以太坊微支付通道原理与实现

以太坊微支付通道原理与实现

线上直接转账需要一定的费用，如果存在大量小额交易的情况下，费用会变的难以承受，因而以太坊引入了微交易支付通道来解决这个问题。以太坊提供了一个票据支付方案，主要依赖于智能合约实现的一对多的账单系统。该账单系统大致上的执行流程如下.

• 1：账单生成，同时提供机制往账单上存入保证金。

• 2：交易发起人生成交易票据

• 3：直接将票据发送给接收人

• 4：接收人兑现票据,从合约转账(尽管某次兑现可能会失败,但是只要票据存在最终还是能够兑现).

这种交易优点在于可以在线下发送任意数量的交易，而只需要两个链上交易（存入保证金，兑现）只要存入保证金，线下通过交换票据进行任意数量的交易，避免了频繁的线上转账，节省了交易费用。

代码结构

.
├── api.go                             //对外接口
├── cheque.go                          //账单
├── cheque_test.go
├── contract       
│   ├── chequebook.go                  //合约go语言接口
│   ├── chequebook.sol                 //合约源码
│   ├── code.go                        //合约byte码
│   ├── mortal.sol                     //合约销毁
│   └── owned.sol                      //hebe权限
└── gencode.go                         //合约byte码生成

合约层

合约自身是接收转账的，用户可以在初始化或者后来增加金额，可以通过cash方法兑现票据，转账金额都会保存在send变量上。

    pragma solidity ^0.4.18;

    import "./mortal.sol";

    /// @title Chequebook for Ethereum micropayments
    /// @author Daniel A. Nagy 
    contract chequebook is mortal {
        // Cumulative paid amount in wei to each beneficiary
        //已经支付的 可以控制双花,防止多次兑换票据
        mapping (address => uint256) public sent;

        /// @notice Overdraft event
        event Overdraft(address deadbeat);

        // Allow sending ether to the chequebook.
        function() public payable { }

        /// @notice Cash cheque
        ///
        /// @param beneficiary beneficiary address
        /// @param amount cumulative amount in wei
        /// @param sig_v signature parameter v
        /// @param sig_r signature parameter r
        /// @param sig_s signature parameter s
        /// The digital signature is calculated on the concatenated triplet of contract address, beneficiary address and cumulative amount
        function cash(address beneficiary, uint256 amount, uint8 sig_v, bytes32 sig_r, bytes32 sig_s) public {
            // Check if the cheque is old.
            // Only cheques that are more recent than the last cashed one are considered.
            require(amount > sent[beneficiary]);
            // Check the digital signature of the cheque.
            bytes32 hash = keccak256(address(this), beneficiary, amount);
            require(owner == ecrecover(hash, sig_v, sig_r, sig_s));
            // Attempt sending the difference between the cumulative amount on the cheque
            // and the cumulative amount on the last cashed cheque to beneficiary.
            uint256 diff = amount - sent[beneficiary];
            if (diff <= this.balance) {
                // update the cumulative amount before sending
                sent[beneficiary] = amount;
                beneficiary.transfer(diff);
            } else {
                // Upon failure, punish owner for writing a bounced cheque.
                // owner.sendToDebtorsPrison();
                Overdraft(owner);
                // Compensate beneficiary.
                selfdestruct(beneficiary);
            }
        }
    }

支付层

账单保存了账本的位置，记账人，所有人等信

    // Chequebook can create and sign cheques from a single contract to multiple beneficiaries.
    // It is the outgoing payment handler for peer to peer micropayments.
    type Chequebook struct {
        path     string                      // path to chequebook file
        prvKey   *ecdsa.PrivateKey           // private key to sign cheque with
        lock     sync.Mutex                  //
        backend  Backend                     // blockchain API
        quit     chan bool                   // when closed causes autodeposit to stop
        owner    common.Address              // owner address (derived from pubkey)
        contract *contract.Chequebook        // abigen binding
        session  *contract.ChequebookSession // abigen binding with Tx Opts

        // persisted fields
        balance      *big.Int                    // not synced with blockchain
        contractAddr common.Address              // contract address
        sent         map[common.Address]*big.Int //tallies for beneficiaries

        txhash    string   // tx hash of last deposit tx
        threshold *big.Int // threshold that triggers autodeposit if not nil
        buffer    *big.Int // buffer to keep on top of balance for fork protection

        log log.Logger // contextual logger with the contract address embedded
    }

票据：合约位置,接收人,金额,签名

    type Cheque struct {
        Contract    common.Address // address of chequebook, needed to avoid cross-contract submission
        Beneficiary common.Address
        Amount      *big.Int // cumulative amount of all funds sent
        Sig         []byte   // signature Sign(Keccak256(contract, beneficiary, amount), prvKey)
    }

票据生成

生成一条支付记录,返回一份签名后的票据,收费这凭借这张票据从合约里面取钱.

    // Issue creates a cheque signed by the chequebook owner's private key. The
    // signer commits to a contract (one that they own), a beneficiary and amount.
    func (self *Chequebook) Issue(beneficiary common.Address, amount *big.Int) (ch *Cheque, err error) {
        defer self.lock.Unlock()
        self.lock.Lock()

        if amount.Sign() <= 0 {
            return nil, fmt.Errorf("amount must be greater than zero (%v)", amount)
        }
        if self.balance.Cmp(amount) < 0 {
            err = fmt.Errorf("insufficient funds to issue cheque for amount: %v. balance: %v", amount, self.balance)
        } else {
            var sig []byte
            sent, found := self.sent[beneficiary]
            if !found {
                sent = new(big.Int)
                self.sent[beneficiary] = sent
            }
            sum := new(big.Int).Set(sent)
            sum.Add(sum, amount)

            sig, err = crypto.Sign(sigHash(self.contractAddr, beneficiary, sum), self.prvKey)
            if err == nil {
                ch = &Cheque{
                    Contract:    self.contractAddr,
                    Beneficiary: beneficiary,
                    Amount:      sum,
                    Sig:         sig,
                }
                sent.Set(sum)
                self.balance.Sub(self.balance, amount) // subtract amount from balance
            }
        }

        // 账单余额少于阈值,自动补充.
        if self.threshold != nil {
            if self.balance.Cmp(self.threshold) < 0 {
                send := new(big.Int).Sub(self.buffer, self.balance)
                self.deposit(send)
            }
        }

        return
    }

存储金额

    func (self *Chequebook) Deposit(amount *big.Int) (string, error) {
        defer self.lock.Unlock()
        self.lock.Lock()
        return self.deposit(amount)
    }

    func (self *Chequebook) deposit(amount *big.Int) (string, error) {
        // since the amount is variable here, we do not use sessions
        depositTransactor := bind.NewKeyedTransactor(self.prvKey)
        depositTransactor.Value = amount
        chbookRaw := &contract.ChequebookRaw{Contract: self.contract}
        //转入金额
        tx, err := chbookRaw.Transfer(depositTransactor)
        if err != nil {
            self.log.Warn("Failed to fund chequebook", "amount", amount, "balance", self.balance, "target", self.buffer, "err", err)
            return "", err
        }
        // assume that transaction is actually successful, we add the amount to balance right away
        self.balance.Add(self.balance, amount)
        self.log.Trace("Deposited funds to chequebook", "amount", amount, "balance", self.balance, "target", self.buffer)
        return tx.Hash().Hex(), nil
    }

兑换票据

// Cash is a convenience method to cash any cheque.
func (self *Chequebook) Cash(ch *Cheque) (txhash string, err error) {
	return ch.Cash(self.session)
}

// Cash cashes the cheque by sending an Ethereum transaction.
func (self *Cheque) Cash(session *contract.ChequebookSession) (string, error) {
	v, r, s := sig2vrs(self.Sig)
    //调用合约的cash方法 提取代币
	tx, err := session.Cash(self.Beneficiary, self.Amount, v, r, s)
	if err != nil {
		return "", err
	}
	return tx.Hash().Hex(), nil
}

其他接口

OutBox：用于在电对点网络中发行票据，提供了保证金存入，票据发行，自动存入保证金等接口。

    type Outbox struct {
        chequeBook  *Chequebook
        beneficiary common.Address
    }

    // Issue creates cheque.
    func (self *Outbox) Issue(amount *big.Int) (swap.Promise, error) {
        return self.chequeBook.Issue(self.beneficiary, amount)
    }

    // AutoDeposit enables auto-deposits on the underlying chequebook.
    func (self *Outbox) AutoDeposit(interval time.Duration, threshold, buffer *big.Int) {
        self.chequeBook.AutoDeposit(interval, threshold, buffer)
    }

InBox:用于在电对点网络中票据兑换，提供了直接兑换，定时兑换，延迟兑换的接口功能。

    // Inbox can deposit, verify and cash cheques from a single contract to a single
    // beneficiary. It is the incoming payment handler for peer to peer micropayments.
    type Inbox struct {
        lock        sync.Mutex
        contract    common.Address              // peer's chequebook contract
        beneficiary common.Address              // local peer's receiving address
        sender      common.Address              // local peer's address to send cashing tx from
        signer      *ecdsa.PublicKey            // peer's public key
        txhash      string                      // tx hash of last cashing tx
        session     *contract.ChequebookSession // abi contract backend with tx opts
        quit        chan bool                   // when closed causes autocash to stop
        maxUncashed *big.Int                    // threshold that triggers autocashing
        cashed      *big.Int                    // cumulative amount cashed
        cheque      *Cheque                     // last cheque, nil if none yet received
        log         log.Logger                  // contextual logger with the contract address embedded
    }

    // Cash attempts to cash the current cheque.
    func (self *Inbox) Cash() (txhash string, err error) {
        if self.cheque != nil {
            txhash, err = self.cheque.Cash(self.session)
            self.log.Trace("Cashing in chequebook cheque", "amount", self.cheque.Amount, "beneficiary", self.beneficiary)
            self.cashed = self.cheque.Amount
        }
        return
    }

转自：（魂祭心） https://my.oschina.net/hunjixin/blog/1807446

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