Building Universal Apps
Universal App Basics

A universal app is a smart contract on ZetaChain that is natively connected to other blockchains like Ethereum, BNB and Bitcoin.

Unlike a regular contract, a universal app can accept contract calls, messages and token transfers from any connected chain. It can also trigger contract calls, and make token transfers on connected chains. These abilities enable universal apps to orchestrate complex multi-step transactions that span across chains.

For example, a Bitcoin user can interact with a universal app to send USDC to a recipient on Ethereum. An Ethereum user can buy an NFT on ZetaChain and send it to to their account on the BNB chain.

Universal apps are deployed on ZetaChain's universal EVM, which extends the EVM with omnichain interoperability features. This means your existing contracts will work out of the box on ZetaChain, and with some modifications, they can gain powerful omnichain capabilities.

Here's an example universal app:

pragma solidity 0.8.7;
import "@zetachain/protocol-contracts/contracts/zevm/interfaces/zContract.sol";
contract UniversalApp is zContract {
    function onCrossChainCall(
        zContext calldata context,
        address zrc20,
        uint256 amount,
        bytes calldata message
    ) external virtual override {
        // ...

Users can call universal apps by interacting with a Gateway contract on a connected chain. Each connected chain has a single Gateway contract that exposes methods to deposit tokens to and call universal apps. Users can pass both data and tokens when calling universal apps.

Gateway is an upcoming unified interface that will replace the TSS address and ERC-20 custody contract. Gateway is scheduled for release towards the end of Q3 2024.

In this example an Ethereum user sends 1 ETH and a message "hello" to a universal app:

A call to a universal app triggers an onCrossChainCall method.

A universal app receives:

  • a message (in this example, "hello") containing arbitrary data. A message can include recipient address, a token address, properties of an NFT to be minted, etc.
  • tokens represented as ZRC-20 (in this example, 1 ZRC-20 ETH).

Each native gas and supported ERC-20 token from connected chains has a corresponding ZRC-20 token on ZetaChain. ZRC-20 tokens are ERC-20-compatible and can be permissionlessly transferred back (withdrawn) to their original chain (ZRC-20 ETH on ZetaChain becomes ETH on Ethereum).

onCrossChainCall also has access to additional context information such as the original sender address and chain ID.

Universal apps can also initiate token transfers and contract calls to connected chains.

In this example a universal app:

  • accepts 6 ETH and "I want BNB" (bytes that represent the destination token) from Ethereum
  • swaps 6 ZRC-20 ETH for ZRC-20 BNB using a decentralized exchange on ZetaChain
  • calls the Gateway contract to withdraw ZRC-20 BNB and call a contract on the BNB chain

Of course, a single universal app call can trigger more than one call to different chains.

With a universal app a complex sequence of transactions and value transfers that span multiple blockchains can be triggered by a single user-signed transaction.

Universal apps are fully-compatible with Bitcoin. The same universal app contract can receive calls from any connected chain.

Users can call universal apps from the Bitcoin network by sending BTC and data to the Gateway address. To make a call to a universal app a Bitcoin user only needs to sign a single transaction using their wallet of choice. Universal apps offer gas abstraction, so end-user are not required to have an account on chains other than Bitcoin or acquire gas tokens.

Incoming calls to universal apps from connected chains do not incur additional gas costs aside from the initial interaction with the Gateway contract.

Outgoing calls from universal apps (like contract calls and token withdrawals) to connected chains require gas. Universal EVM offers tools to query gas, and it's up to each universal app to make sure that the contract has the right amount of ZRC-20 gas token to cover the gas fees.

In practice it means that a universal app would swap a fraction of the incoming ZRC-20 token to the ZRC-20 of the gas token of the chain to which an outgoing call is made. When an outgoing call or withdrawal is made, a required amount of ZRC-20 gas token is deducted to cover the gas fees on the destination chain.

For end user this system abstracts away the complexities of cross-chain gas. As long as the user provides a sufficient amount as input (in the token of their choosing), the gas fees will be covered.

  • Universal apps:
    • can receive contract calls and tokens from users and contracts on connected chains
    • can trigger contract calls and token transfers to connected chains
    • can automatically handle gas for cross-chain transactions
    • are fully compatible with EVM chains (like Ethereum and BNB), Bitcoin. Support for Polygon, Solana, Cosmos (through IBC) and other chains is coming soon.
  • Native gas and supported ERC-20 tokens are represented as ZRC-20 tokens. A ZRC-20 token can be permissionlessly withdrawn back to its original chain.