TwentyOne
First Flight #29
Beginner FriendlyGameFiFoundrySolidity
100 EXP
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Submission Details
Severity: low
Invalid

Weak Randomness in Card Generation

sauronsol

Summary:

The TwentyOne contract uses predictable values (block.timestamp, msg.sender, block.prevrandao)
for random card generation which can be manipulated.

Vulnerability Details:

The following test demonstrates that card generation is predictable:

function test_PredictableRandomness() public {
// Setup
vm.startPrank(player1);
// Set fixed values that attacker can predict
uint256 fixedTimestamp = 1234;
uint256 fixedPrevrandao = 5678;
// Set block values before game starts
vm.warp(fixedTimestamp);
vm.prevrandao(fixedPrevrandao);
// Start game
twentyOne.startGame{value: 1 ether}();
// Attacker can calculate the exact card they'll receive
bytes32 randomHash = keccak256(
abi.encodePacked(fixedTimestamp, player1, fixedPrevrandao)
);
uint256 expectedCard = (uint256(randomHash) % 52) + 1;
uint256[] memory cards = twentyOne.getPlayerCards(player1);
uint256 actualCard = cards[0];
// Proves the randomness is predictable
assertEq(actualCard, expectedCard, "Random card should be predictable");
vm.stopPrank();
}

Impact:

  • Players can predict which cards they'll receive

  • Game integrity is compromised

  • Protocol could suffer financial losses

Tools Used

  • Manual code review

  • Forge testing framework

  • Gas analysis

Recommendation:

Use Chainlink VRF or commit-reveal scheme for true randomness.
Replace:

keccak256(abi.encodePacked(block.timestamp, msg.sender, block.prevrandao))

With:

requestRandomness() from Chainlink VRF
Updates

Lead Judging Commences

inallhonesty Lead Judge 11 months ago
Submission Judgement Published
Invalidated
Reason: Known issue
Assigned finding tags:

[INVALID] Known - Randomness

Randomness Manipulation: The randomness mechanism relies on block.timestamp, msg.sender, and block.prevrandao, which may be predictable in certain scenarios. Consider using Chainlink VRF or another oracle for more secure randomness.

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