Home/Blockchain/Verifiable Randomness/Implementation

🛠️ Implementation: Build a Lottery

Discover how to integrate Chainlink VRF in smart contracts

Generate provably fair random numbers on-chain

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Implementation Guide

Implementing verifiable randomness depends on your application's requirements: security level, cost constraints, and latency tolerance. Chainlink VRF offers the easiest integration for most projects, while commit-reveal is ideal for low-cost applications, and VDF provides maximum security for high-value scenarios.

Implementation Comparison

Choose your implementation approach based on security, cost, and complexity requirements.

Chainlink VRF v2

Complexity

Medium

Gas Cost

~200k gas

External Cost

$5-10 per request

Setup

Subscription required

Smart Contract Code

Solidity ^0.8.0

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;

import "@chainlink/contracts/src/v0.8/VRFConsumerBaseV2.sol";
import "@chainlink/contracts/src/v0.8/interfaces/VRFCoordinatorV2Interface.sol";

contract RandomNFT is VRFConsumerBaseV2 {
    VRFCoordinatorV2Interface COORDINATOR;
    uint64 subscriptionId;
    bytes32 keyHash;
    uint32 callbackGasLimit = 100000;
    uint16 requestConfirmations = 3;
    uint32 numWords = 1;
    
    mapping(uint256 => address) public requestToSender;
    
    constructor(uint64 _subscriptionId) 
        VRFConsumerBaseV2(0x271682DEB8C4E0901D1a1550aD2e64D568E69909) 
    {
        COORDINATOR = VRFCoordinatorV2Interface(
            0x271682DEB8C4E0901D1a1550aD2e64D568E69909
        );
        subscriptionId = _subscriptionId;
        keyHash = 0x8af398995b04c28e9951adb9721ef74c74f93e6a478f39e7e0777be13527e7ef;
    }
    
    function requestRandomNFT() external returns (uint256 requestId) {
        requestId = COORDINATOR.requestRandomWords(
            keyHash,
            subscriptionId,
            requestConfirmations,
            callbackGasLimit,
            numWords
        );
        requestToSender[requestId] = msg.sender;
        return requestId;
    }
    
    function fulfillRandomWords(
        uint256 requestId,
        uint256[] memory randomWords
    ) internal override {
        uint256 randomTokenId = randomWords[0] % 10000;
        address nftOwner = requestToSender[requestId];
        _mintNFT(nftOwner, randomTokenId);
    }
}

Gas Cost Estimator

Estimated Gas

100,000

gas units

Cost @ 50 gwei

$10.00

assuming $2000 ETH

Requests per ETH

200

at current gas price

📝 Best Practices

Choose the right solution

Low-value gaming: commit-reveal. High-value DeFi: Chainlink VRF. Maximum security: VDF

Handle callback failures

VRF callbacks can fail. Implement fallback mechanisms and re-request logic

Don't trust block.prevrandao alone

Validators can manipulate. Always combine with VRF or commit-reveal

Monitor subscription balance

Chainlink VRF requires funded subscription. Set up alerts to avoid service interruption

Production Checklist

Security Audit

Audit randomness implementation by reputable firm

Test on Testnet

Verify VRF requests and callbacks work correctly

Fallback Mechanisms

Handle edge cases: callback failures, non-reveals, timeouts

Monitoring & Alerts

Track request status, subscription balance, callback success rate

Documentation

Document randomness source and verification process for transparency

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