# Storing array of random numbers from Chainlink VRFv2 for later use

**URL:** <https://forum.openzeppelin.com/t/storing-array-of-random-numbers-from-chainlink-vrfv2-for-later-use/28769>\
**Category:** Smart Contracts\
**Created:** [May 16, 2022, 3:53pm UTC](https://forum.openzeppelin.com/t/storing-array-of-random-numbers-from-chainlink-vrfv2-for-later-use/28769 "2022-05-16T15:53:46Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![luxxoh](https://sea2.discourse-cdn.com/flex022/user_avatar/forum.openzeppelin.com/luxxoh/32/14659_2.png) [@luxxoh](https://forum.openzeppelin.com/u/luxxoh)\
**Post date:** [May 16, 2022, 3:53pm UTC](https://forum.openzeppelin.com/t/storing-array-of-random-numbers-from-chainlink-vrfv2-for-later-use/28769/1 "2022-05-16T15:53:46Z")

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I have a scenario in which I need to generate some on-chain random attribute for an NFT after it's minted with the max supply being 10k tokens. In the testing I've done on Rinkeby, if I call Chainlink VRFv2 to request one new random number for each mint it's costing around .35 LINK (so ~$2.50 at time of writing), meaning after all 10k are minted I would have spent $25k in gas.

Instead of calling Chainlink during each mint, couldn't I pre-request the random numbers (e.g before the mint starts) and do something like the following?:

```auto
// Array to store all random numbers
uint256[] private allRandomNums;

// Call this before minting begins
function requestAllRandomNumbers() public onlyOwner {
      COORDINATOR.requestRandomWords(
            keyHash,
            subscriptionId,
            10, // confirmations
            250000, // callback gas limit
            10000 // numwords
        );
    }

function fulfillRandomWords(uint256 requestId, uint256[] memory randomWords) internal virtual override {
      allRandomNums = randomWords;
}

function mint() public {
    uint256 tokenId = totalSupply();
    _mint(...);

    uint256 randomNum = allRandomNums[tokenId];
    // Set some attribute randomly using randomNum
    
}

```

Would this leave the door open for a reroll attack since the minting and attribute setting are all done within `mint()` (ala Meebits)? Is there anything else I am overlooking?

Tagging you @PatrickAlphaC since you seem to be the go-to for Chainlink questions! Any insight would be greatly appreciated.

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<div class="post-metadata">

**Author:** ![PatrickAlphaC](https://sea2.discourse-cdn.com/flex022/user_avatar/forum.openzeppelin.com/patrickalphac/32/4556_2.png) [@PatrickAlphaC](https://forum.openzeppelin.com/u/PatrickAlphaC)\
**Post date:** [May 16, 2022, 4:10pm UTC](https://forum.openzeppelin.com/t/storing-array-of-random-numbers-from-chainlink-vrfv2-for-later-use/28769/2 "2022-05-16T16:10:38Z")

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Yep, this leaves the door open for a reroll attack ☹

Someone could easily see all the random numbers and try to compute for whatever number they want. Who is minting the NFTs here? Is it the users?

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<div class="post-metadata">

**Author:** ![luxxoh](https://sea2.discourse-cdn.com/flex022/user_avatar/forum.openzeppelin.com/luxxoh/32/14659_2.png) [@luxxoh](https://forum.openzeppelin.com/u/luxxoh)\
**Post date:** [May 16, 2022, 4:22pm UTC](https://forum.openzeppelin.com/t/storing-array-of-random-numbers-from-chainlink-vrfv2-for-later-use/28769/3 "2022-05-16T16:22:28Z")

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@PatrickAlphaC Yeah they would be user-minted. Would a valid workaround be to have the random attribute set in a separate transaction, such as a "reveal" function (although obviously, this would incur additional gas on the part of the person minting)?

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<div class="post-metadata">

**Author:** ![PatrickAlphaC](https://sea2.discourse-cdn.com/flex022/user_avatar/forum.openzeppelin.com/patrickalphac/32/4556_2.png) [@PatrickAlphaC](https://forum.openzeppelin.com/u/PatrickAlphaC)\
**Post date:** [May 16, 2022, 8:47pm UTC](https://forum.openzeppelin.com/t/storing-array-of-random-numbers-from-chainlink-vrfv2-for-later-use/28769/4 "2022-05-16T20:47:07Z")

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Yes... but depending on how you do the reveal it could be "un-randomized".