ERC721 NFT Bridge

Scroll Alpha Testnet is now deprecated.

Please visit our new documentation for the Scroll Sepolia Testnet at https://docs.scroll.io/

Deposit ERC721 tokens from Goerli

NFT bridging from Goerli to Scroll Alpha is done via the L1ERC721Gateway contract instead of using a router. The contract address is deployed on Scroll Alpha at 0x1C441Dfc5C2eD7A2AA8636748A664E59CB029157. Similarly to bridging ERC20 tokens, we use the depositERC721 function to send tokens to Scroll Alpha, and we can later retrieve them back to Goerli using withdrawERC721 on the L2ERC721Gateway contract deployed on Scroll.

NFT contracts on both Goerli and Scroll 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.

depositERC721 is a payable function. The amount of ETH sent to this function will be used to pay for L2 fees. If the amount is not enough, the transaction will not be sent. All excess eth will be sent back to the sender. 0.00001 ETH should be more than enough to process a token deposit.

Creating a ScrollERC721 token on L2

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has to grant permission to the Gateway on Scroll Alpha to mint when a token is deposited and burn when the token is withdrawn.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

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 Goerli and Scroll Alpha 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 deployed at 0x8Fee20e0C0Ef16f2898a8073531a857D11b9C700 is used to send tokens from Scroll Alpha to Goerli. 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.

withdrawERC721 is a payable function, and the amount of ETH sent to this function will be used to pay for L1 fees. If the amount is not enough, the transaction will not be sent. All excess ETH will be sent back to sender. Fees depend on Goerli activity but 0.005 ETH should be enough to process a token withdrawal.

Make sure the transaction won't revert on Goerli while sending from Scroll Alpha. There is no way to recover the NFT bridged if you're transaction reverts on Goerli. All assets are burnt on Scroll Alpha when the transaction is sent, and it's impossible to mint them again.

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 Goerli to a recipient's account on Scroll Alpha.

Parameter	Description
token	The address of ERC721 NFT contract on Goerli.
to	The address of recipient's account on Scroll Alpha.
tokenId	The NFT id to deposit.
gasLimit	Gas limit required to complete the deposit on Scroll Alpha.

updateTokenMapping

function updateTokenMapping(
    address _l1Token,
    address _l2Token
) external;

Update the mapping that connects an NFT contract from Goerli to Scroll Alpha.

Parameter	Description
_l1Token	The address of ERC721 token in L1.
_l2Token	The address of corresponding ERC721 token in L2.

L2ERC721Gateway API

withdrawERC721

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

Send an ERC721 NFT from Scroll Alpha to a recipient's account on Goerli.

Parameter	Description
token	The address of ERC721 NFT token contract on Scroll Alpha.
to	The address of recipient's account on Goerli.
tokenId	The NFT id to deposit.
gasLimit	Unused, but included for potential forward compatibility considerations.

updateTokenMapping

function updateTokenMapping(
    address _l1Token,
    address _l2Token
) external;

Update the mapping that connects an NFT contract from Scroll Alpha to Goerli.

Parameter	Description
_l2Token	The address of ERC721 token in L2.
_l1Token	The address of corresponding ERC721 token in L1.