Lesson 5: Blockchain Security

Secure Smart Contract Example

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/security/ReentrancyGuard.sol";
import "@openzeppelin/contracts/access/Ownable.sol";
import "@openzeppelin/contracts/utils/math/SafeMath.sol";

contract SecureVault is ReentrancyGuard, Ownable {
    using SafeMath for uint256;
    
    mapping(address => uint256) public balances;
    
    event Deposit(address indexed user, uint256 amount);
    event Withdrawal(address indexed user, uint256 amount);
    
    function deposit() external payable nonReentrant {
        require(msg.value > 0, "Amount must be greater than 0");
        balances[msg.sender] = balances[msg.sender].add(msg.value);
        emit Deposit(msg.sender, msg.value);
    }
    
    function withdraw(uint256 amount) external nonReentrant {
        require(amount > 0, "Amount must be greater than 0");
        require(balances[msg.sender] >= amount, "Insufficient balance");
        
        balances[msg.sender] = balances[msg.sender].sub(amount);
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success, "Transfer failed");
        
        emit Withdrawal(msg.sender, amount);
    }
}