区块链智能合约美链攻击分析以及安全库的使用(代码片段)

1.美链攻击过程

美链代币BEC为发行在以太坊上的ERC20代币，其具体合约的代码在该链接中合约代码。
向美链发起攻击的交易链接为攻击交易hash。

function batchTransfer(address[] _receivers, uint256 _value) public whenNotPaused returns (bool) 
    uint cnt = _receivers.length;
    uint256 amount = uint256(cnt) * _value;
    require(cnt > 0 && cnt <= 20);
    require(_value > 0 && balances[msg.sender] >= amount);

    balances[msg.sender] = balances[msg.sender].sub(amount);
    for (uint i = 0; i < cnt; i++) 
        balances[_receivers[i]] = balances[_receivers[i]].add(_value);
        Transfer(msg.sender, _receivers[i], _value);
    
    return true;

美链攻击的具体流程分为以下四步：
1.首先构造一个_value值，使得_receivers.len() * _value产生向上溢出，结果为一个极小值amount。比如使用_receivers.len()为2，_value值为2**255，那么在上述代码中计算得到的amount值为0。
2.amount，value通过require验证。
3.在msg.sender中减去amount数量的代币。
4.为每个_receivers账户增加value数量的代币。
其中value为一个极大值，相当于对美链的BEC代币进行了增发，对应的_receivers账户可以获取大量BECtoken。

2.solidity中上溢与下溢

在solidity中变量进行+、-、*运算时会产生溢出，如加法运算和乘法运算会产生向上溢出，减法运算产生向下溢出。如2**255变量乘以2，获得的结果为0。具体溢出情况可参考以下合约代码：

pragma solidity 0.4.20;
contract TestFlow 
    uint256 public zero = 0;
    uint256 public max = 2**256 - 1;
    uint256 public mm = 2**255;

    function subUnderFlow() public constant returns (uint) 
        uint256 a =  zero - 1;
        return a;
    

    function addOverFlow() public constant returns (uint) 
        uint256 a =  max + 1;
        return a;
    

    function mulOverFlow() public constant returns (uint) 
        uint256 a =  mm * 2;
        return a;
    

3.定义安全库

在对数字进行运算时，要采用安全函数，保证结果正确且不发生溢出。安全的数学运算库的使用如下所示。
第一步：导入SafeMath库文件

pragma solidity >=0.4.22 <0.6.0;
 
/**
 * @title SafeMath
 * @dev Unsigned math operations with safety checks that revert on error
 */
 
library SafeMath 
    /**
     * @dev Multiplies two unsigned integers, reverts on overflow.
     */
    function mul(uint256 a, uint256 b) internal pure returns (uint256) 
        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
        // benefit is lost if 'b' is also tested.
        // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
        if (a == 0) 
            return 0;
        
 
        uint256 c = a * b;
        require(c / a == b);
 
        return c;
    
 
    /**
     * @dev Integer division of two unsigned integers truncating the quotient, reverts on division by zero.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) 
        // Solidity only automatically asserts when dividing by 0
        require(b > 0);
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold
 
        return c;
    
 
    /**
     * @dev Subtracts two unsigned integers, reverts on overflow (i.e. if subtrahend is greater than minuend).
     */
    function sub(uint256 a, uint256 b) internal pure returns (uint256) 
        require(b <= a);
        uint256 c = a - b;
 
        return c;
    
 
    /**
     * @dev Adds two unsigned integers, reverts on overflow.
     */
    function add(uint256 a, uint256 b) internal pure returns (uint256) 
        uint256 c = a + b;
        require(c >= a);
 
        return c;
    
 
    /**
     * @dev Divides two unsigned integers and returns the remainder (unsigned integer modulo),
     * reverts when dividing by zero.
     */
    function mod(uint256 a, uint256 b) internal pure returns (uint256) 
        require(b != 0);
        return a % b;
    

第二步，引用

pragma solidity >=0.4.22 <0.6.0;
 
import "./safemath.sol";
 
contract jisuan
    uint a=2;
    uint b=5;
    uint c=8;
    //引入safemath库
    using SafeMath for uint256;
    
    //加法
    function addNum()public view returns(uint d)
        d=a.add(b);
    
    //减法
    function subNum()public view returns(uint d)
        d=b.sub(a);
    
    //乘法
    function mulNum()public view returns(uint d)
        d=a.mul(b);
    
    //除法
    function divNum()public view returns(uint d)
        d=c.div(a);