploy new contracts for...

  • IPFS logo

    How to use IPFS in your Dapp?

    Using the interplanetary file system in your frontend and contracts

    You may have heard about IPFS before, the Interplanetary File System. The concept has existed for quite some time now, but with IPFS you'll get a more reliable data storage, thanks to their internal use of blockchain technology. Filecoin is a new system that is incentivizing storage for IPFS...

  • tiny-kitten

    Downsizing contracts to fight the contract size limit

    What can you do to prevent your contracts from getting too large?

    Why is there a limit? On November 22, 2016 the Spurious Dragon hard-fork introduced EIP-170 which added a smart contract size limit of 24.576 kb. For you as a Solidity developer this means when you add more and more functionality to your contract, at some point you will reach the limit and when...

  • EXTCODEHASH

    Using EXTCODEHASH to secure your systems

    How to safely integrate anyone's smart contract

    What is the EXTCODEHASH? The EVM opcode EXTCODEHASH was added on February 28, 2019 via EIP-1052. Not only does it help to reduce external function calls for compiled Solidity contracts, it also adds additional functionality. It gives you the hash of the code from an address. Since only contract...

  • Uniswap

    Using the new Uniswap v2 in your contracts

    What's new in Uniswap v2 and how to integrate Uniswap v2

    Note : For Uniswap 3 check out the tutorial here. What is UniSwap? If you're not familiar with Uniswap yet, it's a fully decentralized protocol for automated liquidity provision on Ethereum. An easier-to-understand description would be that it's a decentralized exchange (DEX) relying on external...

  • Continuous Integration

    Solidity and Truffle Continuous Integration Setup

    How to setup Travis or Circle CI for Truffle testing along with useful plugins.

    Continuous integration (CI) with Truffle is great for developing once you have a basic set of tests implemented. It allows you to run very long tests, ensure all tests pass before merging a pull request and to keep track of various statistics using additional tools. We will use the Truffle...

  • Devcon 6

    Upcoming Devcon 2021 and other events

    The Ethereum Foundation just announced the next Devcon in 2021 in Colombia

    Biggest virtual hackathon almost finished First of all, the current HackMoney event has come to an end and it has been a massive success. One can only imagine what kind of cool projects people have built in a 30 days hackathon. All final projects can be seen at:...

  • ERC-2020

    The Year of the 20: Creating an ERC20 in 2020

    How to use the latest and best tools to create an ERC-20 token contract

    You know what an ERC-20 is, you probably have created your own versions of it several times (if not, have a look at: ERC-20). But how would you start in 2020 using the latest tools? Let's create a new ERC-2020 token contract with some basic functionality which focuses on simplicity and latest...

  • hiring

    How to get a Solidity developer job?

    There are many ways to get a Solidity job and it might be easier than you think!

    You have mastered the basics of Solidity, created your first few useful projects and now want to get your hands on some real-world projects. Getting a Solidity developer job might be easier than you think. There are generally plenty of options to choose from and often times not a lot of...

  • People making fun

    Design Pattern Solidity: Mock contracts for testing

    Why you should make fun of your contracts

    Mock objects are a common design pattern in object-oriented programming. Coming from the old French word 'mocquer' with the meaning of 'making fun of', it evolved to 'imitating something real' which is actually what we are doing in programming. Please only make fun of your smart contracts if you...

  • React and Ethereum

    Kickstart your Dapp frontend development with create-eth-app

    An overview on how to use the app and its features

    Last time we looked at the big picture of Solidity and already mentioned the create-eth-app. Now you will find out how to use it, what features are integrated and additional ideas on how to expand on it. Started by Paul Razvan Berg, the founder of sablier, this app will kickstart your frontend...

  • Solidity Overview

    The big picture of Solidity and Blockchain development in 2020

    Overview of the most important technologies, services and tools that you need to know

    Now, I do not know about you, but I remember when I first started with Solidity development being very confused by all the tools and services and how they work in connection with one another. If you are like me, this overview will help you understand the big picture of Solidity development. As I...

  • Design Pattern Solidity: Free up unused storage

    Why you should clean up after yourself

    You may or may not be used to a garbage collectors in your previous programming language. There is no such thing in Solidity and even if there was a similar concept, you would still be better off managing state data yourself. Only you as a programmer can know exactly which data will not be used...

  • How to setup Solidity Developer Environment on Windows

    What you need to know about developing on Windows

    Using Windows for development, especially for Solidity development, can be a pain sometimes, but it does not have to be. Once you have configured your environment properly, it can actually be extremely efficient and Windows is a very, very stable OS, so your overall experience can be amazing. The...

  • Avoiding out of gas for Truffle tests

    How you do not have to worry about gas in tests anymore

    You have probably seen this error message a lot of times: Error: VM Exception while processing transaction: out of gas Disclaimer : Unfortunately, this does not always actually mean what it is saying when using Truffle , especially for older versions. It can occur for various reasons and might be...

  • Design Pattern Solidity: Stages

    How you can design stages in your contract

    Closely related to the concept of finite-state machines, this pattern will help you restrict functions in your contract. You will find a lot of situations where it might be useful. Any time a contract should allow function calls only in certain stages. Let's look at an example: contract Pool {...

  • Web3 1.2.5: Revert reason strings

    How to use the new feature

    A new Web3 version was just released and it comes with a new feature that should make your life easier. With the latest version 1.2.5, you can now see the the revert reason if you use the new handleRevert option. You can activate it easily by using web3.eth.handleRevert = true . Now when you use...

  • Gaining back control of the internet

    How Ocelot is decentralizing cloud computing

    I recently came across an ambitious company that will completely redefine the way we are using the internet. Or rather, the way we are using its underlying infrastructure which ultimately is the internet. While looking at their offering, I also learned how to get anonymous cloud machines, you...

  • Devcon 5 - Review

    Impressions from the conference

    I had a lot to catch up on after Devcon. Also things didn't go quite as planned, so please excuse my delayed review! This year's Devcon was certainly stormy with a big typhoon warning already on day 1. Luckily (for us, not the people in Tokyo), it went right past Osaka. Nevertheless, a lot of...

  • Devcon 5 - Information, Events, Links, Telegram

    What you need to know

    Devcon 5 is coming up soon and there are already lots of events available, information about Osaka and more. Here is a short overview: Events Events Calendar Events Google Docs Events Kickback Most events are in all three, but if you really want to see all, you will have to look at all three...

  • Design Pattern Solidity: Off-chain beats on-chain

    Why you should do as much as possible off-chain

    As you might have realized, Ethereum transactions are anything but cheap. In particular, if you are computing complex things or storing a lot of data. That means sometimes we cannot put all logic inside Solidity. Instead, we can utilize off-chain computations to help us. A very simple example...

  • Design Pattern Solidity: Initialize Contract after Deployment

    How to use the Initializable pattern

    There are a few reasons why you might want to initialize a contract after deployment and not directly by passing constructor arguments. But first let's look at an example: contract MyCrowdsale { uint256 rate; function initialize(uint256 _rate) public { rate = _rate; } } What's the advantage over...

  • Consensys Blockchain Jobs Report

    What the current blockchain job market looks like

    Consensys published their blockchain jobs report which you can checkout in their Blockchain Developer Job Kit. The most interesting aspects are Blockchain developer jobs have been growing at a rate of 33x of the previous year according to LinkedIns jobs report Typical salary is about...

  • Provable — Randomness Oracle

    How the Oraclize random number generator works

    One particularly interesting approach by Provable is the usage of a hardware security device, namely the Ledger Nano S. It uses a trusted execution environment to generate random numbers and provides a Provable Connector Contract as interface. How to use the Provable Randomness Oracle? Use the...

  • Solidity Design Patterns: Multiply before Dividing

    Why the correct order matters!

    There has been a lot of progress since the beginning of Ethereum about best practices in Solidity. Unfortunately, I have the feeling that most of the knowledge is within the circle of experienced people and there aren’t that many online resources about it. That is why I would like to start this...

  • Devcon 5 Applications closing in one week

    Devcon 5 Applications closing

    Watch out for the Devcon 5 applications. You only have one week left to apply either as Buidler Student Scholarship Press Devcon is by far the biggest and most impressive Ethereum conference in the world. And it's full of developers! I am especially excited about the cool location this year in...

  • Randomness and the Blockchain

    How to achieve secure randomness for Solidity smart contracts?

    When we talk about randomness and blockchain, these are really two problems: 1. How to generate randomness in smart contracts? 2. How to produce randomness for proof-of-stake (POS) systems? Or more generally, how to produce trusted randomness in public distributed systems? There is some overlap...