Chain Fusion Starter Project

Table of Contents

• Overview
  • What is a Coprocessor?
  • Why Use ICP as a Coprocessor for Ethereum?
• Getting Started
  • In the Cloud
  • Locally
  • Manual Setup
• Architecture
  • EVM Smart Contract
  • Chain Fusion Canister
• Development
  • Interacting with the EVM Smart Contract
  • Leveraging storage.rs for Stable Memory
  • Read from EVM Smart Contracts
  • Sending Transactions to EVM Smart Contracts
• Use Cases
• Additional Resources

Overview

This project demonstrates how to use the Internet Computer (ICP) as a coprocessor for EVM smart contracts. The coprocessor listens to events emitted by an EVM smart contract, processes them, and optionally sends the results back. This starter project is a proof of concept and should not be used in production environments.

To get a better understanding of how the coprocessor works, make sure you check out the recorded workshops in the Additional Resources section.

What is a coprocessor?

The concept of coprocessors originated in computer architecture as a technique to enhance performance. Traditional computers rely on a single central processing unit (CPU) to handle all computations. However, as workloads grew more complex, the CPU became overloaded.

Coprocessors were introduced to offload specific tasks from the CPU to specialized hardware. Similarly, in the EVM ecosystem, smart contracts often face computational constraints. Coprocessors and stateful Layer 2 solutions extend the capabilities of the EVM by offloading specific tasks to more powerful environments.

Read more about coprocessors in the context of Ethereum in the article "A Brief Intro to Coprocessors".

Why Use ICP as a Coprocessor for Ethereum?

Canister smart contracts on ICP can securely read from EVM smart contracts (using HTTPS Outcalls or the EVM RPC canister) and write to them (using Chain-key Signatures, i.e., Threshold ECDSA). This eliminates the need for additional parties to relay messages between the networks, and no extra work is required on the EVM side to verify computation results as the EVM smart contract just needs to check for the proper sender.

Moreover, canister smart contracts have numerous capabilities that can extend smart contract functionality:

• WASM Runtime, which is more efficient than the EVM and allows programming in Rust, JavaScript, and other traditional languages.
• 400 GiB of memory with low storage costs.
• Long-running computations including AI inference.
• HTTPS Outcalls for interacting with other chains and traditional web services.
• Chain-key signatures for signing transactions for other chains, including Ethereum and Bitcoin.
• Timers for syncing with EVM events and scheduling tasks.
• Unbiasable randomness provided by threshold BLS signatures.
• Ability to serve web content directly from canisters.
• The reverse gas model frees end users from paying for every transaction.
• ~1-2 second finality.
• Multi-block transactions.

Getting Started

To deploy the project locally, run ./deploy.sh from the project root. This script will:

• Start anvil
• Start dfx
• Deploy the EVM contract
• Generate a number of jobs to be processed
• Deploy the coprocessor canister

Check the deploy.sh script comments for detailed deployment steps.

In the Cloud

Locally

Ensure Docker and VS Code are installed and running, then click the button below:

Manual Setup

Ensure the following are installed on your system:

• Node.js >= 21
• Foundry
• Caddy
• DFX >= 0.18

Run these commands in a new, empty project directory:

git clone https://github.com/letmejustputthishere/chain-fusion-starter.git
cd chain-fusion-starter

Architecture

This starter project involves multiple canisters working together to process events emitted by an EVM smart contract. The contracts involved are:

• EVM Smart Contract: Emits events such as NewJob when specific functions are called. It also handles callbacks from the chain_fusion canister with the results of the processed jobs.
• Chain Fusion Canister (chain_fusion): Listens to events emitted by the EVM smart contract, processes them, and sends the results back to the EVM smart contract.
• EVM RPC Canister: Facilitates communication between the Internet Computer and EVM-based blockchains by making RPC calls to interact with the EVM smart contract.

The full flow of how these canisters interact can be found in the following sequence diagram:

EVM Smart Contract

The contracts/Coprocessor.sol contract emits a NewJob event when the newJob function is called, transferring ETH to the chain_fusion canister to pay it for job processing and transaction fees (this step is optional and can be customized to fit your use case).

// Function to create a new job
function newJob() public payable {
    // Require at least 0.01 ETH to be sent with the call
    require(msg.value >= 0.01 ether, "Minimum 0.01 ETH not met");

    // Forward the ETH received to the coprocessor address
    // to pay for the submission of the job result back to the EVM
    // contract.
    coprocessor.transfer(msg.value);

    // Emit the new job event
    emit NewJob(job_id);

    // Increment job counter
    job_id++;
}

The callback function writes processed results back to the contract:

function callback(string calldata _result, uint256 _job_id) public {
    require(
        msg.sender == coprocessor,
        "Only the coprocessor can call this function"
    );
    jobs[_job_id] = _result;
}

For local deployment, see the deploy.sh script and script/Coprocessor.s.sol.

Chain Fusion Canister

The chain_fusion canister listens to NewJob events by periodically calling the eth_getLogs RPC method via the EVM RPC canister. Upon receiving an event, it processes the job and sends the results back to the EVM smart contract via the EVM RPC canister, signing the transaction with threshold ECDSA.

The Job processing logic is in canisters/chain_fusion/src/job.rs:

pub async fn job(event_source: LogSource, event: LogEntry) {
    mutate_state(|s| s.record_processed_log(event_source.clone()));
    // because we deploy the canister with topics only matching
    // NewJob events we can safely assume that the event is a NewJob.
    let new_job_event = NewJobEvent::from(event);
    // this calculation would likely exceed an ethereum blocks gas limit
    // but can easily be calculated on the IC
    let result = fibonacci(20);
    // we write the result back to the evm smart contract, creating a signature
    // on the transaction with chain key ecdsa and sending it to the evm via the
    // evm rpc canister
    submit_result(result.to_string(), new_job_event.job_id).await;
    println!("Successfully ran job #{:?}", &new_job_event.job_id);
}

Development

All coprocessing logic resides in canisters/chain_fusion/src/job.rs. Developers can focus on writing jobs to process EVM smart contract events without altering the code for fetching events or sending transactions.

Interacting with the EVM Smart Contract

If you want to check that the chain_fusion canister really processed the events, you can either look at the logs output by running ./deploy.sh – keep an eye open for the Successfully ran job message – or you can call the EVM contract to get the results of the jobs. To do this, run:

cast call 0x5fbdb2315678afecb367f032d93f642f64180aa3 "getResult(uint)(string)" <job_id>

where <job_id> is the ID of the job you want to get the result for. This should always return "6765" for processed jobs, which is the 20th Fibonacci number, and "" for unprocessed jobs.

If you want to create more jobs, simply run:

cast send 0x5fbdb2315678afecb367f032d93f642f64180aa3 "newJob()" --private-key=0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80 --value 0.01ether

Note that the Chain Fusion Canister only scrapes logs every 3 minutes, so you may need to wait a few minutes before seeing the new job processed.

Leveraging storage.rs for Stable Memory

The storage.rs module allows you to store data in stable memory, providing up to 400 GiB of available storage. In this starter template, stable memory can used to store assets that can then be served via HTTP.

To use this feature, you need to uncomment the section in lib.rs that handles HTTP requests. This enables the canister to serve stored assets. Here is the code snippet to uncomment:

// Uncomment this if you need to serve stored assets from `storage.rs` via HTTP requests

// #[ic_cdk::query]
// fn http_request(req: HttpRequest) -> HttpResponse {
//     if let Some(asset) = get_asset(&req.path().to_string()) {
//         let mut response_builder = HttpResponseBuilder::ok();

//         for (name, value) in asset.headers {
//             response_builder = response_builder.header(name, value);
//         }

//         response_builder
//             .with_body_and_content_length(asset.body)
//             .build()
//     } else {
//         HttpResponseBuilder::not_found().build()
//     }
// }

By enabling this code, you can serve web content directly from the canister, leveraging the stable memory for storing large amounts of data efficiently.

Reading from and writing to EVM Smart Contracts

To send transactions to the EVM, this project uses the ic-evm-utils crate. This crate provides functionality for constructing, signing and sending transactions to EVM networks, leveraging the evm-rpc-canister-types crate for data types and constants.

Key Functions:

• sign_eip1559_transaction: This function signs a EIP-1559 transaction.

• eth_call: This function sends a call to an arbitrary EVM smart contract to read data from it. It constructs a JSON-RPC call to the EVM RPC canister, which then forwards the call to the EVM smart contract.

• erc20_balance_of: The erc20_balance_of function demonstrates how to construct and send a call to an ERC20 contract to query the balance of a specific address. It uses the eth_call function to send the call and parse the response. You can refer to the erc20_balance_of function in the eth_call.rs module to understand how to implement similar read operations for other types of EVM smart contracts.

• send_raw_transaction: This function sends a raw transaction to an EVM smart contract. It constructs a transaction, signs it with the canister's private key, and sends it to the EVM network.

• transfer_eth: The transfer_eth function demonstrates how to transfer ETH from a canister-owned EVM address to another address. It covers creating a transaction, signing it with the canister's private key, and sending it to the EVM network. transfer_eth uses the send_raw_transaction function to send the transaction.

• contract_interaction: The contract_interaction function demonstrates how to interact with arbitrary EVM smart contracts. It constructs a transaction based on the desired contract interaction, signs it with the canister's private key, and sends it to the EVM network. contract_interaction uses the send_raw_transaction function to send the transaction. The submit_result function in this starter project leverages this function to send the results of processed jobs back to the EVM smart contract.

Use Cases

Examples leveraging the chain fusion starter logic:

• On-chain asset and metadata creation for ERC721 NFT contracts
• Ethereum Donations Streamer
• Recurring Transactions on Ethereum

Build your own use case and share it with the community!

Some ideas you could explore:

• A referral canister that distributes rewards to users based on their interactions with an EVM smart contract
• A ckNFT canister that mints an NFT on the ICP when an EVM helper smart contract emits a ReceivedNft, similar to the EthDepositHelper contract the ckETH minter uses. This could enable users to trade NFTs on the ICP without having to pay gas fees on Ethereum.
• Decentralized DCA (dollar cost average) service for decentralized exchanges like Uniswap deployed on EVM chains
• Price oracles for DeFi applications via exchange rate canister
• Prediction market resolution
• Soulbound NFT metadata and assets stored in a canister
• An on-chain managed passive index fund (e.g. top 10 ERC20 tokens traded on Uniswap)
• An on-chain donations stream

Additional Resources

• DappCon24 Workshop
• ETHPrague24 Workshop
• Using Cast

For more details and discussions, visit the DFINITY Developer Forum or follow @cryptoschindler on Twitter.