Solidity by Example

Voting

The following contract is quite complex, but showcases a lot of Solidity’s features. It implements a voting contract. Of course, the main problems of electronic voting is how to assign voting rights to the correct persons and how to prevent manipulation. We will not solve all problems here, but at least we will show how delegated voting can be done so that vote counting is automatic and completely transparent at the same time.

The idea is to create one contract per ballot, providing a short name for each option. Then the creator of the contract who serves as chairperson will give the right to vote to each address individually.

The persons behind the addresses can then choose to either vote themselves or to delegate their vote to a person they trust.

At the end of the voting time, winningProposal() will return the proposal with the largest number of votes.

pragma solidity >=0.4.22 <0.6.0;

/// @title Voting with delegation.
contract Ballot {
    // This declares a new complex type which will
    // be used for variables later.
    // It will represent a single voter.
    struct Voter {
        uint weight; // weight is accumulated by delegation
        bool voted;  // if true, that person already voted
        address delegate; // person delegated to
        uint vote;   // index of the voted proposal
    }

    // This is a type for a single proposal.
    struct Proposal {
        bytes32 name;   // short name (up to 32 bytes)
        uint voteCount; // number of accumulated votes
    }

    address public chairperson;

    // This declares a state variable that
    // stores a `Voter` struct for each possible address.
    mapping(address => Voter) public voters;

    // A dynamically-sized array of `Proposal` structs.
    Proposal[] public proposals;

    /// Create a new ballot to choose one of `proposalNames`.
    constructor(bytes32[] memory proposalNames) public {
        chairperson = msg.sender;
        voters[chairperson].weight = 1;

        // For each of the provided proposal names,
        // create a new proposal object and add it
        // to the end of the array.
        for (uint i = 0; i < proposalNames.length; i++) {
            // `Proposal({...})` creates a temporary
            // Proposal object and `proposals.push(...)`
            // appends it to the end of `proposals`.
            proposals.push(Proposal({
                name: proposalNames[i],
                voteCount: 0
            }));
        }
    }

    // Give `voter` the right to vote on this ballot.
    // May only be called by `chairperson`.
    function giveRightToVote(address voter) public {
        // If the first argument of `require` evaluates
        // to `false`, execution terminates and all
        // changes to the state and to Ether balances
        // are reverted.
        // This used to consume all gas in old EVM versions, but
        // not anymore.
        // It is often a good idea to use `require` to check if
        // functions are called correctly.
        // As a second argument, you can also provide an
        // explanation about what went wrong.
        require(
            msg.sender == chairperson,
            "Only chairperson can give right to vote."
        );
        require(
            !voters[voter].voted,
            "The voter already voted."
        );
        require(voters[voter].weight == 0);
        voters[voter].weight = 1;
    }

    /// Delegate your vote to the voter `to`.
    function delegate(address to) public {
        // assigns reference
        Voter storage sender = voters[msg.sender];
        require(!sender.voted, "You already voted.");

        require(to != msg.sender, "Self-delegation is disallowed.");

        // Forward the delegation as long as
        // `to` also delegated.
        // In general, such loops are very dangerous,
        // because if they run too long, they might
        // need more gas than is available in a block.
        // In this case, the delegation will not be executed,
        // but in other situations, such loops might
        // cause a contract to get "stuck" completely.
        while (voters[to].delegate != address(0)) {
            to = voters[to].delegate;

            // We found a loop in the delegation, not allowed.
            require(to != msg.sender, "Found loop in delegation.");
        }

        // Since `sender` is a reference, this
        // modifies `voters[msg.sender].voted`
        sender.voted = true;
        sender.delegate = to;
        Voter storage delegate_ = voters[to];
        if (delegate_.voted) {
            // If the delegate already voted,
            // directly add to the number of votes
            proposals[delegate_.vote].voteCount += sender.weight;
        } else {
            // If the delegate did not vote yet,
            // add to her weight.
            delegate_.weight += sender.weight;
        }
    }

    /// Give your vote (including votes delegated to you)
    /// to proposal `proposals[proposal].name`.
    function vote(uint proposal) public {
        Voter storage sender = voters[msg.sender];
        require(!sender.voted, "Already voted.");
        sender.voted = true;
        sender.vote = proposal;

        // If `proposal` is out of the range of the array,
        // this will throw automatically and revert all
        // changes.
        proposals[proposal].voteCount += sender.weight;
    }

    /// @dev Computes the winning proposal taking all
    /// previous votes into account.
    function winningProposal() public view
            returns (uint winningProposal_)
    {
        uint winningVoteCount = 0;
        for (uint p = 0; p < proposals.length; p++) {
            if (proposals[p].voteCount > winningVoteCount) {
                winningVoteCount = proposals[p].voteCount;
                winningProposal_ = p;
            }
        }
    }

    // Calls winningProposal() function to get the index
    // of the winner contained in the proposals array and then
    // returns the name of the winner
    function winnerName() public view
            returns (bytes32 winnerName_)
    {
        winnerName_ = proposals[winningProposal()].name;
    }
}

Possible Improvements

Currently, many transactions are needed to assign the rights to vote to all participants. Can you think of a better way?

Blind Auction

In this section, we will show how easy it is to create a completely blind auction contract on Ethereum. We will start with an open auction where everyone can see the bids that are made and then extend this contract into a blind auction where it is not possible to see the actual bid until the bidding period ends.

Simple Open Auction

The general idea of the following simple auction contract is that everyone can send their bids during a bidding period. The bids already include sending money / ether in order to bind the bidders to their bid. If the highest bid is raised, the previously highest bidder gets her money back. After the end of the bidding period, the contract has to be called manually for the beneficiary to receive their money - contracts cannot activate themselves.

pragma solidity >=0.4.22 <0.6.0;

contract SimpleAuction {
    // Parameters of the auction. Times are either
    // absolute unix timestamps (seconds since 1970-01-01)
    // or time periods in seconds.
    address payable public beneficiary;
    uint public auctionEndTime;

    // Current state of the auction.
    address public highestBidder;
    uint public highestBid;

    // Allowed withdrawals of previous bids
    mapping(address => uint) pendingReturns;

    // Set to true at the end, disallows any change.
    // By default initialized to `false`.
    bool ended;

    // Events that will be emitted on changes.
    event HighestBidIncreased(address bidder, uint amount);
    event AuctionEnded(address winner, uint amount);

    // The following is a so-called natspec comment,
    // recognizable by the three slashes.
    // It will be shown when the user is asked to
    // confirm a transaction.

    /// Create a simple auction with `_biddingTime`
    /// seconds bidding time on behalf of the
    /// beneficiary address `_beneficiary`.
    constructor(
        uint _biddingTime,
        address payable _beneficiary
    ) public {
        beneficiary = _beneficiary;
        auctionEndTime = now + _biddingTime;
    }

    /// Bid on the auction with the value sent
    /// together with this transaction.
    /// The value will only be refunded if the
    /// auction is not won.
    function bid() public payable {
        // No arguments are necessary, all
        // information is already part of
        // the transaction. The keyword payable
        // is required for the function to
        // be able to receive Ether.

        // Revert the call if the bidding
        // period is over.
        require(
            now <= auctionEndTime,
            "Auction already ended."
        );

        // If the bid is not higher, send the
        // money back.
        require(
            msg.value > highestBid,
            "There already is a higher bid."
        );

        if (highestBid != 0) {
            // Sending back the money by simply using
            // highestBidder.send(highestBid) is a security risk
            // because it could execute an untrusted contract.
            // It is always safer to let the recipients
            // withdraw their money themselves.
            pendingReturns[highestBidder] += highestBid;
        }
        highestBidder = msg.sender;
        highestBid = msg.value;
        emit HighestBidIncreased(msg.sender, msg.value);
    }

    /// Withdraw a bid that was overbid.
    function withdraw() public returns (bool) {
        uint amount = pendingReturns[msg.sender];
        if (amount > 0) {
            // It is important to set this to zero because the recipient
            // can call this function again as part of the receiving call
            // before `send` returns.
            pendingReturns[msg.sender] = 0;

            if (!msg.sender.send(amount)) {
                // No need to call throw here, just reset the amount owing
                pendingReturns[msg.sender] = amount;
                return false;
            }
        }
        return true;
    }

    /// End the auction and send the highest bid
    /// to the beneficiary.
    function auctionEnd() public {
        // It is a good guideline to structure functions that interact
        // with other contracts (i.e. they call functions or send Ether)
        // into three phases:
        // 1. checking conditions
        // 2. performing actions (potentially changing conditions)
        // 3. interacting with other contracts
        // If these phases are mixed up, the other contract could call
        // back into the current contract and modify the state or cause
        // effects (ether payout) to be performed multiple times.
        // If functions called internally include interaction with external
        // contracts, they also have to be considered interaction with
        // external contracts.

        // 1. Conditions
        require(now >= auctionEndTime, "Auction not yet ended.");
        require(!ended, "auctionEnd has already been called.");

        // 2. Effects
        ended = true;
        emit AuctionEnded(highestBidder, highestBid);

        // 3. Interaction
        beneficiary.transfer(highestBid);
    }
}

Blind Auction

The previous open auction is extended to a blind auction in the following. The advantage of a blind auction is that there is no time pressure towards the end of the bidding period. Creating a blind auction on a transparent computing platform might sound like a contradiction, but cryptography comes to the rescue.

During the bidding period, a bidder does not actually send her bid, but only a hashed version of it. Since it is currently considered practically impossible to find two (sufficiently long) values whose hash values are equal, the bidder commits to the bid by that. After the end of the bidding period, the bidders have to reveal their bids: They send their values unencrypted and the contract checks that the hash value is the same as the one provided during the bidding period.

Another challenge is how to make the auction binding and blind at the same time: The only way to prevent the bidder from just not sending the money after they won the auction is to make her send it together with the bid. Since value transfers cannot be blinded in Ethereum, anyone can see the value.

The following contract solves this problem by accepting any value that is larger than the highest bid. Since this can of course only be checked during the reveal phase, some bids might be invalid, and this is on purpose (it even provides an explicit flag to place invalid bids with high value transfers): Bidders can confuse competition by placing several high or low invalid bids.

pragma solidity >0.4.23 <0.6.0;

contract BlindAuction {
    struct Bid {
        bytes32 blindedBid;
        uint deposit;
    }

    address payable public beneficiary;
    uint public biddingEnd;
    uint public revealEnd;
    bool public ended;

    mapping(address => Bid[]) public bids;

    address public highestBidder;
    uint public highestBid;

    // Allowed withdrawals of previous bids
    mapping(address => uint) pendingReturns;

    event AuctionEnded(address winner, uint highestBid);

    /// Modifiers are a convenient way to validate inputs to
    /// functions. `onlyBefore` is applied to `bid` below:
    /// The new function body is the modifier's body where
    /// `_` is replaced by the old function body.
    modifier onlyBefore(uint _time) { require(now < _time); _; }
    modifier onlyAfter(uint _time) { require(now > _time); _; }

    constructor(
        uint _biddingTime,
        uint _revealTime,
        address payable _beneficiary
    ) public {
        beneficiary = _beneficiary;
        biddingEnd = now + _biddingTime;
        revealEnd = biddingEnd + _revealTime;
    }

    /// Place a blinded bid with `_blindedBid` =
    /// keccak256(abi.encodePacked(value, fake, secret)).
    /// The sent ether is only refunded if the bid is correctly
    /// revealed in the revealing phase. The bid is valid if the
    /// ether sent together with the bid is at least "value" and
    /// "fake" is not true. Setting "fake" to true and sending
    /// not the exact amount are ways to hide the real bid but
    /// still make the required deposit. The same address can
    /// place multiple bids.
    function bid(bytes32 _blindedBid)
        public
        payable
        onlyBefore(biddingEnd)
    {
        bids[msg.sender].push(Bid({
            blindedBid: _blindedBid,
            deposit: msg.value
        }));
    }

    /// Reveal your blinded bids. You will get a refund for all
    /// correctly blinded invalid bids and for all bids except for
    /// the totally highest.
    function reveal(
        uint[] memory _values,
        bool[] memory _fake,
        bytes32[] memory _secret
    )
        public
        onlyAfter(biddingEnd)
        onlyBefore(revealEnd)
    {
        uint length = bids[msg.sender].length;
        require(_values.length == length);
        require(_fake.length == length);
        require(_secret.length == length);

        uint refund;
        for (uint i = 0; i < length; i++) {
            Bid storage bidToCheck = bids[msg.sender][i];
            (uint value, bool fake, bytes32 secret) =
                    (_values[i], _fake[i], _secret[i]);
            if (bidToCheck.blindedBid != keccak256(abi.encodePacked(value, fake, secret))) {
                // Bid was not actually revealed.
                // Do not refund deposit.
                continue;
            }
            refund += bidToCheck.deposit;
            if (!fake && bidToCheck.deposit >= value) {
                if (placeBid(msg.sender, value))
                    refund -= value;
            }
            // Make it impossible for the sender to re-claim
            // the same deposit.
            bidToCheck.blindedBid = bytes32(0);
        }
        msg.sender.transfer(refund);
    }

    // This is an "internal" function which means that it
    // can only be called from the contract itself (or from
    // derived contracts).
    function placeBid(address bidder, uint value) internal
            returns (bool success)
    {
        if (value <= highestBid) {
            return false;
        }
        if (highestBidder != address(0)) {
            // Refund the previously highest bidder.
            pendingReturns[highestBidder] += highestBid;
        }
        highestBid = value;
        highestBidder = bidder;
        return true;
    }

    /// Withdraw a bid that was overbid.
    function withdraw() public {
        uint amount = pendingReturns[msg.sender];
        if (amount > 0) {
            // It is important to set this to zero because the recipient
            // can call this function again as part of the receiving call
            // before `transfer` returns (see the remark above about
            // conditions -> effects -> interaction).
            pendingReturns[msg.sender] = 0;

            msg.sender.transfer(amount);
        }
    }

    /// End the auction and send the highest bid
    /// to the beneficiary.
    function auctionEnd()
        public
        onlyAfter(revealEnd)
    {
        require(!ended);
        emit AuctionEnded(highestBidder, highestBid);
        ended = true;
        beneficiary.transfer(highestBid);
    }
}

Safe Remote Purchase

pragma solidity >=0.4.22 <0.6.0;

contract Purchase {
    uint public value;
    address payable public seller;
    address payable public buyer;
    enum State { Created, Locked, Inactive }
    State public state;

    // Ensure that `msg.value` is an even number.
    // Division will truncate if it is an odd number.
    // Check via multiplication that it wasn't an odd number.
    constructor() public payable {
        seller = msg.sender;
        value = msg.value / 2;
        require((2 * value) == msg.value, "Value has to be even.");
    }

    modifier condition(bool _condition) {
        require(_condition);
        _;
    }

    modifier onlyBuyer() {
        require(
            msg.sender == buyer,
            "Only buyer can call this."
        );
        _;
    }

    modifier onlySeller() {
        require(
            msg.sender == seller,
            "Only seller can call this."
        );
        _;
    }

    modifier inState(State _state) {
        require(
            state == _state,
            "Invalid state."
        );
        _;
    }

    event Aborted();
    event PurchaseConfirmed();
    event ItemReceived();

    /// Abort the purchase and reclaim the ether.
    /// Can only be called by the seller before
    /// the contract is locked.
    function abort()
        public
        onlySeller
        inState(State.Created)
    {
        emit Aborted();
        state = State.Inactive;
        seller.transfer(address(this).balance);
    }

    /// Confirm the purchase as buyer.
    /// Transaction has to include `2 * value` ether.
    /// The ether will be locked until confirmReceived
    /// is called.
    function confirmPurchase()
        public
        inState(State.Created)
        condition(msg.value == (2 * value))
        payable
    {
        emit PurchaseConfirmed();
        buyer = msg.sender;
        state = State.Locked;
    }

    /// Confirm that you (the buyer) received the item.
    /// This will release the locked ether.
    function confirmReceived()
        public
        onlyBuyer
        inState(State.Locked)
    {
        emit ItemReceived();
        // It is important to change the state first because
        // otherwise, the contracts called using `send` below
        // can call in again here.
        state = State.Inactive;

        // NOTE: This actually allows both the buyer and the seller to
        // block the refund - the withdraw pattern should be used.

        buyer.transfer(value);
        seller.transfer(address(this).balance);
    }
}

Micropayment Channel

In this section we will learn how to build a simple implementation of a payment channel. It use cryptographics signatures to make repeated transfers of Ether between the same parties secure, instantaneous, and without transaction fees. To do it we need to understand how to sign and verify signatures, and setup the payment channel.

Creating and verifying signatures

Imagine Alice wants to send a quantity of Ether to Bob, i.e. Alice is the sender and the Bob is the recipient. Alice only needs to send cryptographically signed messages off-chain (e.g. via email) to Bob and it will be very similar to writing checks.

Signatures are used to authorize transactions, and they are a general tool that is available to smart contracts. Alice will build a simple smart contract that lets her transmit Ether, but in a unusual way, instead of calling a function herself to initiate a payment, she will let Bob do that, and therefore pay the transaction fee. The contract will work as follows:

  1. Alice deploys the ReceiverPays contract, attaching enough Ether to cover the payments that will be made.
  2. Alice authorizes a payment by signing a message with their private key.
  3. Alice sends the cryptographically signed message to Bob. The message does not need to be kept secret (you will understand it later), and the mechanism for sending it does not matter.
  4. Bob claims their payment by presenting the signed message to the smart contract, it verifies the authenticity of the message and then releases the funds.

Creating the signature

Alice does not need to interact with Ethereum network to sign the transaction, the process is completely offline. In this tutorial, we will sign messages in the browser using web3.js and MetaMask. In particular, we will use the standard way described in EIP-762, as it provides a number of other security benefits.

/// Hashing first makes a few things easier
var hash = web3.sha3("message to sign");
web3.personal.sign(hash, web3.eth.defaultAccount, function () {...});

Note that the web3.personal.sign prepends the length of the message to the signed data. Since we hash first, the message will always be exactly 32 bytes long, and thus this length prefix is always the same, making everything easier.

What to Sign

For a contract that fulfills payments, the signed message must include:

  1. The recipient’s address
  2. The amount to be transferred
  3. Protection against replay attacks

A replay attack is when a signed message is reused to claim authorization for a second action. To avoid replay attacks we will use the same as in Ethereum transactions themselves, a so-called nonce, which is the number of transactions sent by an account. The smart contract will check if a nonce is used multiple times.

There is another type of replay attacks, it occurs when the owner deploys a ReceiverPays smart contract, performs some payments, and then destroy the contract. Later, she decides to deploy the RecipientPays smart contract again, but the new contract does not know the nonces used in the previous deployment, so the attacker can use the old messages again.

Alice can protect against it including the contract’s address in the message, and only messages containing contract’s address itself will be accepted. This functionality can be found in the first two lines of the claimPayment() function in the full contract at the end of this chapter.

Packing arguments

Now that we have identified what information to include in the signed message, we are ready to put the message together, hash it, and sign it. For simplicity, we just concatenate the data. The ethereumjs-abi library provides a function called soliditySHA3 that mimics the behavior of Solidity’s keccak256 function applied to arguments encoded using abi.encodePacked. Putting it all together, here is a JavaScript function that creates the proper signature for the ReceiverPays example:

// recipient is the address that should be paid.
// amount, in wei, specifies how much ether should be sent.
// nonce can be any unique number to prevent replay attacks
// contractAddress is used to prevent cross-contract replay attacks
function signPayment(recipient, amount, nonce, contractAddress, callback) {
    var hash = "0x" + ethereumjs.ABI.soliditySHA3(
        ["address", "uint256", "uint256", "address"],
        [recipient, amount, nonce, contractAddress]
    ).toString("hex");

    web3.personal.sign(hash, web3.eth.defaultAccount, callback);
}

Recovering the Message Signer in Solidity

In general, ECDSA signatures consist of two parameters, r and s. Signatures in Ethereum include a third parameter called v, that can be used to recover which account’s private key was used to sign in the message, the transaction’s sender. Solidity provides a built-in function ecrecover that accepts a message along with the r, s and v parameters and returns the address that was used to sign the message.

Extracting the Signature Parameters

Signatures produced by web3.js are the concatenation of r, s and v, so the first step is splitting those parameters back out. It can be done on the client, but doing it inside the smart contract means only one signature parameter needs to be sent rather than three. Splitting apart a byte array into component parts is a little messy. We will use inline assembly to do the job in the splitSignature function (the third function in the full contract at the end of this chapter).

Computing the Message Hash

The smart contract needs to know exactly what parameters were signed, and so it must recreate the message from the parameters and use that for signature verification. The functions prefixed and recoverSigner do this and their use can be found in the claimPayment function.

The full contract

pragma solidity >=0.4.24 <0.6.0;

contract ReceiverPays {
    address owner = msg.sender;

    mapping(uint256 => bool) usedNonces;

    constructor() public payable {}

    function claimPayment(uint256 amount, uint256 nonce, bytes memory signature) public {
        require(!usedNonces[nonce]);
        usedNonces[nonce] = true;

        // this recreates the message that was signed on the client
        bytes32 message = prefixed(keccak256(abi.encodePacked(msg.sender, amount, nonce, this)));

        require(recoverSigner(message, signature) == owner);

        msg.sender.transfer(amount);
    }

    /// destroy the contract and reclaim the leftover funds.
    function kill() public {
        require(msg.sender == owner);
        selfdestruct(msg.sender);
    }

    /// signature methods.
    function splitSignature(bytes memory sig)
        internal
        pure
        returns (uint8 v, bytes32 r, bytes32 s)
    {
        require(sig.length == 65);

        assembly {
            // first 32 bytes, after the length prefix.
            r := mload(add(sig, 32))
            // second 32 bytes.
            s := mload(add(sig, 64))
            // final byte (first byte of the next 32 bytes).
            v := byte(0, mload(add(sig, 96)))
        }

        return (v, r, s);
    }

    function recoverSigner(bytes32 message, bytes memory sig)
        internal
        pure
        returns (address)
    {
        (uint8 v, bytes32 r, bytes32 s) = splitSignature(sig);

        return ecrecover(message, v, r, s);
    }

    /// builds a prefixed hash to mimic the behavior of eth_sign.
    function prefixed(bytes32 hash) internal pure returns (bytes32) {
        return keccak256(abi.encodePacked("\x19Ethereum Signed Message:\n32", hash));
    }
}

Writing a Simple Payment Channel

Alice will now build a simple but complete implementation of a payment channel. Payment channels use cryptographic signatures to make repeated transfers of Ether securely, instantaneously, and without transaction fees.

What is a Payment Channel?

Payment channels allow participants to make repeated transfers of Ether without using transactions. This means that the delays and fees associated with transactions can be avoided. We are going to explore a simple unidirectional payment channel between two parties (Alice and Bob). Using it involves three steps:

  1. Alice funds a smart contract with Ether. This “opens” the payment channel.
  2. Alice signs messages that specify how much of that Ether is owed to the recipient. This step is repeated for each payment.
  3. Bob “closes” the payment channel, withdrawing their portion of the Ether and sending the remainder back to the sender.

Not ethat only steps 1 and 3 require Ethereum transactions, step 2 means that the sender transmits a cryptographically signed message to the recipient via off chain ways (e.g. email). This means only two transactions are required to support any number of transfers.

Bob is guaranteed to receive their funds because the smart contract escrows the Ether and honors a valid signed message. The smart contract also enforces a timeout, so Alice is guaranteed to eventually recover their funds even if the recipient refuses to close the channel. It is up to the participants in a payment channel to decide how long to keep it open. For a short-lived transaction, such as paying an internet cafe for each minute of network access, or for a longer relationship, such as paying an employee an hourly wage, a payment could last for months or years.

Opening the Payment Channel

To open the payment channel, Alice deploys the smart contract, attaching the Ether to be escrowed and specifying the intendend recipient and a maximum duration for the channel to exist. It is the function SimplePaymentChannel in the contract, that is at the end of this chapter.

Making Payments

Alice makes payments by sending signed messages to Bob. This step is performed entirely outside of the Ethereum network. Messages are cryptographically signed by the sender and then transmitted directly to the recipient.

Each message includes the following information:

  • The smart contract’s address, used to prevent cross-contract replay attacks.
  • The total amount of Ether that is owed the recipient so far.

A payment channel is closed just once, at the end of a series of transfers. Because of this, only one of the messages sent will be redeemed. This is why each message specifies a cumulative total amount of Ether owed, rather than the amount of the individual micropayment. The recipient will naturally choose to redeem the most recent message because that is the one with the highest total. The nonce per-message is not needed anymore, because the smart contract will only honor a single message. The address of the smart contract is still used to prevent a message intended for one payment channel from being used for a different channel.

Here is the modified javascript code to cryptographically sign a message from the previous chapter:

function constructPaymentMessage(contractAddress, amount) {
    return ethereumjs.ABI.soliditySHA3(
        ["address", "uint256"],
        [contractAddress, amount]
    );
}

function signMessage(message, callback) {
    web3.personal.sign(
        "0x" + message.toString("hex"),
        web3.eth.defaultAccount,
        callback
    );
}

// contractAddress is used to prevent cross-contract replay attacks.
// amount, in wei, specifies how much Ether should be sent.

function signPayment(contractAddress, amount, callback) {
    var message = constructPaymentMessage(contractAddress, amount);
    signMessage(message, callback);
}

Closing the Payment Channel

When Bob is ready to receive their funds, it is time to close the payment channel by calling a close function on the smart contract. Closing the channel pays the recipient the Ether they are owed and destroys the contract, sending any remaining Ether back to Alice. To close the channel, Bob needs to provide a message signed by Alice.

The smart contract must verify that the message contains a valid signature from the sender. The process for doing this verification is the same as the process the recipient uses. The Solidity functions isValidSignature and recoverSigner work just like their JavaScript counterparts in the previous section. The latter is borrowed from the ReceiverPays contract in the previous chapter.

The close function can only be called by the payment channel recipient, who will naturally pass the most recent payment message because that message carries the highest total owed. If the sender were allowed to call this function, they could provide a message with a lower amount and cheat the recipient out of what they are owed.

The function verifies the signed message matches the given parameters. If everything checks out, the recipient is sent their portion of the Ether, and the sender is sent the rest via a selfdestruct. You can see the close function in the full contract.

Channel Expiration

Bob can close the payment channel at any time, but if they fail to do so, Alice needs a way to recover their escrowed funds. An expiration time was set at the time of contract deployment. Once that time is reached, Alice can call claimTimeout to recover their funds. You can see the claimTimeout function in the full contract.

After this function is called, Bob can no longer receive any Ether, so it is important that Bob closes the channel before the expiration is reached.

The full contract

pragma solidity >=0.4.24 <0.6.0;

contract SimplePaymentChannel {
    address payable public sender;      // The account sending payments.
    address payable public recipient;   // The account receiving the payments.
    uint256 public expiration;  // Timeout in case the recipient never closes.

    constructor (address payable _recipient, uint256 duration)
        public
        payable
    {
        sender = msg.sender;
        recipient = _recipient;
        expiration = now + duration;
    }

    function isValidSignature(uint256 amount, bytes memory signature)
        internal
        view
        returns (bool)
    {
        bytes32 message = prefixed(keccak256(abi.encodePacked(this, amount)));

        // check that the signature is from the payment sender
        return recoverSigner(message, signature) == sender;
    }

    /// the recipient can close the channel at any time by presenting a
    /// signed amount from the sender. the recipient will be sent that amount,
    /// and the remainder will go back to the sender
    function close(uint256 amount, bytes memory signature) public {
        require(msg.sender == recipient);
        require(isValidSignature(amount, signature));

        recipient.transfer(amount);
        selfdestruct(sender);
    }

    /// the sender can extend the expiration at any time
    function extend(uint256 newExpiration) public {
        require(msg.sender == sender);
        require(newExpiration > expiration);

        expiration = newExpiration;
    }

    /// if the timeout is reached without the recipient closing the channel,
    /// then the Ether is released back to the sender.
    function claimTimeout() public {
        require(now >= expiration);
        selfdestruct(sender);
    }

    /// All functions below this are just taken from the chapter
    /// 'creating and verifying signatures' chapter.

    function splitSignature(bytes memory sig)
        internal
        pure
        returns (uint8 v, bytes32 r, bytes32 s)
    {
        require(sig.length == 65);

        assembly {
            // first 32 bytes, after the length prefix
            r := mload(add(sig, 32))
            // second 32 bytes
            s := mload(add(sig, 64))
            // final byte (first byte of the next 32 bytes)
            v := byte(0, mload(add(sig, 96)))
        }

        return (v, r, s);
    }

    function recoverSigner(bytes32 message, bytes memory sig)
        internal
        pure
        returns (address)
    {
        (uint8 v, bytes32 r, bytes32 s) = splitSignature(sig);

        return ecrecover(message, v, r, s);
    }

    /// builds a prefixed hash to mimic the behavior of eth_sign.
    function prefixed(bytes32 hash) internal pure returns (bytes32) {
        return keccak256(abi.encodePacked("\x19Ethereum Signed Message:\n32", hash));
    }
}

Note: The function splitSignature is very simple and does not use all security checks. A real implementation should use a more rigorously tested library, such as openzepplin’s version of this code.

Verifying Payments

Unlike in our previous chapter, messages in a payment channel aren’t redeemed right away. The recipient keeps track of the latest message and redeems it when it’s time to close the payment channel. This means it’s critical that the recipient perform their own verification of each message. Otherwise there is no guarantee that the recipient will be able to get paid in the end.

The recipient should verify each message using the following process:

  1. Verify that the contact address in the message matches the payment channel.
  2. Verify that the new total is the expected amount.
  3. Verify that the new total does not exceed the amount of Ether escrowed.
  4. Verify that the signature is valid and comes from the payment channel sender.

We’ll use the ethereumjs-util library to write this verifications. The final step can be done a number of ways, but if it’s being done in JavaScript. The following code borrows the constructMessage function from the signing JavaScript code above:

// this mimics the prefixing behavior of the eth_sign JSON-RPC method.
function prefixed(hash) {
    return ethereumjs.ABI.soliditySHA3(
        ["string", "bytes32"],
        ["\x19Ethereum Signed Message:\n32", hash]
    );
}

function recoverSigner(message, signature) {
    var split = ethereumjs.Util.fromRpcSig(signature);
    var publicKey = ethereumjs.Util.ecrecover(message, split.v, split.r, split.s);
    var signer = ethereumjs.Util.pubToAddress(publicKey).toString("hex");
    return signer;
}

function isValidSignature(contractAddress, amount, signature, expectedSigner) {
    var message = prefixed(constructPaymentMessage(contractAddress, amount));
    var signer = recoverSigner(message, signature);
    return signer.toLowerCase() ==
        ethereumjs.Util.stripHexPrefix(expectedSigner).toLowerCase();
}