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

The `TwentyOne` contract does not handle aces as it should according to classic blackjack rules

phylax

Summary

In classic blackjack a ace has a value of either 1 or 11 depending on what is most advantagous for the player or dealer. The TwentyOnecontract handles aces as having only the value of 1.

Vulnerability Details

Not handling aces as 1 or 11 according to regular blackjack rules leads to unintended gameplay by the player.

Impact

A player may loose the game when thinking the hand is a winner.

Tools Used

Manual code review

Recommendations

  1. Identify aces in the hand.

  2. Initially count aces as 11.

  3. Check if hand is bust.

  4. If hand is busted subtract 10 from hand.

Adjust playerHand function to handle aces correctly:

function playersHand(address player) public view returns (uint256) {
uint256 playerTotal = 0;
+ uint256 acesCount = 0;
for (uint256 i = 0; i < playersDeck[player].playersCards.length; i++) {
uint256 cardValue = playersDeck[player].playersCards[i] % 13;
​
+ if (cardValue == 1) {
+ playerTotal += 11;
+ acesCount += 1;
+ } else if (cardValue >= 10 || cardValue == 0) {
- if (cardValue == 0 || cardValue >= 10) {
playerTotal += 10;
} else {
playerTotal += cardValue;
}
}
​
// Adjust for aces if total is over 21
+ while (playerTotal > 21 && acesCount > 0) {
+ playerTotal -= 10; // Change an ace from 11 to 1
+ acesCount -= 1;
+ }
​
return playerTotal;
}

Adjust dealerHand function to handle aces correctly:

function dealersHand(address player) public view returns (uint256) {
uint256 dealerTotal = 0;
+ uint256 acesCount = 0;
​
for (uint256 i = 0; i < dealersDeck[player].dealersCards.length; i++) {
uint256 cardValue = dealersDeck[player].dealersCards[i] % 13;
​
+ if (cardValue == 0) {
+ dealerTotal += 11;
+ acesCount += 1;
+ } else if (cardValue >= 10 || cardValue == 0) {
- if (cardValue >= 10) {
dealerTotal += 10;
} else {
dealerTotal += cardValue;
}
}
​
// Adjust for aces if total is over 21
+ while (dealerTotal > 21 && acesCount > 0) {
+ dealerTotal -= 10; // Change an ace from 11 to 1
+ acesCount -= 1;
+ }
​
return dealerTotal;
}
Updates

Lead Judging Commences

inallhonesty Lead Judge 11 months ago
Submission Judgement Published
Validated
Assigned finding tags:

Wrong ace value

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