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.

/// @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`.
    function Ballot(bytes32[] proposalNames)
    {
        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 `proposal.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)
    {
        if (msg.sender != chairperson || voters[voter].voted)
            // `throw` terminates and reverts all changes to
            // the state and to Ether balances. It is often
            // a good idea to use this if functions are
            // called incorrectly. But watch out, this
            // will also consume all provided gas.
            throw;
        voters[voter].weight = 1;
    }

    /// Delegate your vote to the voter `to`.
    function delegate(address to)
    {
        // assigns reference
        Voter sender = voters[msg.sender];
        if (sender.voted)
            throw;
        // Forward the delegation as long as
        // `to` also delegated.
        while (voters[to].delegate != address(0) &&
               voters[to].delegate != msg.sender)
            to = voters[to].delegate;
        // We found a loop in the delegation, not allowed.
        if (to == msg.sender)
            throw;
        // Since `sender` is a reference, this
        // modifies `voters[msg.sender].voted`
        sender.voted = true;
        sender.delegate = to;
        Voter 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)
    {
        Voter sender = voters[msg.sender];
        if (sender.voted) throw;
        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() constant
            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;
            }
        }
    }
}

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 his money - contracts cannot activate themselves.

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

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

    // Set to true at the end, disallows any change
    bool ended;

    // Events that will be fired 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`.
    function SimpleAuction(uint _biddingTime,
                           address _beneficiary) {
        beneficiary = _beneficiary;
        auctionStart = now;
        biddingTime = _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() {
        // No arguments are necessary, all
        // information is already part of
        // the transaction.
        if (now > auctionStart + biddingTime)
            // Revert the call if the bidding
            // period is over.
            throw;
        if (msg.value <= highestBid)
            // If the bid is not higher, send the
            // money back.
            throw;
        if (highestBidder != 0)
            highestBidder.send(highestBid);
        highestBidder = msg.sender;
        highestBid = msg.value;
        HighestBidIncreased(msg.sender, msg.value);
    }

    /// End the auction and send the highest bid
    /// to the beneficiary.
    function auctionEnd() {
        if (now <= auctionStart + biddingTime)
            throw; // auction did not yet end
        if (ended)
            throw; // this function has already been called
        AuctionEnded(highestBidder, highestBid);
        // We send all the money we have, because some
        // of the refunds might have failed.
        beneficiary.send(this.balance);
        ended = true;
    }

    function () {
        // This function gets executed if a
        // transaction with invalid data is sent to
        // the contract or just ether without data.
        // We revert the send so that no-one
        // accidentally loses money when using the
        // contract.
        throw;
    }
}

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 he 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 at least as large as the 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.

contract BlindAuction
{
    struct Bid
    {
        bytes32 blindedBid;
        uint deposit;
    }
    address public beneficiary;
    uint public auctionStart;
    uint public biddingEnd;
    uint public revealEnd;
    bool public ended;

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

    address public highestBidder;
    uint public highestBid;

    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) { if (now >= _time) throw; _ }
    modifier onlyAfter(uint _time) { if (now <= _time) throw; _ }

    function BlindAuction(uint _biddingTime,
                            uint _revealTime,
                            address _beneficiary)
    {
        beneficiary = _beneficiary;
        auctionStart = now;
        biddingEnd = now + _biddingTime;
        revealEnd = biddingEnd + _revealTime;
    }

    /// Place a blinded bid with `_blindedBid` = sha3(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)
        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[] _values, bool[] _fake,
                    bytes32[] _secret)
        onlyAfter(biddingEnd)
        onlyBefore(revealEnd)
    {
        uint length = bids[msg.sender].length;
        if (_values.length != length || _fake.length != length ||
                    _secret.length != length)
            throw;
        uint refund;
        for (uint i = 0; i < length; i++)
        {
            var bid = bids[msg.sender][i];
            var (value, fake, secret) =
                    (_values[i], _fake[i], _secret[i]);
            if (bid.blindedBid != sha3(value, fake, secret))
                // Bid was not actually revealed.
                // Do not refund deposit.
                continue;
            refund += bid.deposit;
            if (!fake && bid.deposit >= value)
                if (placeBid(msg.sender, value))
                    refund -= value;
            // Make it impossible for the sender to re-claim
            // the same deposit.
            bid.blindedBid = 0;
        }
        msg.sender.send(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 != 0)
            // Refund the previously highest bidder.
            highestBidder.send(highestBid);
        highestBid = value;
        highestBidder = bidder;
        return true;
    }

    /// End the auction and send the highest bid
    /// to the beneficiary.
    function auctionEnd()
        onlyAfter(revealEnd)
    {
        if (ended) throw;
        AuctionEnded(highestBidder, highestBid);
        // We send all the money we have, because some
        // of the refunds might have failed.
        beneficiary.send(this.balance);
        ended = true;
    }

    function () { throw; }
}

Safe Remote Purchase

contract Purchase
{
    uint public value;
    address public seller;
    address public buyer;
    enum State { Created, Locked, Inactive }
    State public state;
    function Purchase()
    {
        seller = msg.sender;
        value = msg.value / 2;
        if (2 * value != msg.value) throw;
    }
    modifier require(bool _condition)
    {
        if (!_condition) throw;
        _
    }
    modifier onlyBuyer()
    {
        if (msg.sender != buyer) throw;
        _
    }
    modifier onlySeller()
    {
        if (msg.sender != seller) throw;
        _
    }
    modifier inState(State _state)
    {
        if (state != _state) throw;
        _
    }
    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()
        onlySeller
        inState(State.Created)
    {
        aborted();
        seller.send(this.balance);
        state = State.Inactive;
    }
    /// Confirm the purchase as buyer.
    /// Transaction has to include `2 * value` ether.
    /// The ether will be locked until confirmReceived
    /// is called.
    function confirmPurchase()
        inState(State.Created)
        require(msg.value == 2 * value)
    {
        purchaseConfirmed();
        buyer = msg.sender;
        state = State.Locked;
    }
    /// Confirm that you (the buyer) received the item.
    /// This will release the locked ether.
    function confirmReceived()
        onlyBuyer
        inState(State.Locked)
    {
        itemReceived();
        buyer.send(value); // We ignore the return value on purpose
        seller.send(this.balance);
        state = State.Inactive;
    }
    function() { throw; }
}

Micropayment Channel

To be written.