构建数字化“分歧终端机”——探索Commit-Reveal Scheme | 火星技术帖

不知道看过电影《非诚勿扰》的人还记不记得“分歧终端机”这项奇葩发明，两片带把手的塑料片组成一个圆筒，有分歧的两个人将手伸入桶中，用石头剪刀布的方式同时出拳，然后拿开塑料片，根据猜拳结果决定听谁的。这玩意儿乍一看很荒谬，但是不得不说确实在一定程度上保证了猜拳的绝对公平性。

对于一个善于猜拳高手来说，哪怕只有短短半秒钟时间，也足够他观察出对手的手势进而一招制敌，赢得猜拳胜利。因此，在玩猜拳游戏的时候，双方都会尽可能的保证同时出拳。然而，在正常情况下，“绝对的同时”是不存在的，于是“分歧终端机”这个奇葩的发明似乎就有了用武之地。可如今，区块链为类似的公平性问题，提供了更好的解决方案——commitment scheme. 

Commitment scheme

Commitment scheme是一种加密算法，它允许某人承诺某个值，同时将其对他人隐藏，并在以后可以对他人显示。commitment scheme中的值具有约束力，一旦提交，任何人都无法更改它们。此方案有两个阶段：一个提交阶段，该阶段中需要选择和指定一个值；另一个是揭示阶段，此阶段显示和检查该值。其实约等于一个数字化可记录版的“分歧终端机”。

如何在以太坊上构建一个“分歧终端机”？

Commitment scheme的功能听上去简单，但适用范围却很广。接下来就让我们手把手的教你，如何用Commitment scheme在以太坊上创建一个数字化版本的“分歧终端机”。对于想要学习区块链技术的人来说，这也不啻于一个有趣又实用的实操学习。

首先，从构造函数和初始化值开始。我们需要为“玩家”提供选择，以猜拳游戏为例，选项为：石头、布和剪刀。之后，要保证它们的功能只在各自的阶段中运行。最后，创建一个表示玩家承诺的结构，并在构造函数中设置一些初始值、状态变量以及事件。

具体操作如下：

contract RockPaperScissors {

    enum Choice {

        None,

        Rock,

        Paper,

        Scissors

    }

    enum Stage {

        FirstCommit,

        SecondCommit,

        FirstReveal,

        SecondReveal,

        Distribute

    }

    struct CommitChoice {

        address playerAddress;

        bytes32 commitment;

        Choice choice;

    }

    event Commit(address player);

    event Reveal(address player, Choice choice);

    event Payout(address player, uint amount);

    // Initialisation args

    uint public bet;

    uint public deposit;

    uint public revealSpan;

    // State vars

    CommitChoice[2] public players;

    uint public revealDeadline;

    Stage public stage = Stage.FirstCommit;

    constructor(uint _bet, uint _deposit, uint _revealSpan) public {

        bet = _bet;

        deposit = _deposit;

        revealSpan = _revealSpan;

    }

}

完成上述步骤之后，接下了需要结合检查功能来构建一个Commit Function. 该功能只被允许在两个提交阶段之一中运行。接着，我们可以确保其固有值随交易一同发送，并且将任何额外资金退回。在完成检查后，我们可以将承诺存储起来，发出Commit event，并进入下一个阶段。

具体操作如下：

function commit(bytes32 commitment) public payable {

    // Only run during commit stages

    uint playerIndex;

    if(stage == Stage.FirstCommit) playerIndex = 0;

    else if(stage == Stage.SecondCommit) playerIndex = 1;

    else revert("both players have already played");

    uint commitAmount = bet + deposit;

    require(commitAmount >= bet, "overflow error");

    require(msg.value >= commitAmount, "value must be greater than commit amount");

    // Return additional funds transferred

    if(msg.value > commitAmount) {

        (bool success, ) = msg.sender.call.value(msg.value - commitAmount)("");

        require(success, "call failed");

    }

    // Store the commitment

    players[playerIndex] = CommitChoice(msg.sender, commitment, Choice.None);

    // Emit the commit event

    emit Commit(msg.sender);

    // If we're on the first commit, then move to the second

    if(stage == Stage.FirstCommit) stage = Stage.SecondCommit;

    // Otherwise we must already be on the second, move to first reveal

    else stage = Stage.FirstReveal;

}

 接着，从检查这步转到揭示功能。此功能仅在揭示阶段之一中运行，且只接受有效选择。然后，找到玩家数据，以便于检查他们承诺的哈希值，确定是否有效。如果该哈希值有效，对其进行存储，触发Reveal（揭示）事件，进入下一个阶段。

 具体操作如下：

function reveal(Choice choice, bytes32 blindingFactor) public {

    // Only run during reveal stages

    require(stage == Stage.FirstReveal || stage == Stage.SecondReveal, "not at reveal stage");

    // Only accept valid choices

    require(choice == Choice.Rock || choice == Choice.Paper || choice == Choice.Scissors, "invalid choice");

    // Find the player index

    uint playerIndex;

    if(players[0].playerAddress == msg.sender) playerIndex = 0;

    else if (players[1].playerAddress == msg.sender) playerIndex = 1;

    // Revert if unknown player

    else revert("unknown player");

    // Find player data

    CommitChoice storage commitChoice = players[playerIndex];

    // Check the hash to ensure the commitment is correct

    require(keccak256(abi.encodePacked(msg.sender, choice, blindingFactor)) == commitChoice.commitment, "invalid hash");

    // Update choice if correct

    commitChoice.choice = choice;

    // Emit reveal event

    emit Reveal(msg.sender, commitChoice.choice);

    if(stage == Stage.FirstReveal) {

        // If this is the first reveal, set the deadline for the second one

        revealDeadline = block.number + revealSpan;

        require(revealDeadline >= block.number, "overflow error");

        // Move to second reveal

        stage = Stage.SecondReveal;

    }

    // If we're on second reveal, move to distribute stage

    else stage = Stage.Distribute;

}

 最后需要的是分配功能。我们需要在确定获胜者之后将奖励支付给他们。此功能也同之前每一步一样，只能在分发阶段或者揭示阶段完成之后才能运行。通过对结果进行检查，判定出获胜的一方，之后计算奖励，触发Payout event，将资金发送到胜者的地址，并为下一次游戏将状态重置。

具体操作如下：

function distribute() public {

    // To distribute we need:

        // a) To be in the distribute stage OR

        // b) Still in the second reveal stage but past the deadline

    require(stage == Stage.Distribute || (stage == Stage.SecondReveal && revealDeadline <= block.number), "cannot yet distribute");

    // Calculate value of payouts for players

    uint player0Payout;

    uint player1Payout;

    uint winningAmount = deposit + 2 * bet;

    require(winningAmount / deposit == 2 * bet, "overflow error");

    // If both players picked the same choice, return their deposits and bets

    if(players[0].choice == players[1].choice) {

        player0Payout = deposit + bet;

        player1Payout = deposit + bet;

    }

    // If only one player made a choice, they win

    else if(players[0].choice == Choice.None) {

        player1Payout = winningAmount;

    }

    else if(players[1].choice == Choice.None) {

        player0Payout = winningAmount;

    }

    else if(players[0].choice == Choice.Rock) {

        assert(players[1].choice == Choice.Paper || players[1].choice == Choice.Scissors);

        if(players[1].choice == Choice.Paper) {

            // Rock loses to paper

            player0Payout = deposit;

            player1Payout = winningAmount;

        }

        else if(players[1].choice == Choice.Scissors) {

            // Rock beats scissors

            player0Payout = winningAmount;

            player1Payout = deposit;

        }

    }

    else if(players[0].choice == Choice.Paper) {

        assert(players[1].choice == Choice.Rock || players[1].choice == Choice.Scissors);

        if(players[1].choice == Choice.Rock) {

            // Paper beats rock

            player0Payout = winningAmount;

            player1Payout = deposit;

        }

        else if(players[1].choice == Choice.Scissors) {

            // Paper loses to scissors

            player0Payout = deposit;

            player1Payout = winningAmount;

        }

    }

    else if(players[0].choice == Choice.Scissors) {

        assert(players[1].choice == Choice.Paper || players[1].choice == Choice.Rock);

        if(players[1].choice == Choice.Rock) {

            // Scissors lose to rock

            player0Payout = deposit;

            player1Payout = winningAmount;

        }

        else if(players[1].choice == Choice.Paper) {

            // Scissors beats paper

            player0Payout = winningAmount;

            player1Payout = deposit;

        }

    }

    else revert("invalid choice");

    // Send the payouts

    if(player0Payout > 0) {

        (bool success, ) = players[0].playerAddress.call.value(player0Payout)("");

        require(success, 'call failed');

        emit Payout(players[0].playerAddress, player0Payout);

    } else if (player1Payout > 0) {

        (bool success, ) = players[1].playerAddress.call.value(player1Payout)("");

        require(success, 'call failed');

        emit Payout(players[1].playerAddress, player1Payout);

    }

    // Reset the state to play again

    delete players;

    revealDeadline = 0;

    stage = Stage.FirstCommit;

}

 完整合约内容可见公众号。

以太坊是一个公共区块链，可因此导致了对于隐私数据的管理困难。而有许多应用程序就像是上面提到的猜拳游戏一样，需要隐藏值才能正常运行，因此，commitment schemes就成为了一个卓绝的解决方案。

踢马河：RaTiO Fintech合伙人，曾任某券商自营操盘手，十余年海外对冲基金和国内大型投资机构基金经理，资深交易建模专家，币圈大咖。

请尊重原创！转载请注明出处。