TwentyOne Analysis Report
TwentyOne Analysis Report
This report was generated by a manual review of the smart contract code. It highlights potential security vulnerabilities, optimizations, and other issues to improve the overall robustness of the code. This review is not a substitute for a formal security audit or extensive testing.
Table of Contents
Summary
Files Summary
| Key | Value |
|---|---|
.sol Files |
1 |
| Total nSLOC | 140 |
Issue Summary
| Category | No. of Issues |
|---|---|
| High | 2 |
| Medium | 2 |
| Low | 5 |
High Issues
H-1: Predictable Randomness Vulnerability
Description
The contract uses block.timestamp, msg.sender, and block.prevrandao for randomness in the drawCard function. These values are predictable and manipulable by miners, leading to potential exploits.
Impact
An attacker or miner could manipulate the randomness to favor specific outcomes, undermining the fairness of the game.
Recommendation
Use a more secure source of randomness, such as Chainlink VRF, which is verifiable and tamper-proof.
Affected Code:
uint256 randomIndex = uint256(keccak256(abi.encodePacked(block.timestamp, msg.sender, block.prevrandao))) % availableCards[player].length;
H-2: Lack of Access Control for Withdrawals
Description
The endGame function pays out 2 ether if the player wins. There is no restriction preventing malicious actors from calling this function directly and exploiting the contract.
Impact
An attacker could exploit the contract to drain funds if they manipulate game conditions.
Recommendation
Implement stricter access control using modifiers to ensure that only valid game participants can trigger payouts.
Medium Issues
M-1: block.prevrandao Dependency
Description
The contract relies on block.prevrandao for randomness, which miners can influence to manipulate outcomes.
Impact
This dependency reduces the randomness quality, allowing miners to exploit the game logic.
Recommendation
Use an external randomness provider like Chainlink VRF instead of on-chain randomness.
M-2: Potential Reentrancy in endGame
Description
The endGame function performs an external Ether transfer (payable(player).transfer(2 ether)) before clearing the state.
Impact
This may expose the contract to reentrancy attacks, allowing malicious contracts to repeatedly call endGame and drain funds.
Recommendation
Perform state changes before external calls, or use the checks-effects-interactions pattern to mitigate reentrancy risks.
Affected Code:
Low Issues
L-1: Solidity Pragma Version
Description
The contract uses pragma solidity ^0.8.13;. Using a wide version range increases the risk of unintended behavior due to compiler changes.
Recommendation
Specify a fixed compiler version for consistency and to avoid unintended bugs.
Affected Code:
L-2: Inefficient State Reset in endGame
Description
The endGame function deletes mappings for players' cards and the deck. This process could be optimized.
Recommendation
Use delete playersDeck[player] and delete dealersDeck[player] instead of accessing individual arrays.
L-3: Missing external Modifier
Description
Several public functions, such as startGame, are never called internally. The external modifier could save gas.
Recommendation
Use the external modifier for functions that are not invoked within the contract.
Examples:
L-4: Lack of Indexed Fields in Events
Description
The events (PlayerLostTheGame, PlayerWonTheGame, etc.) do not include indexed fields, making it harder for off-chain tools to filter events.
Recommendation
Add indexed parameters to events, such as address indexed player.
L-5: Magic Numbers for Ether Values
Description
The contract hardcodes Ether values (e.g., 1 ether, 2 ether).
Recommendation
Define constants for these values for better readability and maintainability.
Example:
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