一个简单的以太坊合约让imtoken支持多签

熟悉比特币和以太坊的人应该都知道，在比特币中有2种类型的地址，1开头的是P2PKH，就是个人地址，3开头的是P2SH，一般是一个多签地址。所以在原生上比特币就支持多签。多签的一个优势就是可以多方对一笔付款达成共识，才能支付成功。比如3个人合伙开公司，他们的对外付款是比特币，为了防止管理财务的人作恶，于是他们可以创建2/3多签的地址，每个人持有一个私钥，对于每一笔付款，必须任意2个人都签名了才能支付出去。

比特币上的这个多签地址在以太坊上是没有原生支持的！以太坊最大的优点是支持图灵完备的智能合约，所以多签功能需要靠智能合约来实现。

为了简化代码，我们的需求是这样的：创建一个AB两个用户创建2/2的多签合约，该合约支持指定的ERC20 Token的支付。当某需要对外付款时，A用户调用合约，发起对C的转账n个Token，B用户也必须调用合约，发起对C的转账n个Token，只有A和B都调用了合约后，合约才会真的付款。其他用户发起转账无效。

根据以上需求，我改了一款极其简单的多签合约。代码如下：

pragma solidity ^0.4.24;

interface Token {
  function balanceOf(address _owner)  public view returns (uint256 );
  function transfer(address _to, uint256 _value) public ;
}

contract MultiSig {
    address private addrA;
    address private addrB;
    address private addrToken;

    struct Permit {
        bool addrAYes;
        bool addrBYes;
    }

    mapping (address => mapping (uint => Permit)) private permits;

    event Transfer(address indexed from, address indexed to, uint256 value);

    constructor(address a, address b, address tokenAddress) public{
        addrA = a;
        addrB = b;
        addrToken = tokenAddress;
    }
    function getAddrs() public view returns(address, address,address) {
      return (addrA, addrB,addrToken);
    }
    function transferTo(address to,  uint amount) public{
        Token token = Token(addrToken);
        require(token.balanceOf(this) >= amount);

        if (msg.sender == addrA) {
            permits[to][amount].addrAYes = true;
        } else if (msg.sender == addrB) {
            permits[to][amount].addrBYes = true;
        } else {
            require(false);
        }

        if (permits[to][amount].addrAYes == true && permits[to][amount].addrBYes == true) {
            token.transfer(to, amount);
            permits[to][amount].addrAYes = false;
            permits[to][amount].addrBYes = false;
        }
        emit Transfer(msg.sender, to, amount);
    }
}

以上代码十分简陋，功能十分有限，而且需要在etherscan或者remix上调用，对用户来说十分不友好，于是我想到了可以按ERC20标准接口对这个多签合约进行改造。改造后的合约看起来像是一个Token，但是本质上是一个多签地址。A B用户都可以使用imtoken或者KCash之类的支持ERC20的钱包APP进行多签，而不需要任何复杂的技能。所以我改写后的多签合约如下：

pragma solidity ^0.4.24;

interface IERC20 {
    function transfer(address to, uint256 value) external returns (bool);

    function approve(address spender, uint256 value) external returns (bool);

    function transferFrom(address from, address to, uint256 value) external returns (bool);

    function totalSupply() external view returns (uint256);

    function balanceOf(address who) external view returns (uint256);

    function allowance(address owner, address spender) external view returns (uint256);

    event Transfer(address indexed from, address indexed to, uint256 value);

    event Approval(address indexed owner, address indexed spender, uint256 value);
}

contract MultiSig is IERC20 {
    address private addrA;
    address private addrB;
    address private addrToken;

    struct Permit {
        bool addrAYes;
        bool addrBYes;
    }

    mapping (address => mapping (uint => Permit)) private permits;

     event Transfer(address indexed from, address indexed to, uint256 value);

    event Approval(address indexed owner, address indexed spender, uint256 value);

    uint public totalSupply = 10*10**26;
    uint8 constant public decimals = 18;
    string constant public name = "MutiSigPTN";
    string constant public symbol = "MPTN";

 function approve(address spender, uint256 value) external returns (bool){
     return false;
 }

    function transferFrom(address from, address to, uint256 value) external returns (bool){
        return false;
    }

    function totalSupply() external view returns (uint256){
          IERC20 token = IERC20(addrToken);
          return token.totalSupply();
    }

    function allowance(address owner, address spender) external view returns (uint256){
        return 0;
    }

    constructor(address a, address b, address tokenAddress) public{
        addrA = a;
        addrB = b;
        addrToken = tokenAddress;
    }
    function getAddrs() public view returns(address, address,address) {
      return (addrA, addrB,addrToken);
    }
    function  transfer(address to,  uint amount)  public returns (bool){
        IERC20 token = IERC20(addrToken);
        require(token.balanceOf(this) >= amount);

        if (msg.sender == addrA) {
            permits[to][amount].addrAYes = true;
        } else if (msg.sender == addrB) {
            permits[to][amount].addrBYes = true;
        } else {
            require(false);
        }

        if (permits[to][amount].addrAYes == true && permits[to][amount].addrBYes == true) {
            token.transfer(to, amount);
            permits[to][amount].addrAYes = false;
            permits[to][amount].addrBYes = false;
        }
        emit Transfer(msg.sender, to, amount);
        return true;
    }
    function balanceOf(address _owner) public view returns (uint) {
        IERC20 token = IERC20(addrToken);
        if (_owner==addrA || _owner==addrB){
            return token.balanceOf(this);
        }
        return 0;
    }
}

这里我需要特别指出的是decimals这个属性必须与所支持的ERC20 Token一致，这样钱包才会算出正确的转账金额。另外imtoken的缓存刷新比较慢，并不是部署了合约后马上就能搜索到的。以上代码都实测没问题。