The Universal NFT and Token standards enable non-fungible tokens (ERC-721 NFTs) and fungible tokens (ERC-20) to be minted on any chain and seamlessly transferred between connected chains.

For simplicity, we'll refer to both Universal NFTs and Universal Tokens as "assets" throughout this document.

When transferring assets between chains, an asset is burned on the source chain. The asset's metadata and information are sent in a message to the asset contract on the destination chain, where a corresponding asset is minted.

Both Universal NFT and Universal Token projects consist of two contracts: Universal and Connected.

The Universal contract is deployed on ZetaChain. It is used to:

• Mint assets on ZetaChain
• Transfer assets from ZetaChain to a connected chain
• Handle incoming asset transfers from a connected chain to ZetaChain
• Handle asset transfers between connected chains

The Connected contract is deployed on one or more connected EVM chains. It is used to:

• Mint assets on a connected chain
• Transfer assets to another connected chain or ZetaChain
• Handle incoming asset transfers from ZetaChain or another connected chain

A Universal contract deployment on ZetaChain is required, while Connected contracts can be deployed as needed to enable asset transfers for specific chains.

A universal asset can be minted on any chain — ZetaChain or any connected EVM chain.

For Universal NFTs, when an asset is minted, it receives a persistent ID that is unique across all chains. This ID remains unchanged when the asset is transferred between chains.

Assets can be transferred from ZetaChain to a connected chain, from a connected chain to ZetaChain, and between connected chains. ZetaChain acts as a hub for cross-chain transactions, so all transfers are routed through it. For example, when transferring an asset from Ethereum to BNB, two cross-chain transactions are initiated: Ethereum → ZetaChain → BNB. This does not affect transfer time or cost, but it allows the system to scale linearly as more chains are added.

Cross-chain asset transfers support reverts. If the transfer fails on the destination chain, the asset is returned to the original sender on the source chain.

Asset contracts only accept cross-chain calls from trusted contracts. Each contract on a connected chain stores the address of the Universal contract on ZetaChain. The Universal contract, in turn, stores the addresses of the connected contracts on other chains. This ensures that only contracts from the same asset collection can participate in cross-chain transfers.

Here’s the flow using a Universal NFT as an example:

Deployment

1. Deploy the Universal contract on ZetaChain. This is a required step, as ZetaChain acts as the hub for all cross-chain transfers, even between connected EVM chains.
2. Deploy a Connected contract on a supported EVM chain (e.g., Ethereum, Base, Polygon, BNB).
3. On a universal contract on ZetaChain, run setConnected(zrc20, connectedAddress), where:
   • zrc20 is the ZRC-20 contract of the gas token for the target EVM chain. This acts as a chain identifier.
   • connectedAddress is the address of the Connected contract on the EVM chain (from step 2).
4. On the connected EVM chain, run ConnectedAsset.setUniversal(universalAddress), where universalAddress is the address of the Universal contract on ZetaChain (from step 1).

You now have trusted asset contracts deployed and linked on ZetaChain and a connected EVM chain.

To enable transfers to additional EVM chains, repeat steps 2 and 3 for each new chain.

setConnected and setUniversal are required to establish a secure trust relationship between the asset contracts. Each contract verifies that cross-chain calls originate from a trusted counterpart.

Gas Fees

• EVM → ZetaChain: No cross-chain fee is charged.
• ZetaChain → EVM: A cross-chain fee is paid in ZETA, based on the ZRC-20 withdraw fee for the destination chain. ZETA is automatically swapped to the destination chain's gas token ZRC-20 to cover execution.
• EVM → EVM: A cross-chain fee is paid in the gas token of the source chain. For example, if transferring an asset from Ethereum to BNB, the fee is paid in ETH. ZetaChain uses ZRC-20 ETH to cover the execution and swaps it for ZRC-20 BNB to call the destination chain.

Revert Handling

If a cross-chain transfer fails on the destination chain (e.g., due to out-of-gas, contract rejection, or network error), the asset is reverted to the original sender.

When a transfer between two connected EVM chains fails, the asset is reverted to the original sender on ZetaChain, rather than the original source chain. This avoids costly return operations to the EVM chain where the transfer started. The sender can then initiate a new transfer to the same or another chain.