Overview

We provide 2 PoCs for Zetachain:

• token: a one-way transfer of a message from a cheap chain to an expensive chain.
• token-back: a back-and-forth transfer of a message from an expensive chain to a cheap chain, which sends some result back to the expensive chain.

Notice in all of our examples, the BNB chain is the cheap chain where we conduct the costly operation, and Ethereum is the expensive chain where we mint an NFT.

Setup and Run

To run a PoC, first cd into the PoC directory. Then do the following:

• run yarn to install required packages
• on one terminal, run npx hardhat localnet. Localnet is a local testnet that allows us to test cross chain contracts.
• on another, run ./scripts/localnet.sh
• Observe the logs on both windows. Our logs on the localnet start with "!!!!!".

What we're passing around

// This is what we are actually sending
abi.encode(
  address zrc20Dest, // The ZRC-20 address of the gas token of the destination chain
  struct Msg { 
    uint16 amount;  // Our input
    bool isResult;  // A boolean that indicates if this is sending from the cheap chain back to the expensive chain
  },
  uint256 gasAmount, //The remaining gas Amount
  address sender  // The original Sender
);

// These below are for demonstration purposes but 
// There is no reason it should not be possible to send the function we want to run
// And switch to it in onCall in the receiving contract via a conditional
struct Request { 
  address zrc20Dest,
  uint256 gasAmount, 
  address sender,
  bytes arguments,
  string function
}

struct Response {
  address zrc20Dest,
  uint256 gasAmount, 
  address sender,
  bytes result
}

PoC 1: Token

This PoC shows that it's possible to transfer message cross chain via the help of a Universal contract deployed on Zetachain. The data flow is B -> Z -> E, where B is BNB, Z is ZetaChain, E is Ethereum.

• We invoke transferCrossChain on the EVMConnectedNFT contract on the BNB chain, with an input of 10.
• The EVMConnectedNFT contract on BNB does a costly computation and sends the result and some estimated gas fee to the ZetaChainUniversal contract on Zetachain.
• TheonCall function of the ZetaChainUniversal contract on Zetachain will be called. Here we swap the estimated gas fee with the targeting zrc tokens. Then, we approve the gateway to use our swapped zrc tokens. Lastly, we invoke gateway.withdrawAndCall, which sends parts of our funds there with a call command and arguments. (For the parts of our funds that we didn't send, these are the gas fee for Zetachain.)
• The onCall function of the EVMConnectedNFT contract on the Ethereum Chain will get invoked. It will either mint an NFT or do nothing based on the forwarded result we got from ZetachainUniversal. Lastly, we send the remaining gas back to the sender in the target token in the destination chain. In our case, it's ZRC20_ETHEREUM.

PoC 2: Token-Back

This PoC shows that it's possible to transfer messages cross chain back and forth. This allows the User to only connect to the expensive chain, acheiving our goal of computational arbitrage. The data flow is E -> Z -> B -> Z -> E, where B is BNB, Z is ZetaChain, E is Ethereum.

• We invoke transferCrossChain on the EVMConnectedNFT contract on the Ethereum chain, with an input of 10.
• The EVMConnectedNFT contract on Ethereum sends a request to BNB via the ZetaChainUniversal contract on Zetachain.
• The EVMConnectedNFT contract on BNB conducts a costly computation and sends the result back to the EVMConnectedNFT contract on Ethereum, potentially triggering a mint.

Notice that in both examples, whenever we invoke a cross chain call, we pass on the remaining gas fees. In PoC 2, after potentially triggering a mint on Ethereum, we send the remaining gas fees to the sender in ZRC20_ETH.

Top 3 Resources

Gateway EVM and ZEVM protocol contracts

Sample Universal Apps and Contracts to be deployed in External Chain

Validator Node Docs