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Complete Guide
Quick Answer
Chainlink VRF provides verifiable randomness that cannot be manipulated by validators. VRF V2.5 with direct funding costs 0.25–2.5 LINK per request (~$2.50–$25 at $10/LINK). For 10,000-item NFT collections: use one VRF request and derive all traits from a single seed to reduce cost to ~$25 total. Alternative: use the subscription model for pre-funded, cost-efficient requests.
Why Not Use block.prevrandao or block.timestamp?
block.prevrandao: validators can slightly influence this value. For high-value randomness: compromised.
block.timestamp: validators can adjust by ±15 seconds. Exploitable for timestamp-dependent randomness.
Chainlink VRF: Generates randomness off-chain with a cryptographic proof. The on-chain verifier confirms the proof before accepting the random value. Cannot be manipulated by validators or anyone else.
VRF V2.5 Direct Funding (No Subscription)
Example
// VRF V2.5: Direct Funding model (pay per request in LINK)
import {VRFConsumerBaseV2Plus} from "@chainlink/contracts/src/v0.8/vrf/dev/VRFConsumerBaseV2Plus.sol";
import {VRFV2PlusClient} from "@chainlink/contracts/src/v0.8/vrf/dev/libraries/VRFV2PlusClient.sol";
contract NFTReveal is VRFConsumerBaseV2Plus, ERC721 {
// Chainlink VRF configuration
address constant VRF_COORDINATOR = 0x9DdfaCa8183c41ad55329BdeeD9F6A8d53168B1B; // Mainnet
bytes32 constant KEY_HASH = 0x8af398995b04c28e9951adb9721ef74c74f93e6a478f39e7e0777be13527e7ef;
uint32 constant CALLBACK_GAS_LIMIT = 100_000;
uint16 constant REQUEST_CONFIRMATIONS = 3;
mapping(uint256 => uint256) public requestIdToTokenId;
mapping(uint256 => uint256) public tokenIdToRevealSeed;
constructor() VRFConsumerBaseV2Plus(VRF_COORDINATOR) ERC721("MyNFT", "MNFT") {}
// After mint: request VRF for this token's trait assignment
function requestReveal(uint256 tokenId) external {
require(ownerOf(tokenId) == msg.sender, "Not owner");
require(tokenIdToRevealSeed[tokenId] == 0, "Already revealed");
uint256 requestId = s_vrfCoordinator.requestRandomWords(
VRFV2PlusClient.RandomWordsRequest({
keyHash: KEY_HASH,
subId: 0,
requestConfirmations: REQUEST_CONFIRMATIONS,
callbackGasLimit: CALLBACK_GAS_LIMIT,
numWords: 1, // One random number per token
extraArgs: VRFV2PlusClient._argsToBytes(
VRFV2PlusClient.ExtraArgsV1({nativePayment: false}) // Pay in LINK
)
})
);
requestIdToTokenId[requestId] = tokenId;
emit RevealRequested(tokenId, requestId);
}
// Chainlink calls this with the random number
function fulfillRandomWords(
uint256 requestId,
uint256[] calldata randomWords
) internal override {
uint256 tokenId = requestIdToTokenId[requestId];
tokenIdToRevealSeed[tokenId] = randomWords[0];
emit TokenRevealed(tokenId, randomWords[0]);
}
// Derive traits from the random seed
function getTraits(uint256 tokenId) public view returns (
string memory background,
string memory body,
string memory accessory
) {
uint256 seed = tokenIdToRevealSeed[tokenId];
require(seed != 0, "Not revealed");
// Use different slices of the seed for different traits
uint256 backgroundSeed = uint256(keccak256(abi.encode(seed, "background"))) % 100;
uint256 bodySeed = uint256(keccak256(abi.encode(seed, "body"))) % 100;
uint256 accessorySeed = uint256(keccak256(abi.encode(seed, "accessory"))) % 100;
background = _getBackground(backgroundSeed);
body = _getBody(bodySeed);
accessory = _getAccessory(accessorySeed);
}
function _getBackground(uint256 seed) internal pure returns (string memory) {
if (seed < 5) return "Gold"; // 5% chance
if (seed < 20) return "Purple"; // 15% chance
if (seed < 50) return "Blue"; // 30% chance
return "White"; // 50% chance
}
event RevealRequested(uint256 tokenId, uint256 requestId);
event TokenRevealed(uint256 tokenId, uint256 seed);
}VRF Subscription Model (Cost Efficient for Many Requests)
Example
// For projects with many VRF requests: use subscription to pre-fund
// Create subscription at vrf.chain.link → get subscriptionId
// Fund subscription with LINK → requests draw from the balance
contract SubscriptionVRF is VRFConsumerBaseV2Plus {
uint256 immutable subscriptionId;
constructor(uint256 _subscriptionId) VRFConsumerBaseV2Plus(VRF_COORDINATOR) {
subscriptionId = _subscriptionId;
}
function requestRandom() internal returns (uint256 requestId) {
return s_vrfCoordinator.requestRandomWords(
VRFV2PlusClient.RandomWordsRequest({
keyHash: KEY_HASH,
subId: subscriptionId, // Use subscription
requestConfirmations: 3,
callbackGasLimit: 100_000,
numWords: 1,
extraArgs: VRFV2PlusClient._argsToBytes(
VRFV2PlusClient.ExtraArgsV1({nativePayment: false})
)
})
);
}
}