- Thenaesh Elango (A0124772E)
- Khairul Rizqi (A0121284N)
- Er Xue Hui (A0107345L)
- Go Hui Shan (A0126000H)
We present a blockchain smart-contract based lottery system implemented on Ethereum. This lottery system enables anyone with an Ethereum account to create or participate in a lottery securely and anonymously.
An Ethereum user may deploy a lottery contract onto the blockchain and become the owner of the lottery. Any Ethereum user can then purchase a ticket using Ether (the cryptocurrency of Ethereum), specifying a particular ticket number. The Ether goes into a common pool in the lottery contract.
The owner of the lottery can then close the lottery. A winning number is picked and the prize pool is divided among the winners equally after a chunk is set aside as the owner's commission. Each winner is then sent his share of the winnings.
The owner can then destroy the contract and claim his commission.
The lottery contract's constructor accepts a single argument uint[2] args,
where uint[0] is the ticket price and uint[1] is the commission of the
owner (expressed as an integer from 0 to 100 representing how many percent
commission the owner takes). Upon deploying the contract, these parameters are
fixed and cannot be changed by anyone, not even the owner.
The lottery contract has a method called placeBet(uint _tixnum). Anyone
with an Ethereum account wishing to participate in the lottery simply sends
a transaction from his account to the lottery contract, including in the
transaction an amount of Ether sufficient to pay the ticket fees. The account
holder is then registered as a player and any excess Ether sent is refunded.
If the amount of Ether sent is not sufficient to pay the transaction fees, the
account holder is not registered in the lottery and all the Ether sent is
refunded. The Ether sent to the contract is tracked in the pool variable.
The lottery contract has a method called closeLottery(). This method, which
is only executed if the caller is the contract owner, picks a winning number.
The winners are then selected from all the players, by checking who purchased
a ticket with the winning number (there may be more that one person who
purchased the ticket with the winning number, and hence more than one winner).
The prize pool is (after setting aside the commission amount for the owner)
divided equally and sent to all the winners.
The lottery contract has a method called kill(), which is only executed if
the caller is the owner. The method destroys the contract using Ethereum's
selfdestruct mechanism and passes the commission to the winner.
A script start.sh is provided to enable easy creation of a test blockchain.
The file CustomGenesis.json contains parameters to be supplied when creating
the blockchain. Adjust the parameters if necessary (especially the block gas
limit), and then run ./start.sh init CustomGenesis.json to initialise the
blockchain.
Ethereum can then be started using ./start.sh console. If a node is already
running, it is possible to attach to it using
./start.sh attach ipc:blockchain_data/geth.ipc instead. Typically, one will
start the console and start the miner in one shell, then attach to that console
in another shell to interact with it. This reduces the need for repeatedly
starting and stopping the miner while experimenting with the contract.
Many utility functions are provided in the script/ directory, and all can be
loaded into the Ethereum console with loadScript('init.js').
One may compile the contract with var contracts = compile("..."); and then
deploy with var lottery = deploy(contracts.Lottery, account_to_deploy_from);.
Assuming the miner is running, the contract will soon be inserted into the
blockchain and acquire an address. If the contract is already deployed at a
known address, it may be retrieved with
getContract(contracts.Lottery, known_address_of_contract);.
A script prepare_contract.sh is provided to help strip newlines from the
contract to enable insertion into the compile function.
The blockchain-based approach to lottery systems yields many benefits over simply hosting a lottery website and using existing mechanisms to transfer money to buy tickets. The benefits may be summed up as follows:
- Anonymity
- Reduced Trust Requirement in Lottery Owner
- Non-Repudiation of Ticket Purchase
We address each of these in turn.
On a traditional client-server-based lottery website, one first needs to authenticate oneself in order to purchase a lottery ticket. One then needs to pay for the ticket using traditional payment mechanisms like credit cards. Upon winning, one certainly needs to authenticate oneself before collecting the prize money.
At every stage, there is no expectation of anonymity in a traditional site. This may be highly undesirable due to the social stigma associated with gambling and the envy of one's peers (with potentially devastating consequences).
Using Ethereum, the only requirement to participate in the lottery is an Ethereum address. While the potential for deanonymisation remains if the Ethereum address is carelessly associated with the user's real identity, it is very much possible to maintain anonymity with an Ethereum address. Ethereum users can thus anonymously buy tickets, and even anonymously set up lotteries, as long as they take care to dissociate their real identities from their Ethereum address.
In a traditional client-server-based lottery website, it may be difficult to trust the party maintaining the website. A malicious lottery-site owner has many means to cheat the players of their money.
In the most extreme case, after accepting money for purchase of lottery tickets, the owner can simply shut down the site and disappear with the money, leaving little recourse for the victims especially if they are located in a different country.
Alternatively, the owner can select a winning number to give an unfair advantage to some player in the lottery.
The problem is that, in the traditional website, the lottery owner has full control over any state in the server, and can modify it in any way. This requires players to trust that the owner will not undertake any malicious activity.
Using Ethereum allows players to do away with trust in the owner, as the owner does not have any special privileges once the contract is created. A player can check the contract's code to make sure that the owner has not inserted any backdoors to enable malicious state changes. The player can then be convinced that the contract does exactly what it specifies, and this is verified each time a transaction is made and a block is mined.
The entire code and specifications of the contract are laid bare on the blockchain for all to see, allowing players to confidently purchase tickets without having to worry about a fraudulent owner.
On the other hand, a traditional client-server based lottery website enables a user to claim he has not purchased a ticket even though he has, or to claim that he has purchased a different ticket from what the website has shown. The player may claim that the owner of the website has changed his ticket number, or defrauded him in some similar manner.
These claims can be made as the state of the lottery system is entirely under the owner's control and the owner may plausibly have made the alleged changes.
With Ethereum, a player's ticket purchase is recorded indelibly in the blockchain, and the player cannot deny having purchased a particular ticket number or claim to have purchased a different ticket number.
We have thus seen how blockchain-based lotteries have several advantages over their traditional counterparts. Their decentralised and anonymous nature allows them to maintain a permanent, indelible record of all transactions (barring an attack on Ethereum itself, such as through a 51% attack), while allowing players to keep their identities hidden. An incorruptible, mechanical third party in the form of the contract itself allows players and lottery owners to not have to worry about being defrauded.
Smart contracts on Ethereum may indeed herald a new age of privacy, especially when government and corporate surveillance of individuals is reaching its peak.