Rock Paper Scissors

Summary

The RockPaperScissors::_finishGame and _handleTie functions calculate the protocol fee using Solidity's integer division. If the totalPot is not perfectly divisible by 100, the remainder will be silently discarded. This leads to unclaimable "dust" ETH that stays permanently locked in the contract and causes accounting discrepancies. Over time, this could accumulate to a noticeable amount, especially as the number of games increases.

Vulnerability Details

Issues Identified

1. Integer Truncation

   • Solidity truncates results when performing division with integers.

   • For example, 3 * 10 / 100 = 0 when ideally you want 0.3 → 1 wei or round up.

2. Locked ETH (Dust)

   • Any leftover remainder from the division will not be included in either fee or prize.

   • This remainder becomes permanently trapped in the contract.

3. Accounting Discrepancy

   • accumulatedFees only tracks the truncated fee, but address(this).balance will include the leftover.

   • This mismatch can create confusion during audits or when calculating withdrawable fees.

Impact

• Lost Funds: Small ETH amounts will be unrecoverable, especially impactful over many games.

• Balance Mismatch: The tracked accumulatedFees will not match the actual contract balance.

• User Trust Impact: Players may be concerned about unexplained leftover balances.

Proof of Concept (PoC)

PoC Explanation

This test creates a game where the total pot is not divisible by 100, ensuring a truncated remainder. The PoC confirms that fee < ceil(fee), proving dust is created.

Expected Output

This confirms a 1 wei discrepancy, which will remain locked in the contract and unaccounted for in accumulatedFees.

Tools Used

• Manual Review

• Foundry Unit Test

Recommendations

1. Use Safe Rounding Up
   Prevent truncation using:

   uint256 fee = (totalPot * PROTOCOL_FEE_PERCENT + 99) / 100;

2. Use Subtractive Fee Calculation

   uint256 prize = totalPot * 90 / 100;

   uint256 fee = totalPot - prize;

3. Add Dust Recovery (Optional)
   Let admin recover any excess ETH not tracked in accumulatedFees.

   function recoverDust() external onlyAdmin {

   uint256 dust = address(this).balance - accumulatedFees;

   require(dust > 0, "No dust");

   (bool success,) = adminAddress.call{value: dust}("");

   require(success, "Recover failed");

   }