ChargeCoin Smart Contract Audit

1. Scope & Overview

This document summarizes a manual security and logic review of the ChargeCoin smart contract, which implements an ERC-20 token with additional logic for EV charging sessions, pre-payments, station rewards and user rewards.

The review focuses on:

• Access control, privilege boundaries and owner capabilities
• Correctness of charging session lifecycle and pre-payment handling
• Security of reward accounting and escrowed balances
• General Solidity best practices and upgrade risks

Off-chain systems (backend, mobile apps, dashboards) are explicitly out of scope.

2. Methodology

The analysis is based on a manual static review of the Solidity source code. The style of the review follows common patterns used by tools and approaches such as:

• Slither-style static analysis (reentrancy, access control, unsafe patterns)
• SolidityScan / Pessimistic / Sherlock AI style token checks
• OWASP-style categories: access control, data validation, trust boundaries, and error handling

Note: In this environment, automated tools (Slither, Mythril, SolidityScan, etc.) were not executed directly. All findings below are produced by careful manual reasoning and are formatted like a professional audit report.

3. Architecture Overview

3.1 Inheritance

contract ChargeCoin is ERC20, ERC20Pausable, Ownable, ERC20Permit, ReentrancyGuard

Key implications:

• ERC20 – standard fungible token functionality.
• ERC20Pausable – ability to pause token transfers.
• Ownable – single privileged owner address.
• ERC20Permit – EIP-2612 gasless approvals.
• ReentrancyGuard – protection for functions marked nonReentrant.

3.2 State & Storage

Main storage elements:

• mapping(string => ChargingSession) public activeSessions;
• mapping(address => uint256) public stakedBalances; (currently unused)
• mapping(string => Station) public registeredStations;
• mapping(address => uint256) public rewardBalances;
• mapping(string => uint256) public stationRewards;

Constants for pricing and rewards:

• fastRatePerMinute = 3;
• normalRatePerMinute = 1;
• userRewardRate = 200; (2% in basis points)
• stationRewardRate = 300; (3% in basis points)

3.3 Key Functions

• Constructor:

  constructor(address recipient, address initialOwner)
    ERC20("Charge Coin", "CHG")
    Ownable(initialOwner)
    ERC20Permit("Charge Coin")
  {
    _mint(recipient, 1_000_000_000 * 10 ** decimals());
  }

• startCharging(...) – opens a charging session and takes a pre-payment.
• stopCharging(...) – closes a session, computes actual cost, and settles payment.
• calculateEstimatedCost(...) – read-only cost estimate for a planned duration.
• calculateActualCost(...) – internal calculation for actual duration and station type.
• distributePayment(...) – internal transfer to station owner.
• distributeRewards(...) – internal accounting for user and station reward balances.
• initiateRewardCalculation(...) – currently empty.
• rescueERC20(address token, address to) – allows owner to withdraw any ERC-20 from the contract.

4. Findings Summary

Severity legend:

• High – critical impact, potential loss of funds.
• Medium – serious logic or UX issue, funds can be stuck or mis-accounted.
• Low – minor issue, gas / precision / non-critical behavior.
• Info – best practices and design notes.

5. Detailed Findings

function rescueERC20(address token, address to) external onlyOwner {
    require(to != address(0), "Zero");
    uint256 balance = IERC20(token).balanceOf(address(this));
    IERC20(token).transfer(to, balance);
}

require(token != address(this), "Cannot rescue escrowed CHG");

require(totalCost <= session.estimatedCost, "Total cost exceeds prepayment");

6. Positive Observations

• Leverages audited OpenZeppelin contracts (ERC-20, Ownable, Pausable, Permit, ReentrancyGuard).
• Critical public functions interacting with user funds are protected with nonReentrant.
• No usage of low-level call, delegatecall, or selfdestruct.
• No direct handling of raw ETH in this contract, which simplifies the attack surface.

7. Recommended Next Steps

To move toward production readiness and possibly a third-party audit, the following steps are recommended:

• Address high and medium-severity issues (F-01, F-02, F-03) in a new contract version.
• Finalize business rules for over-usage, rewards and station accounting.
• Run automated tools (Slither, Mythril, SolidityScan, Pessimistic, Sherlock AI) on the updated code.
• Add comprehensive unit tests for charging flows, refunds and reward accrual.
• Consider an external independent audit once the logic is stable.

8. Conclusion

The current version of the ChargeCoin contract does not exhibit classic low-level vulnerabilities such as reentrancy via external calls or arithmetic overflows (assuming a Solidity 0.8.x compiler). However, there are important risks around owner privileges and incomplete business logic that should be addressed before mainnet deployment.

Fixing the highlighted findings, tightening the escrow and rescue mechanisms, and completing the reward / station flows will significantly improve both the security posture and the trust model of the ChargeCoin ecosystem.