This tutorial will guide you through the process of creating a smart contract on Base that utilizes Supra dVRF to serve random numbers using an onchain randomness generation mechanism directly within your smart contracts.

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Objectives

By the end of this tutorial you should be able to do the following:
  • Set up a smart contract project for Base using Foundry
  • Install the Supra dVRF as a dependency
  • Use Supra dVRF within your smart contract
  • Deploy and test your smart contracts on Base

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Prerequisites

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Foundry

This tutorial requires you to have Foundry installed.
  • From the command-line (terminal), run: curl -L https://foundry.paradigm.xyz | bash
  • Then run foundryup, to install the latest (nightly) build of Foundry
For more information, see the Foundry Book installation guide.

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Coinbase Wallet

In order to deploy a smart contract, you will first need a wallet. You can create a wallet by downloading the Coinbase Wallet browser extension.

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Wallet funds

Deploying contracts to the blockchain requires a gas fee. Therefore, you will need to fund your wallet with ETH to cover those gas fees. For this tutorial, you will be deploying a contract to the Base Sepolia test network. You can fund your wallet with Base Sepolia ETH using one of the faucets listed on the Base Network Faucets page.

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Supra wallet registration

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What is a Verifiable Random Function (VRF)?

A Verifiable Random Function (VRF) provides a solution for generating random outcomes in a manner that is both decentralized and verifiably recorded onchain. VRFs are crucial for applications where randomness integrity is paramount, such as in gaming or prize drawings. Supra dVRF provides a decentralized VRF that ensures that the outcomes are not only effectively random but also responsive, scalable, and easily verifiable, thereby addressing the unique needs of onchain applications for trustworthy and transparent randomness.

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Creating a project

Before you can begin writing smart contracts for Base, you need to set up your development environment by creating a Foundry project. To create a new Foundry project, first create a new directory: 
mkdir myproject
Then run: 
cd myproject
forge init
This will create a Foundry project, which has the following basic layout: 
.
├── foundry.toml
├── script
 │   └── Counter.s.sol
├── src
 │   └── Counter.sol
└── test
    └── Counter.t.sol

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Writing the Smart Contract

Once your Foundry project has been created, you can now start writing a smart contract. The Solidity code below defines a basic contract named RNGContract. The smart contract’s constructor takes in a single address and assigns it to a member variable named supraAddr. This address corresponds to the contract address of the Supra Router Contract that will be used to generate random numbers. The contract address of the Supra Router Contract on Base Sepolia testnet is 0x99a021029EBC90020B193e111Ae2726264a111A2. The contract also assigns the contract deployer (msg.sender) to a member variable named supraClientAddress. This should be the client wallet address that is registered and whitelisted to use Supra VRF (see: Prerequisites). 
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract RNGContract {
    address supraAddr;
    address supraClientAddress;

    constructor(address supraSC) {
        supraAddr = supraSC;
        supraClientAddress = msg.sender;
    }
}
In your project, add the code provided above to a new file named src/ExampleContract.sol, and delete the src/Counter.sol contract that was generated with the project. (you can also delete the test/Counter.t.sol and script/Counter.s.sol files). The following sections will guide you step-by-step on how to update your contract to generate random numbers using the Supra Router contract.

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Adding the Supra Router Contract interface

In order to help your contract (the requester contract) interact with the Supra Router contract and understand what methods it can call, you will need to add the following interface to your contract file. 
interface ISupraRouter {
	function generateRequest(string memory _functionSig, uint8 _rngCount, uint256 _numConfirmations, uint256 _clientSeed, address _clientWalletAddress) external returns(uint256);
	function generateRequest(string memory _functionSig, uint8 _rngCount, uint256 _numConfirmations, address _clientWalletAddress) external returns(uint256);
}
The ISupraRouter interface defines a generateRequest function. This function is used to create a request for random numbers. The generateRequest function is defined twice, because one of the definitions allows for an optional _clientSeed (defaults to 0) for additional unpredictability.
Alternatively, you can add the ISupraRouter interface in a separate interface file and inherit the interface in your contract.

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Adding a request function

Once you have defined the ISupraRouter, you are ready to add the logic to your smart contract for requesting random numbers. For Supra dVRF, adding logic for requesting random numbers requires two functions:
  • A request function
  • A callback function
The request function is a custom function defined by the developer. There are no requirements when it comes to the signature of the request function. The following code is an example of a request function named rng that requests random numbers using the Supra Router Contract. Add this function to your smart contract: 
function rng() external returns (uint256) {
    // Amount of random numbers to request
    uint8 rngCount = 5;
    // Amount of confirmations before the request is considered complete/final
    uint256 numConfirmations = 1;
    uint256 nonce = ISupraRouter(supraAddr).generateRequest(
        "requestCallback(uint256,uint256[])",
        rngCount,
        numConfirmations,
        supraClientAddress
    );
    return nonce;
    // store nonce if necessary (e.g., in a hashmap)
    // this can be used to track parameters related to the request in a lookup table
    // these can be accessed inside the callback since the response from supra will include the nonce
}
The rng function above requests 5 random numbers (defined by rngCount), and waits 1 confirmation (defined by numConfirmations) before considering the result to be final. It makes this request by calling the generateRequest function of the Supra Router contract, while providing a callback function with the signature requestCallback(uint256,uint256[]).

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Adding a callback function

As seen in the previous section, the generateRequest method of the Supra Router contract expects a signature for a callback function. This callback function must be of the form: uint256 nonce, uint256[] calldata rngList, and must include validation code, such that only the Supra Router contract can call the function. To do this, add the following callback function (requestCallback) to your smart contract: 
function requestCallback(uint256 _nonce ,uint256[] _rngList) external {
    require(msg.sender == supraAddr, "Only the Supra Router can call this function.");
    uint8 i = 0;
    uint256[] memory x = new uint256[](rngList.length);
    rngForNonce[nonce] = x;
    for(i=0; i<rngList.length;i++){
        rngForNonce[nonce][i] = rngList[i] % 100;
    }
}
Once a random number is generated, requestCallback is executed by the Supra Router. The code above stores the resulting random numbers list in a map named rngForNonce using the _nonce argument. Because of this, you will need to add the following mapping to your contract: 
mapping (uint256 => uint256[] ) rngForNonce;

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Adding a function to view the result

To fetch resulting random numbers based on their associated nonce, you add a third function: 
function viewRngForNonce(uint256 nonce) external view returns (uint256[] memory) {
    return rngForNonce[nonce];
}

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Final smart contract code

After following all the steps above, your smart contract code should look like the following: 
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

interface ISupraRouter {
    function generateRequest(string memory _functionSig, uint8 _rngCount, uint256 _numConfirmations, uint256 _clientSeed, address _clientWalletAddress) external returns (uint256);
    function generateRequest(string memory _functionSig, uint8 _rngCount, uint256 _numConfirmations, address _clientWalletAddress) external returns (uint256);
}

contract RNGContract {
    address supraAddr;
    address supraClientAddress;

    mapping (uint256 => uint256[]) rngForNonce;

    constructor(address supraSC) {
        supraAddr = supraSC;
        supraClientAddress = msg.sender;
    }

    function rng() external returns (uint256) {
        // Amount of random numbers to request
        uint8 rngCount = 5;
        // Amount of confirmations before the request is considered complete/final
        uint256 numConfirmations = 1;
        uint256 nonce = ISupraRouter(supraAddr).generateRequest(
            "requestCallback(uint256,uint256[])",
            rngCount,
            numConfirmations,
            supraClientAddress
        );
        return nonce;
    }

    function requestCallback(uint256 _nonce, uint256[] memory _rngList) external {
        require(msg.sender == supraAddr, "Only the Supra Router can call this function.");
        uint8 i = 0;
        uint256[] memory x = new uint256[](_rngList.length);
        rngForNonce[_nonce] = x;
        for (i = 0; i < _rngList.length; i++) {
            rngForNonce[_nonce][i] = _rngList[i] % 100;
        }
    }

    function viewRngForNonce(uint256 nonce) external view returns (uint256[] memory) {
        return rngForNonce[nonce];
    }
}
You must whitelist this smart contract under the wallet address you registered with Supra, and deposit funds to be paid for the gas fees associated with transactions for your callback function.Follow the guidance steps provided by Supra for whitelisting your contract and depositing funds.If you have not yet registered your wallet with Supra, see the Prerequisites section.

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Compiling the Smart Contract

To compile your smart contract code, run: 
forge build

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Deploying the smart contract

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Setting up your wallet as the deployer

Before you can deploy your smart contract to the Base network, you will need to set up a wallet to be used as the deployer. To do so, you can use the cast wallet import command to import the private key of the wallet into Foundry’s securely encrypted keystore: 
cast wallet import deployer --interactive
After running the command above, you will be prompted to enter your private key, as well as a password for signing transactions.
For instructions on how to get your private key from Coinbase Wallet, visit the Coinbase Wallet documentation.It is critical that you do NOT commit this to a public repo.
To confirm that the wallet was imported as the deployer account in your Foundry project, run: 
cast wallet list

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Setting up environment variables for Base Sepolia

To setup your environment for deploying to the Base network, create an .env file in the home directory of your project, and add the RPC URL for the Base Sepolia testnet, as well as the Supra Router contract address for Base Sepolia testnet: 
BASE_SEPOLIA_RPC="https://sepolia.base.org"
ISUPRA_ROUTER_ADDRESS=0x99a021029EBC90020B193e111Ae2726264a111A2
Once the .env file has been created, run the following command to load the environment variables in the current command line session: 
source .env

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Deploying the smart contract to Base Sepolia

With your contract compiled and environment setup, you are ready to deploy the smart contract to the Base Sepolia Testnet! For deploying a single smart contract using Foundry, you can use the forge create command. The command requires you to specify the smart contract you want to deploy, an RPC URL of the network you want to deploy to, and the account you want to deploy with. To deploy the RNGContract smart contract to the Base Sepolia test network, run the following command: 
forge create ./src/RNGContract.sol:RNGContract --rpc-url $BASE_SEPOLIA_RPC --constructor-args $ISUPRA_ROUTER_ADDRESS --account deployer
When prompted, enter the password that you set earlier, when you imported your wallet’s private key.
Your wallet must be funded with ETH on the Base Sepolia Testnet to cover the gas fees associated with the smart contract deployment. Otherwise, the deployment will fail.To get testnet ETH for Base Sepolia, see the prerequisites.
After running the command above, the contract will be deployed on the Base Sepolia test network. You can view the deployment status and contract by using a block explorer.

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Interacting with the Smart Contract

Foundry provides the cast command-line tool that can be used to interact with the smart contract that was deployed and call the getLatestPrice() function to fetch the latest price of ETH. To call the getLatestPrice() function of the smart contract, run: 
cast call <DEPLOYED_ADDRESS> --rpc-url $BASE_SEPOLIA_RPC "rng()"
You should receive a nonce value. You can use this nonce value to call the viewRngForNonce(uint256) function to get the resulting list of randomly generated numbers: 
cast call <DEPLOYED_ADDRESS> --rpc-url $BASE_SEPOLIA_RPC "viewRngForNonce(uint256)" <NONCE>

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Conclusion

Congratulations! You have successfully deployed and interacted with a smart contract that generates a list of random numbers using Supra dVRF on the Base blockchain network. To learn more about VRF and using Supra dVRF to generate random numbers within your smart contracts on Base, check out the following resources: