ERC721 NFT Bridge

Deposit ERC721 tokens from L1

NFT bridging from L1 to L2 is done via the L1ERC721Gateway contract instead of using a router. Similarly to bridging ERC20 tokens, we use the depositERC721 function to send tokens to L2, and we can later retrieve them back to L1 using withdrawERC721 on the L2ERC721Gateway contract deployed on L2.

NFT contracts on both L1 and l2 must be launched and connected through the Gateways to enable bridging. This means deposit and withdraw transactions will revert if a contract on either L1 or L2 is missing or not mapped through the updateTokenMapping function.

Creating a ScrollERC721 token on L2

To deposit an ERC721 token to L2, a token contract compatible with the IScrollERC721 standard has to be launched and mapped on a L1ERC721Gateway and L2ERC721Gateway on both L1 and L2, respectively. This contract has to grant permission to the Gateway on L2 to mint when a token is deposited and burn when the token is withdrawn.

The following interface is the IScrollERC721 needed for deploying ERC721 tokens compatible with the L2ERC721Gateway on L2.

interface IScrollERC721 {
  /// @notice Return the address of Gateway the token belongs to.
  function gateway() external view returns (address);

  /// @notice Return the address of counterpart token.
  function counterpart() external view returns (address);

  /// @notice Mint some token to recipient's account.
  /// @dev Gateway Utilities, only gateway contract can call
  /// @param _to The address of recipient.
  /// @param _tokenId The token id to mint.
  function mint(address _to, uint256 _tokenId) external;

  /// @notice Burn some token from account.
  /// @dev Gateway Utilities, only gateway contract can call
  /// @param _tokenId The token id to burn.
  function burn(uint256 _tokenId) external;
}

Adding ERC721 NFTs to the Scroll Bridge

All assets can be bridged securely and permissionlessly through Gateway contracts deployed by any developer. However, Scroll also manages an ERC721 Gateway contract where all NFTs created by the community are welcome. Being part of the Scroll-managed Gateway means you won’t need to deploy the Gateway contracts, and your token will appear in the Scroll frontend. To be part of the Scroll Gateway, you must contact the Scroll team to add the NFT on both L1 and L2 Gateway contracts. To do so, follow the instructions on the token lists repository to add a new token to the Scroll official frontend.

Withdraw ERC721 tokens from Scroll

The L2ERC721Gateway contract is used to send tokens from L2 to L1. Before bridging, the L2ERC721Gateway contract has to be approved by the NFT contract. Once that is done, withdrawERC721 can be called to perform the asset bridge.

L1ERC721Gateway API

Please visit the npm library for the complete Scroll contract API documentation.

depositERC721

function depositERC721(address _token, address _to, uint256 _tokenId, uint256 _gasLimit) external payable;

Deposit an ERC721 NFT from L1 to a recipient’s account on L2.

updateTokenMapping

function updateTokenMapping(address _l1Token, address _l2Token) external;

Update the mapping that connects an NFT contract from L1 to L2.

L2ERC721Gateway API

withdrawERC721

function withdrawERC721(address _token, address _to, uint256 _tokenId, uint256 _gasLimit) external payable;

Send an ERC721 NFT from L2 to a recipient’s account on L1.

updateTokenMapping

function updateTokenMapping(address _l1Token, address _l2Token) external;

Update the mapping that connects an NFT contract from L2 to L1.