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Bridging assets

Bridging assets

Introduction

Bridging is implemented by having two contracts (one deployed to L1, and the second deployed to L2) communicating with each other using L1 <-> L2 interoperability.

Developers are free to build their own bridge for any token. However, we provide our default bridges (one for ETH and one for ERC20 tokens), which can be used for basic bridging.

Note

Addresses of tokens on L2 will always differ from the same token L1 address.

Default bridges

You can get the default bridge addresses using the zks_getBridgeContracts endpoint or getDefaultBridgeAddresses method of Provider. Similar methods are available in the other SDKs.

Add tokens to the bridge

While the zkSync standard bridge can be used without permission from a smart contract perspective, the UI only displays tokens that have been added to our SDK.

If you would like to add a token, submit a request by filling out the token request formopen in new window. Our team will review your request, and get back to you if we have any outstanding questions before adding the token to our list.

Deposits (to L2)

Users must call the deposit method on the L1 bridge contract, which triggers the following actions:

  • The user's L1 tokens will be sent to the L1 bridge and become locked there.
  • The L1 bridge initiates a transaction to the L2 bridge using L1 -> L2 communication.
  • Within the L2 transaction, tokens will be minted and sent to the specified address on L2.
    • If the token does not exist on zkSync yet, a new contract is deployed for it. Given the L2 token address is deterministic (based on the original L1 address, name and symbol), it doesn't matter who is the first person bridging it, the new L2 address will be the same.
  • For every executed L1 -> L2 transaction, there will be an L2 -> L1 log message confirming its execution.
  • Lastly, the finalizeDepositmethod is called and it finalizes the deposit and mints funds on L2.

Deposit ETH

Here is an example of how to deposit ETH with the deposit method from the Deployer class.

import { Wallet, Provider, utils } from "zksync-web3";
import * as ethers from "ethers";
import { HardhatRuntimeEnvironment } from "hardhat/types";
import { Deployer } from "@matterlabs/hardhat-zksync-deploy";

// load env file
import dotenv from "dotenv";
dotenv.config();

// HTTPS RPC endpoint
const MAINNET_RPC_ENDPOINT = "";

// Amount in ETH
const AMOUNT = "0.1";

const WALLET_PRIV_KEY = process.env.WALLET_PRIV_KEY || "";

if (!WALLET_PRIV_KEY) {
  throw new Error("Wallet private key is not configured in env file");
}

export default async function (hre: HardhatRuntimeEnvironment) {
  console.log(`Running script to deposit ETH in L2`);

  // Initialize the wallet.
  const provider = new Provider(MAINNET_RPC_ENDPOINT);

  const wallet = new Wallet(WALLET_PRIV_KEY, provider);

  // Create deployer object
  const deployer = new Deployer(hre, wallet);

  // Deposit ETH to L2
  const depositHandle = await deployer.zkWallet.deposit({
    to: deployer.zkWallet.address,
    token: utils.ETH_ADDRESS,
    amount: ethers.utils.parseEther(AMOUNT),
  });
  console.log(`Deposit transaction sent ${depositHandle.hash}`);
  console.log(`Waiting for deposit to be processed in L2...`);
  // Wait until the deposit is processed on zkSync Era
  await depositHandle.wait();
  console.log(`ETH available in L2`);
}

Deposit ERC20 tokens

To deposit ERC20 tokens, use the same method but pass the approveERC20: true option. Here's an example:

import { Wallet, Provider } from "zksync-web3";
import * as ethers from "ethers";
import { HardhatRuntimeEnvironment } from "hardhat/types";
import { Deployer } from "@matterlabs/hardhat-zksync-deploy";

// load env file
import dotenv from "dotenv";
dotenv.config();

// HTTPS RPC endpoint
const MAINNET_RPC_ENDPOINT = "";

// Token address
const TOKEN_ADDRESS = "";

// Amount of tokens
const AMOUNT = "5";

const WALLET_PRIV_KEY = process.env.WALLET_PRIV_KEY || "";

if (!WALLET_PRIV_KEY) {
  throw new Error("Wallet private key not configured in env file");
}

export default async function (hre: HardhatRuntimeEnvironment) {
  console.log(`Running script to bridge ERC20 to L2`);

  // Initialize the wallet.
  const provider = new Provider(
    // @ts-ignore
    MAINNET_RPC_ENDPOINT || hre.config.networks.zkSyncTestnet.ethNetwork
  );

  const wallet = new Wallet(WALLET_PRIV_KEY, provider);

  // Create deployer object
  const deployer = new Deployer(hre, wallet);

  // Deposit ERC20 tokens to L2
  const depositHandle = await deployer.zkWallet.deposit({
    to: deployer.zkWallet.address,
    token: TOKEN_ADDRESS,
    amount: ethers.utils.parseEther(AMOUNT), // assumes ERC20 has 18 decimals
    // performs the ERC20 approve action
    approveERC20: true,
  });

  console.log(`Deposit transaction sent ${depositHandle.hash}`);
  console.log(`Waiting for deposit to be processed in L2...`);
  // Wait until the deposit is processed on zkSync
  await depositHandle.wait();
  console.log(`ERC20 tokens available in L2`);
}

To run this script, configure your hardhat.config.ts file as explained in this guide, or use the command npx zksync-cli create-project PROJECT_NAME to scaffold a new project.

Once your hardhat.config.ts file is configured, place the script files in the deploy folder and run them with the following command:

yarn hardhat deploy-zksync --script SCRIPT_FILENAME.ts

The log message described above is not yet fully supported by our SDK but is available on the L1 bridge contract.

Withdrawals (to L1)

Tips

  • To provide additional security during the Alpha phase, withdrawals in zkSync Era take 24 hours.
  • For more information, read the withdrawal delay guide.

Users must call the withdraw method on the L2 bridge contract, which will trigger the following actions:

  • L2 tokens will be burned.
  • An L2 -> L1 message with the information about the withdrawal will be sent.
  • After that, the withdrawal action will be available to be finalized by anyone in the L1 bridge (by proving the inclusion of the L2 -> L1 message, which is done when calling the finalizeWithdrawal method on the L1 bridge contract).
  • After the method is called, the funds are unlocked from the L1 bridge and sent to the withdrawal recipient.

Note

On the testnet environment, we automatically finalize all withdrawals, i.e., for every withdrawal, we will take care of it by making an L1 transaction that proves the inclusion for each message.

Custom bridges on L1 and L2

To build a custom bridge, create a regular Solidity contract which extends the correct interface for the layer. The interfaces provide access to the zkSync Era SDK deposit and withdraw implementations.