Starknet: An NFT contract with Nile and L1 <-> L2 reputation messaging

• Starknet employs an account-based model and a distinct L1–L2 messaging architecture to enable cross-chain interactions with Ethereum.
• This tutorial walks through deploying a Starknet-based NFT contract (with a reputation system) and a corresponding L1 contract on Ethereum Sepolia.
• You’ll mint NFTs on L2, send messages from L2 to L1 (to withdraw reputation points), and send messages back from L1 to L2 (to raise an NFT’s reputation).
• Tools used include Cairo, OpenZeppelin’s Nile, the Starknet CLI, Remix, and MetaMask (for L1) plus ArgentX (for L2).

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Main article

• Get you familiar with the foundational Starknet concepts: the account model and the L1 <-> L2 messaging.
• Do a hands-on walkthrough with the tooling available on Starknet, the smart contracts, and the communication between the L1 Ethereum network and the L2 Starknet network.

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Prerequisites

• Chainstack account to deploy an Ethereum Sepolia node and a Starknet testnet node.
• Cairo to compile Starknet contracts and to interact with the network through the Starknet CLI.
• OpenZeppelin Nile to create and deploy an NFT contract.
• OpenZeppelin Cairo contracts to use the ERC-721 contract library in Cairo.
• MetaMask to deploy the L1 contract and interact with the contract.
• ArgentX to interact with the L2 NFT contract.

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Overview

1. With Chainstack, create a public chain project.
2. With Chainstack, deploy an Ethereum Sepolia testnet node.
3. With Chainstack, deploy a Starknet testnet node.
4. With Chainstack, access your Ethereum Sepolia testnet node credentials and your Starknet testnet node credentials.
5. Set up your MetaMask with the Ethereum Sepolia testnet node.
6. Deploy the L1 contract on the Ethereum Sepolia testnet and verify the contract.
7. With the Starknet CLI, deploy an account on the Starknet testnet.
8. With Nile, deploy the L2 NFT contract on the Starknet network.
9. Mint an NFT on L2.
10. Increase and decrease the reputation of the minted NFT on L2.
11. On L1, withdraw the reputation points of the minted L2 NFT.
12. On L1, increase the reputation of the minted L2 NFT.

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Step-by-step

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Create a public chain project

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Join the Ethereum Sepolia and Starknet testnets

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Get the node access and credentials

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Set up MetaMask

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Set up an account on Starknet

new_account --wallet starkware.starknet.wallets.open_zeppelin.OpenZeppelinAccount --network=alpha-sepolia

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Set up the L2 NFT contract

• Starknet L1 core contract — the default Starknet contract deployed on L1. See the official core contract addresses.
• Starknet L2 NFT contract — your tutorial contract in Cairo that you will deploy on the Starknet testnet. This contract will communicate with your Ethereum L1 contract.
• Ethereum L1 contract — your tutorial contract in Solidity that you will deploy on the Ethereum Sepolia testnet. This contract will:
  • Consume messages from your L2 contract through the Starknet L1 core contract.
  • Send messages to your L2 contract.

nile init

pip install cairo-nile openzeppelin-cairo-contracts

%lang starknet

from openzeppelin.token.erc721.ERC721_Mintable_Burnable import constructor

%lang starknet

from openzeppelin.token.erc721.ERC721_Mintable_Burnable import constructor

from starkware.cairo.common.alloc import alloc
from starkware.cairo.common.cairo_builtins import HashBuiltin
from starkware.starknet.common.messages import send_message_to_l1

const L1_CONTRACT_ADDRESS = (
    L1_MESSAGING_CONTRACT)
const MESSAGE_REP_DOWN = 0

# A mapping from a nftId (L1 Ethereum address) to their reputation.
@storage_var
func reputation(nftId : felt) -> (res : felt):
end

@view
func get_rep{syscall_ptr : felt*, pedersen_ptr : HashBuiltin*, range_check_ptr}(
    nftId : felt
) -> (reputation : felt):
    let (res) = reputation.read(nftId=nftId)
    return (res)
end

@external
func rep_up{syscall_ptr : felt*, pedersen_ptr : HashBuiltin*, range_check_ptr}(
    nftId : felt, points : felt
):
    let (res) = reputation.read(nftId=nftId)
    reputation.write(nftId, res + points)
    return ()
end

@external
func rep_down{syscall_ptr : felt*, pedersen_ptr : HashBuiltin*, range_check_ptr}(
    nftId : felt, points : felt
):

    let (res) = reputation.read(nftId=nftId)
    tempvar new_reputation = res - points

    # Update the new reputation.
    reputation.write(nftId, new_reputation)

    # Send the rep down message.
    let (message_payload : felt*) = alloc()
    assert message_payload[0] = MESSAGE_REP_DOWN
    assert message_payload[1] = nftId
    assert message_payload[2] = points
    send_message_to_l1(to_address=L1_CONTRACT_ADDRESS, payload_size=3, payload=message_payload)

    return ()
end

@l1_handler
func repUp{syscall_ptr : felt*, pedersen_ptr : HashBuiltin*, range_check_ptr}(
    from_address : felt, nftId : felt, points : felt
):
    # Make sure the message was sent by the intended L1 contract.
    assert from_address = L1_CONTRACT_ADDRESS

    # Read the current reputation.
    let (res) = reputation.read(nftId=nftId)

    # Compute and update the new reputation.
    tempvar new_reputation = res + points
    reputation.write(nftId, new_reputation)

    return ()
end

• The NFT part of the contract is based on the OpenZeppelin ERC721 Mintable Burnable library.
• nftId is used to identify the NFT token ID when it is minted.
• rep_up can be called by any contract on L2 to increase the reputation of the NFT.
• rep_down can be called by any contract on L2 to decrease the reputation of the NFT and send the reputation points as a message to the L1 contract.
• get_rep can be called by any contract on L2 to get the current reputation of the NFT.
• l1_handler processes the message from the L1 contract.

​

Set up and deploy the L1 contract

pragma solidity ^0.8.7;

interface IStarknetCore {
    /**
      Sends a message to an L2 contract.

      Returns the hash of the message.
    */
    function sendMessageToL2(
        uint256 toAddress,
        uint256 selector,
        uint256[] calldata payload
    ) external returns (bytes32);

    /**
      Consumes a message that was sent from an L2 contract.

      Returns the hash of the message.
    */
    function consumeMessageFromL2(uint256 fromAddress, uint256[] calldata payload)
        external
        returns (bytes32);
}

/**
  Contract for L1 <-> L2 interaction between an L2 Starknet contract and this L1 solidity
  contract.
*/
contract L1L2ERC721Rep {
    // The Starknet core contract.
    IStarknetCore starknetCore;

    mapping(uint256 => uint256) public nftIdIdleReputationPoints;

    uint256 constant MESSAGE_REP_DOWN = 0;

    // The selector of the "repUp" l1_handler.
    uint256 constant REPUP_SELECTOR =
        481301234104709516967081079511443560691305293629011359495317790738588668414;

    /**
      Initializes the contract state.
    */
    constructor(IStarknetCore starknetCore_) {
        starknetCore = starknetCore_;
    }

    function withdrawToIdle(
        uint256 l2ContractAddress,
        uint256 nftId,
        uint256 points
    ) external {
        // Construct the rep down message's payload.
        uint256[] memory payload = new uint256[](3);
        payload[0] = MESSAGE_REP_DOWN;
        payload[1] = nftId;
        payload[2] = points;

        // Consume the message from the Starknet core contract.
        // This will revert the (Ethereum) transaction if the message does not exist.
        starknetCore.consumeMessageFromL2(l2ContractAddress, payload);

        // Update the L1 reputation points of the NFT.
        nftIdIdleReputationPoints[nftId] += points;
    }

    function repUp(
        uint256 l2ContractAddress,
        uint256 nftId,
        uint256 points
    ) external {
        require(points < 2**64, "Invalid points.");
        require(points <= nftIdIdleReputationPoints[nftId], "Insufficient nftId's reputation points.");

        // Update the L1 reputation points of the NFT.
        nftIdIdleReputationPoints[nftId] -= points;

        // Construct the repUp message's payload.
        uint256[] memory payload = new uint256[](2);
        payload[0] = nftId;
        payload[1] = points;

        // Send the message to the Starknet core contract.
        starknetCore.sendMessageToL2(l2ContractAddress, REPUP_SELECTOR, payload);
    }
}

• The contract has the interface to communicate with the L1 Starknet core contract.
• The constructor of the contract takes the L1 Starknet core contract address.
• The withdrawToIdle external function withdraws the NFT reputation points sent to L1 using the L2 rep_down function. Mechanically, once you hit the L2 rep_down function, the L2 state is pushed as a message into the L1 Starknet core contract. The rep_down message then sits in the L1 Starknet core contract until it gets consumed by triggering the withdrawToIdle function in your L1 contract.
• The contract has the repUp external function of the L2 contract computed for REPUP_SELECTOR. To compute:

1. Create a Python script with the function name (repUp):
   py

   Copy

   from starkware.starknet.compiler.compile import \
       get_selector_from_name
   
   print(get_selector_from_name('repUp'))
   

2. Run the script. The reputation points withdrawn to an account through withdrawToIdle can then be used in the L1 repUp function to send a message to the L2 NFT contract to increase the NFT reputation. If this seems a little complicated at this point, do not worry—just keep following the tutorial and you will have a full hands-on walkthrough, which will help understand each of the steps better. In Remix, compile the L1L2ERC721Rep.sol contract. In Remix, on the deployment tab:
   • In Environment, select Injected Provider - Metamask.
   • In Contract, select L1L2ERC721Rep.
   • In Deploy, provide 0xde29d060D45901Fb19ED6C6e959EB22d8626708e. This is the address of the L1 Starknet core contract on the Ethereum Sepolia testnet.
   Clicking Deploy will engage your MetaMask instance connected to the Ethereum Sepolia testnet and deploy the contract.

1. Find the contract by the address and click Contract > Verify and Publish.
2. For Compiler Type, select Solidity (Single file).
3. For Compiler Version, select v0.8.7.
4. For Open Source License Type, select No License (None).
5. For Optimization, select No.
6. In the Solidity Contract Code field, paste the entirety of your L1L2ERC721Rep.sol contract.
7. Click Verify and Publish.

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Deploy the L2 NFT contract

nile compile

nile deploy CONTRACT_NAME NFT_NAME_INT NFT_TICKER_INT OWNER_ADDRESS --network sepolia

• CONTRACT_NAME — the name of the contract, which is NFT_with_reputation for this tutorial.
• NFT_NAME_INT — Nile accepts only integers for the deployment constructor. Provide any NFT collection name in integer format. See below for the conversion script.
• NFT_TICKER_INT — Any NFT collection ticker in integer format.

# string_to_integer.py
import sys

for arg in sys.argv[1:]:
    print(int.from_bytes(arg.encode(), byteorder="big"))

$ python3 string_to_integer.py NFTwithRep
369641944198362097149296

nile deploy NFT_with_reputation 369641944198362097149296 336187380304 0x02a152cb1a753a858c950bb710d957e7f3f78d87435831e7fae394f9233e60fa --network goerli

starknet tx_status --hash 0x635132bdc45cfe7f7f3b458bba7e739043f133ee1315ae1e67da70f7ed53782 --network=alpha-goerli

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Interact with the L2 contract

1. Use Voyager to open your deployed contract.
2. Click Write Contract.
3. Click Connect ArgentX Wallet and make sure you connect with the account address that you provided as the owner when deploying the contract.
4. In mint, provide a deployed account to mint an NFT to in the to field and any ID in tokenId. This will be the ID of the minted NFT that you will assign the reputation points to.
5. Click Transact.

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L2 -> L1 messaging

• Starknet core mainnet: 0xc662c410C0ECf747543f5bA90660f6ABeBD9C8c4
• Starknet core testnet: 0xde29d060D45901Fb19ED6C6e959EB22d8626708e

1. Use Voyager to open your deployed contract.
2. Click Write Contract.
3. In rep_up, provide the ID of your minted NFT in nftId and any integer for reputation points in points. For example, assign 1500.
4. Click Transact.

1. Use Voyager to open your deployed contract.
2. Click Write Contract.
3. In rep_down, provide the ID of your minted NFT in nftId and any integer for reputation points in points. For example, remove 700 points.
4. Click Transact.

• RECEIVED — the transaction is received by the Starknet node.
• PENDING – the transaction is valid and is in a pending block.
• ACCEPTED_ON_L2 – the transaction is in an accepted block on L2.
• ACCEPTED_ON_L1 — the transaction is pushed onto L2 as a message and ready to be consumed on L1.

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L1 -> L2 messaging

1. Use Etherscan to open your contract.
2. Click Write Contract > Connect to Web3.
3. In withdrawToIdle, provide:
   • l2ContractAddress — the address of your NFT contract deployed on L2
   • nftId — the ID of the minted NFT. For this tutorial, it is 1.
   • points — the number of reputation points you removed from the NFT contract on L2. For this tutorial, it is 700.
4. Click Write.

1. Use Etherscan to open your contract.
2. Click Write Contract > Connect to Web3.
3. In repUp, provide:
   • l2ContractAddress — the address of your NFT contract deployed on L2
   • nftId — the ID of the minted NFT. For this tutorial, it is 1.
   • points — the number of reputation points you want to put back in the NFT reputation on the L2 contract. For this tutorial, it is 300.
4. Click Write.

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Conclusion

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About the author

Ake

Director of Developer Experience @ Chainstack Talk to me all things Web320 years in technology | 8+ years in Web3 full time years experienceTrusted advisor helping developers navigate the complexities of blockchain infrastructure