High Stakes Roulette on Ethereum
Learn by Example: Building a secure High Stakes Roulette
It's always best to learn with examples. So let's build a little online casino on the blockchain. We'll also make it secure enough to allow playing in really high stakes by adding a secure randomness generator.
Let's discuss the overall design first.
Designing the contract
Before we program anything as developers, we properly plan and design our system. In our Roulette we need to figure out how we can create a random number, how we manage funds and how to handle timeouts.
Securely generating a random number
At the core of the contract will be a commitment scheme. If you want to learn more about randomness on the blockchain, check out my previous tutorial here. But in summary, there is no good direct source for randomness in the blockchain, since all code has to run deterministically.
Solution for low stakes: Using a future block hash is a possible solution, but miners have some influence on this value. They can choose to not publish a new block, foregoing the block reward. But if they are meanwhile playing a very high stakes game of Roulette, withholding a block might be the better strategy for them.
Solution for high stakes: So for a high stakes setup we need a better randomness generator. Luckily in a setup with two participants (bank and player), we can use the commitment scheme. Each player commits to secret random number by first sending the keccak256 commitment hash of that number. Once both hashes are in the contract, players can securely reveal the actual random number. The contract verifies that keccak256(randomNumber) == commitment hash, ensuring both parties cannot change the random number anymore. The final random number will be randomNumberBank XOR randomNumberPlayer. More details about this are explained in the linked tutorial above.
Improving the high stakes design
Given this high stakes design, we can improve upon it for our blockchain casino. The improvements can be done in two ways:
- Having the bank pre-commit to a secret number sending the hash is actually sufficient. When the bank is committed, a player can directly reveal his value. This reduces a game round by one step, avoiding the player to also send a commitment hash.
- Assuming a player will not just play a single round, we can build a chain of commitment hashes. This chain is computed as keccak256(keccak256(keccak256( ... (randomNumber) ... ))). You basically calculate the hash of the hash of the hash, and so on, for millions of times. The last hash is later sent as the first commitment, while all other intermediate hashes are stored for later usage. Any secret number reveal from the bank will then automatically be the commitment for the next round.
Using those two improvements, a single game round is then reduced to
- Player sending a secret number along with his bet. For simplicity we will only allow betting on red or black, but it should be easy to extend the functionality.
- The bank sending the reveal which will automatically trigger the payout.
Funds Management
We don't want to send out funds for every single game round. So just like in the real world, our casino will have its own funds management. Players and the bank deposit funds in the contract and receive in-game funds. They can withdraw any unlocked funds or deposit more funds whenever they want.
Handling timeouts
In the commitment scheme has one way to manipulate a result: not revealing and thus preventing a round from finishing. To handle this case, we need an additional function for players that checks if the bank has not sent a reveal within a defined time. In that case the player wins automatically.
Managing funds in the contract
We can declare a simple mapping in the storage:
mapping (address => uint256) public registeredFunds;In here we can add the deposited amount for an address upon sending ETH, or likewise remove the withdrawn amount upon taking ETH out. You could similarly just use an ERC-20 token instead of ETH.
We use the .call method instead of .transfer, as transfer is not a recommended way anymore for sending ETH.
function depositFunds() external payable {
require(msg.value > 0, "Must send ETH");
registeredFunds[msg.sender] += msg.value;
}
function withdrawFunds() external {
require(registeredFunds[msg.sender] > 0);
uint256 funds = registeredFunds[msg.sender];
registeredFunds[msg.sender] = 0;
(bool success, ) = msg.sender.call{value: funds}("");
require(success, "ETH transfer failed");
}Placing a bet
Next up let's create the actual game. A single round will be defined by our GameRound struct. We'll have the values for generating the random number:
- bankHash
- bankSecretValue
- userValue
As well as the choice by the user if he bet on red or on black (better coding style may be to use an enum here with the two values RED and BLACK). And the size potential money to win as lockedFunds. For red/black bets this will be double the bet size. And we'll also need to store the time of placing the bet for the timeout function.
struct GameRound {
bytes32 bankHash;
uint256 bankSecretValue;
uint256 userValue;
bool hasUserBetOnRed;
uint256 timeWhenSecretUserValueSubmitted;
uint256 lockedFunds;
}Now with that we can create a placeBet function. We'll make sure that the game is in the correct state and that enough funds exist for the bank and the player. We'll store the bet, lock the funds and store the time for the timeout.
Why do we store one bank hash per player? You may wonder why we don't just use one bank hash across all games. This seems tempting as it reduces the complexity for the bank. Unfortunately it would allow full manipulation of the randomness. Imagine multiple players betting at the same time. Now the bank could decide for which player to send the current reveal for. To prevent this we would need to enforce a strict order for each reveal according to the time of the bet. That would ultimately end up being more complex than having one hash per player.
function placeBet(bool hasUserBetOnRed, uint256 userValue, uint256 betAmount) external {
require(gameRounds[msg.sender].bankHash != 0x0, "Bank hash not yet set");
require(gameRounds[msg.sender].userValue == 0, "Already placed bet");
require(registeredFunds[bankAddress] >= betAmount, "Not enough bank funds");
require(registeredFunds[msg.sender] >= betAmount, "Not enough user funds");
gameRounds[msg.sender].userValue = userValue;
gameRounds[msg.sender].hasUserBetOnRed = hasUserBetOnRed;
gameRounds[msg.sender].lockedFunds = betAmount * 2;
gameRounds[userAddress].timeWhenSecretUserValueSubmitted = block.timestamp;
registeredFunds[msg.sender] -= _betAmount;
registeredFunds[bankAddress] -= _betAmount;
}You may have noticed that the bank hash would be empty before the first round. So we need two extra functions which are only ever used once at the beginning by a player. With initializeGame a player can request the bank to call setInitialBankHash.
function initializeGame() external {
require(!hasRequestedGame[msg.sender],"Already requested");
hasRequestedGame[msg.sender] = true;
emit NewGameRequest(msg.sender);
}The bank would have a server running that listens to the NewGameRequest event. Upon receipt of the event, it will set its initial bank hash.
function setInitialBankHash(
bytes32 bankHash,
address user
) external onlyOwner {
require(
gameRounds[user].bankHash == 0x0,
"Bank hash already set"
);
gameRounds[user].bankHash = bankHash;
}Sending the bank reveal
Now for the actual game, the bank needs to reveal the value. We require that the game round is indeed in the state for the bank to send the value. Also we ensure that the hashedReveal equals gameRounds[userAddress].bankHash, therefore enforcing that the bank cannot manipulate the randomness.
function sendBankSecretValue(uint256 bankSecretValue, address user) external {
require(gameRounds[userAddress].userValue != 0, "User has no value set");
require(gameRounds[userAddress].bankSecretValue == 0, "Already revealed");
bytes32 hashedReveal = keccak256(abi.encodePacked(bankSecretValue));
require(hashedReveal == gameRounds[userAddress].bankHash, "Bank reveal not matching commitment");
gameRounds[userAddress].bankSecretValue = bankSecretValue;
_evaluateBet(user);
_resetContractFor(user);
gameRounds[userAddress].bankHash = bytes32(bankSecretValue);
}Then we evaluate the bet to find out who won. Lastly we reset the data for the next round which includes automatically setting the bank hash to the current secret number (according to our design described at the start).
function _resetContractFor(address user) private {
gameRounds[user] = GameRound(0x0, 0, 0, false, 0, 0);
}
function _evaluateBet(address user) private {
uint256 random = gameRounds[user].bankSecretValue
^ gameRounds[user].userValue;
uint256 number = random % ROULETTE_NUMBER_COUNT;
uint256 winningAmount = gameRounds[user].lockedFunds;
bool isNeitherRedNorBlack = number == 0;
bool isRed = isNumberRed[number];
bool hasUserBetOnRed = gameRounds[user].hasUserBetOnRed;
address winner;
if (isNeitherRedNorBlack) winner = bankAddress;
else if (isRed == hasUserBetOnRed) winner = userAddress;
else winner = bankAddress;
registeredFunds[winner] += winningAmount;
}Evaluating a bet
As for the actual bet evaluation, we now have two randomly chosen numbers by player and bank. Using bitwise OR we can compute a final random number.
Using random % ROULETTE_NUMBER_COUNT, or in other words computing the random number modulo 37, we will get a random number between 0 and 36 with any number having the same chance to be chosen.
Now for evaluating the winner we have three cases:
- Number is 0: Bank wins (the bank profit)
- Color was guessed correctly by player
- Color was not guessed correctly by player
To determine if the color is red, we can use a storage bool[37] isNumberRed array with the definitions.
Handling the timeout
Now given the pre-defined timeout time TIMEOUT_FOR_BANK_REVEAL (for example 2 days), we can check for any timeouts. If the game is indeed waiting for the bank to send the reveal and is waiting for more than the timeout time, a player can call checkBankSecretValueTimeout and will automatically win the game round.
function checkBankSecretValueTimeout() external {
require(gameRounds[msg.sender].bankHash != 0, "Bank hash not set");
require(gameRounds[msg.sender].bankSecretValue == 0, "Bank secret is set");
require(gameRounds[msg.sender].userValue != 0, "User value not set");
uint256 timeout = (gameRounds[msg.sender].timeWhenSecretUserValueSubmitted + TIMEOUT_FOR_BANK_REVEAL);
require(block.timestamp > timeout, "Timeout not yet reached");
registeredFunds[msg.sender] += gameRounds[msg.sender].lockedFunds;
_resetContractFor(msg.sender);
hasRequestedGame[msg.sender] = false;
}Fully working example + missing components
You can find a fully working example here. You should know though that this is only the contract side. As a bank provider you would need a backend server running that handles the logic for listening to new bets and sending the commitment hashes. Further gas improvements are possible by allowing the bank to submit multiple hashes for multiple players at the same time.
Also a nice frontend interface for the players is always welcome.
Solidity Developer
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How to get a Solidity developer job?
There are many ways to get a Solidity job and it might be easier than you think!
You have mastered the basics of Solidity, created your first few useful projects and now want to get your hands on some real-world projects. Getting a Solidity developer job might be easier than you think. There are generally plenty of options to choose from and often times not a lot of...
Design Pattern Solidity: Mock contracts for testing
Why you should make fun of your contracts
Mock objects are a common design pattern in object-oriented programming. Coming from the old French word 'mocquer' with the meaning of 'making fun of', it evolved to 'imitating something real' which is actually what we are doing in programming. Please only make fun of your smart contracts if you...
Kickstart your Dapp frontend development with create-eth-app
An overview on how to use the app and its features
Last time we looked at the big picture of Solidity and already mentioned the create-eth-app. Now you will find out how to use it, what features are integrated and additional ideas on how to expand on it. Started by Paul Razvan Berg, the founder of sablier, this app will kickstart your frontend...
The big picture of Solidity and Blockchain development in 2020
Overview of the most important technologies, services and tools that you need to know
Now, I do not know about you, but I remember when I first started with Solidity development being very confused by all the tools and services and how they work in connection with one another. If you are like me, this overview will help you understand the big picture of Solidity development. As I...
Design Pattern Solidity: Free up unused storage
Why you should clean up after yourself
You may or may not be used to a garbage collectors in your previous programming language. There is no such thing in Solidity and even if there was a similar concept, you would still be better off managing state data yourself. Only you as a programmer can know exactly which data will not be used...
How to setup Solidity Developer Environment on Windows
What you need to know about developing on Windows
Using Windows for development, especially for Solidity development, can be a pain sometimes, but it does not have to be. Once you have configured your environment properly, it can actually be extremely efficient and Windows is a very, very stable OS, so your overall experience can be amazing. The...
Avoiding out of gas for Truffle tests
How you do not have to worry about gas in tests anymore
You have probably seen this error message a lot of times: Error: VM Exception while processing transaction: out of gas Disclaimer : Unfortunately, this does not always actually mean what it is saying when using Truffle , especially for older versions. It can occur for various reasons and might be...
Design Pattern Solidity: Stages
How you can design stages in your contract
Closely related to the concept of finite-state machines, this pattern will help you restrict functions in your contract. You will find a lot of situations where it might be useful. Any time a contract should allow function calls only in certain stages. Let's look at an example: contract Pool {...
Web3 1.2.5: Revert reason strings
How to use the new feature
A new Web3 version was just released and it comes with a new feature that should make your life easier. With the latest version 1.2.5, you can now see the the revert reason if you use the new handleRevert option. You can activate it easily by using web3.eth.handleRevert = true . Now when you use...
Gaining back control of the internet
How Ocelot is decentralizing cloud computing
I recently came across an ambitious company that will completely redefine the way we are using the internet. Or rather, the way we are using its underlying infrastructure which ultimately is the internet. While looking at their offering, I also learned how to get anonymous cloud machines, you...
Devcon 5 - Review
Impressions from the conference
I had a lot to catch up on after Devcon. Also things didn't go quite as planned, so please excuse my delayed review! This year's Devcon was certainly stormy with a big typhoon warning already on day 1. Luckily (for us, not the people in Tokyo), it went right past Osaka. Nevertheless, a lot of...
Devcon 5 - Information, Events, Links, Telegram
What you need to know
Devcon 5 is coming up soon and there are already lots of events available, information about Osaka and more. Here is a short overview: Events Events Calendar Events Google Docs Events Kickback Most events are in all three, but if you really want to see all, you will have to look at all three...
Design Pattern Solidity: Off-chain beats on-chain
Why you should do as much as possible off-chain
As you might have realized, Ethereum transactions are anything but cheap. In particular, if you are computing complex things or storing a lot of data. That means sometimes we cannot put all logic inside Solidity. Instead, we can utilize off-chain computations to help us. A very simple example...
Design Pattern Solidity: Initialize Contract after Deployment
How to use the Initializable pattern
There are a few reasons why you might want to initialize a contract after deployment and not directly by passing constructor arguments. But first let's look at an example: contract MyCrowdsale { uint256 rate; function initialize(uint256 _rate) public { rate = _rate; } } What's the advantage over...
Consensys Blockchain Jobs Report
What the current blockchain job market looks like
Consensys published their blockchain jobs report which you can checkout in their Blockchain Developer Job Kit. The most interesting aspects are Blockchain developer jobs have been growing at a rate of 33x of the previous year according to LinkedIns jobs report Typical salary is about...
Provable — Randomness Oracle
How the Oraclize random number generator works
One particularly interesting approach by Provable is the usage of a hardware security device, namely the Ledger Nano S. It uses a trusted execution environment to generate random numbers and provides a Provable Connector Contract as interface. How to use the Provable Randomness Oracle? Use the...
Solidity Design Patterns: Multiply before Dividing
Why the correct order matters!
There has been a lot of progress since the beginning of Ethereum about best practices in Solidity. Unfortunately, I have the feeling that most of the knowledge is within the circle of experienced people and there aren’t that many online resources about it. That is why I would like to start this...
Devcon 5 Applications closing in one week
Devcon 5 Applications closing
Watch out for the Devcon 5 applications. You only have one week left to apply either as Buidler Student Scholarship Press Devcon is by far the biggest and most impressive Ethereum conference in the world. And it's full of developers! I am especially excited about the cool location this year in...
Randomness and the Blockchain
How to achieve secure randomness for Solidity smart contracts?
When we talk about randomness and blockchain, these are really two problems: 1. How to generate randomness in smart contracts? 2. How to produce randomness for proof-of-stake (POS) systems? Or more generally, how to produce trusted randomness in public distributed systems? There is some overlap...