Verifiable Randomness Function (VRF)

Undeniably random numbers in your smart contract

A detailed example of how to use Orakl Network VRF can be found at example repository vrf-consumer.

What is Verifiable Randomness Function?

A Verifiable Randomness Function (VRF) is a cryptographic function that generates a random value, or output, based on some input data (called the "seed"). Importantly, the VRF output is verifiable, meaning that anyone who has access to the VRF output and the seed can verify that the output was generated correctly.

In the context of the blockchain, VRFs can be used to provide a source of randomness that is unpredictable and unbiased. This can be useful in various decentralized applications (dApps) that require randomness as a key component, such as in randomized auctions or as part of a decentralized games.

Orakl Network VRF allows smart contracts to use VRF to generate verifiably random values, which can be used in various dApps that require randomness. Orakl Network VRF can be used with two different account types that support prepayment method:

Permanent Account allows consumers to prepay for VRF services, and then use those funds when interacting with Orakl Network. Permanent account is currently a recommended way to request for VRF. You can learn more about prepayment payment method or permanent account, go to developer's guide on how to use Prepayment.

Temporary Account allows user to pay directly for VRF without any extra prerequisites. This approach is great for infrequent use, or for users that do not want to hassle with Temporary Account settings and want to use VRF as soon as possible.

In the rest of this document, we describe both Permanent Account and Temporary Account approaches that can be used to request VRF.

Permanent Account (recommended)

We assume that at this point you have already created permanent account through Prepayment smart contract, deposited $KAIA, and assigned consumer(s) to it. If not, please read how to do all the above, in order to be able to continue in this guide.

After you created account (and obtained accId), deposited some $KAIA and assigned at least one consumer, you can use it to request and fulfill random words.

User smart contract that wants to use Orakl Network VRF has to inherit from VRFConsumerBase abstract smart contract.

import "@bisonai/orakl-contracts/src/v0.1/VRFConsumerBase.sol";
contract VRFConsumer is VRFConsumerBase {
    ...
}

Initialization

VRF smart contract (VRFCoordinator) is used both for requesting random words and for request fulfillments as well. We recommend you to bond IVRFCoordinator interface with VRFCoordinator address passed as a constructor parameter, and use it for random words requests (requestRandomWords).

import "@bisonai/orakl-contracts/src/v0.1/VRFConsumerBase.sol";
import "@bisonai/orakl-contracts/src/v0.1/interfaces/IVRFCoordinator.sol";

contract VRFConsumer is VRFConsumerBase {
  IVRFCoordinator COORDINATOR;

  constructor(address coordinator) VRFConsumerBase(coordinator) {
      COORDINATOR = IVRFCoordinator(coordinator);
  }
}

Get estimated service fee

The estimateFee function calculates the estimated service fee for a random words request based on the provided parameters.

function estimateFee(
    uint64 reqCount,
    uint8 numSubmission,
    uint32 callbackGasLimit
) public view returns (uint256) {
    uint256 serviceFee = calculateServiceFee(reqCount) * numSubmission;
    uint256 maxGasCost = tx.gasprice * callbackGasLimit;
    return serviceFee + maxGasCost;
}

Let's understand the purpose and arguments of this function:

  • reqCount: This is a uint64 value representing the number of previous requests made. By providing the accId, you can obtain the reqCount by invoking the external function getReqCount() of the Prepayment contract

  • numSubmission: This is a uint8 value representing the number of submissions for the request. The value of numSubmission for a VRF request is always 1.

  • callbackGasLimit: This is a uint32 value representing the gas limit allocated for the callback function.

By calling the estimateFee function with the appropriate arguments, users can get an estimation of the total fee required for their request. This can be useful for spending required amount for each request.

Request random words

Request for random words must be called from a contract that has been approved through addConsumer function of Prepayment smart contract. If the smart contract has not been approved, the request is rejected through InvalidConsumer error. If account (specified by accId) does not exist (InvalidAccount error), does not have balance high enough, or uses an unregistered keyHash (InvalidKeyHash error) request is rejected as well.

function requestRandomWords(
    bytes32 keyHash,
    uint64 accId,
    uint32 callbackGasLimit,
    uint32 numWords
)
    public
    onlyOwner
    returns (uint256 requestId)
{
   requestId = COORDINATOR.requestRandomWords(
       keyHash,
       accId,
       callbackGasLimit,
       numWords
   );
}

Below, you can find an explanation of requestRandomWords function and its arguments defined at VRFCoordinator smart contract:

  • keyHash: a bytes32 value representing the hash of the key used to generate the random words, also used to choose a trusted VRF provider.

  • accId: a uint64 value representing the ID of the account associated with the request.

  • callbackGasLimit: a uint32 value representing the gas limit for the callback function that executes after the confirmations have been received.

  • numWords: a uint32 value representing the number of random words requested. (maximum 500)

The function call requestRandomWords() on COORDINATOR contract passes keyHash, accId, callbackGasLimit, and numWords as arguments. After a successful execution of this function, you obtain an ID (requestId) that uniquely defines your request. Later, when your request is fulfilled, the ID (requestId) is supplied together with random words to be able to make a match between requests and fulfillments when there is more than one request.

Fulfill random words

fulfillRandomWords is a virtual function of VRFConsumerBase smart contract, and therefore must be overridden. This function is called by VRFCoordinator when fulfilling the request. callbackGasLimit parameter defined during VRF request denotes the amount of gas required for execution of this function.

function fulfillRandomWords(
    uint256 /* requestId */,
    uint256[] memory randomWords
)
    internal
    override
{
    // requestId should be checked if it matches the expected request.
    // Generate random value between 1 and 50.
    sRandomWord = (randomWords[0] % 50) + 1;
}

The arguments of fulfillRandomWords function are explained below:

  • requestId: a uint256 value representing the ID of the request

  • randomWords: an array of uint256 values representing the random words generated in response to the request

This function is executed from previously defined COORDINATOR contract. After receiving random value(s) (randomWords) in range of uint256 data type, it takes the first random element and limits it to a range between 1 and 50. The result is saved in the storage variable s_randomResult.

Temporary Account

Temporary Account is an alternative type of account which does not require a user to create account, deposit $KAIA, and assign consumer before being able to utilize VRF functionality. Request for VRF with Temporary Account is only a little bit different compared to Permanent Account, however, the fulfillment function is exactly same.

User smart contract that wants to use Orakl Network VRF has to inherit from VRFConsumerBase abstract smart contract.

import "@bisonai/orakl-contracts/src/v0.1/VRFConsumerBase.sol";
contract VRFConsumer is VRFConsumerBase {
    ...
}

Initialization With Temporary Account

There is no difference in initializing VRF user contract that request for VRF with Permanent Account or Temporary Account.

VRF smart contract (VRFCoordinator) is used both for requesting random words and for request fulfillments as well. We recommend you to bond IVRFCoordinator interface with VRFCoordinator address passed as a constructor parameter, and use it for random words requests (requestRandomWords).

import "@bisonai/orakl-contracts/src/v0.1/VRFConsumerBase.sol";
import "@bisonai/orakl-contracts/src/v0.1/interfaces/IVRFCoordinator.sol";

contract VRFConsumer is VRFConsumerBase {
  IVRFCoordinator COORDINATOR;

  constructor(address coordinator) VRFConsumerBase(coordinator) {
      COORDINATOR = IVRFCoordinator(coordinator);
  }
}

Request random words with Temporary Account (consumer)

The request for random words using Temporary Account is very similar to request using Permanent Account. The only difference is that with a Temporary Account user has to send $KAIA together with call using value property. There are several checks that have to pass in order to successfully request for VRF. You can read about them in one of the previous subsections called Request random words.

function requestRandomWords(
    bytes32 keyHash,
    uint32 callbackGasLimit,
    uint32 numWords,
    address refundRecipient
)
    public
    payable
    onlyOwner
    returns (uint256 requestId)
{
  requestId = COORDINATOR.requestRandomWords{value: msg.value}(
    keyHash,
    callbackGasLimit,
    numWords,
    refundRecipient
  );
}

This function calls the requestRandomWords() function defined in COORDINATOR contract, and passes keyHash, callbackGasLimit, numWords and refundRecipient as arguments. The payment for service is sent through msg.value to the requestRandomWords() in COORDINATOR contract. If the payment is larger than expected payment, exceeding payment is returned to the refundRecipient address. Eventually, it generates a request for random words. To accurately specify msg.value for the requestRandomWords function, please refer to the explanation on how to estimate the service fee.

In the section below, you can find more detailed explanation of how request for random words using temporary account works.

Cancel request and receive refund (consumer)

In the previous section, we explained that $KAIA is sent together with request for VRF to VRFCoordinator which passes the $KAIA deposit to Prepayment contract. The $KAIA payment stays in the Prepayment contract until the request is fulfilled.

In rare cases, it is possible that request cannot be fulfilled, and consumer does not receive requested random words. To refund deposited $KAIA in such cases, one must first cancel request by calling cancelRequest inside of VRFCoordinator and then withdraw $KAIA (withdrawTemporary) from temporary account inside of Prepayment contract. In both cases, consumer smart contract has to be the sender (msg.sender). Our consumer smart contract therefore has to include such auxiliary function(s) to make appropriate calls. If we do not add such functions to consumer contract, it will not be possible to cancel request and withdraw funds deposited to temporary account. Deposited funds will be then forever locked inside of Prepayment contract.

The code listing below is an example of function inside of consumer contract to cancel and withdraw funds from temporary account.

function cancelAndWithdraw(
    uint256 requestId,
    uint64 accId,
    address refundRecipient
) external onlyOwner {
    COORDINATOR.cancelRequest(requestId);
    address prepaymentAddress = COORDINATOR.getPrepaymentAddress();
    IPrepayment(prepaymentAddress).withdrawTemporary(accId, payable(refundRecipient));
}

Request random words with Temporary Account (coordinator)

The following function is defined in VRFCoordinator contract.

function requestRandomWords(
    bytes32 keyHash,
    uint32 callbackGasLimit,
    uint32 numWords,
    address refundRecipient
) external payable nonReentrant onlyValidKeyHash(keyHash) returns (uint256) {
    uint64 reqCount = 0;
    uint8 numSubmission = 1;
    uint256 fee = estimateFee(reqCount, numSubmission, callbackGasLimit);
    if (msg.value < fee) {
        revert InsufficientPayment(msg.value, fee);
    }

    uint64 accId = sPrepayment.createTemporaryAccount(msg.sender);
    bool isDirectPayment = true;
    uint256 requestId = requestRandomWords(
        keyHash,
        accId,
        callbackGasLimit,
        numWords,
        isDirectPayment
    );
    sPrepayment.depositTemporary{value: fee}(accId);

    // Refund extra $KAIA
    uint256 remaining = msg.value - fee;
    if (remaining > 0) {
        (bool sent, ) = refundRecipient.call{value: remaining}("");
        if (!sent) {
            revert RefundFailure();
        }
    }

    return requestId;
}

This function first calculates a fee for the request by calling estimateFee() function. Then, it create a temporary account inside of Prepayment contract with sPrepayment.createTemporaryAccount(msg.sender) call. In the next step, we request for random words by requestRandomWords function. The function has several validation steps, therefore we included requesting for random words before depositing the required fee to the account (sPrepayment.depositTemporary{value: fee}(accId)). If the amount of $KAIA passed by msg.value to the requestRandomWords is larger than required fee, the remaining amount is sent back to the refundRecipient address. Finally, the function returns requestId that is generated by the internal requestRandomWords() call.

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