Provably Fair Gaming: Commit-Reveal Schemes Explained

Introduction

One of the core promises of blockchain gaming is fairness. Unlike traditional online games where players must trust a centralized operator, blockchain games aim to provide transparent, tamper-proof mechanisms to ensure no party can cheat. A popular cryptographic technique to guarantee fairness in random number generation and game outcomes is the commit-reveal scheme.

In this article, I’ll break down how commit-reveal schemes work, why they are essential for provably fair gaming, and how I’ve applied this approach in my own project, yoss.gg, a zero-rake P2P USDC coin flip game built on the Base L2.

What is a Commit-Reveal Scheme?

A commit-reveal scheme is a two-phase cryptographic protocol that allows one party to commit to a value without revealing it immediately, then reveal the value later in a verifiable way. This scheme prevents cheating by ensuring that the committed value cannot be changed after the commitment, but is hidden until the reveal phase.

How does it work?

1. Commit Phase: The player (or system) generates a secret value (e.g., a random number or guess). They then create a cryptographic hash of this value combined with some randomness (often called a nonce) and submit this hash to the blockchain or counterparty.

2. Reveal Phase: After the opposing party has revealed their value or after some event triggers, the original player reveals their secret value and nonce. Anyone can verify that the hash of the revealed data matches the original commitment hash.

If the hashes don’t match, it means cheating or tampering occurred.

Why is Commit-Reveal Crucial in Blockchain Gaming?

In many games, especially those involving random outcomes or wagers, there is a risk that one party could manipulate the results once they see the other party’s move. Commit-reveal schemes mitigate this by:

• Preventing front-running: Because commitments are hashes, others cannot predict or change moves based on what’s committed.
• Ensuring unpredictability: Neither party can alter their committed value after seeing the other’s choice.
• Providing transparency: The blockchain records the commitment and reveal, allowing anyone to verify fairness.

This makes commit-reveal schemes a foundation for provably fair mechanisms in decentralized gaming.

Real-World Example: Coin Flip on yoss.gg

At yoss.gg, I wanted to build a simple yet fair peer-to-peer coin flip game where users bet USDC with zero rake. The challenge was ensuring neither player could cheat by manipulating the coin flip outcome after seeing the other’s guess.

Here’s how the commit-reveal scheme plays out:

1. Player 1 (Commit): Generates a secret random seed and commits to it by hashing the seed combined with a nonce. This commitment is stored on-chain.
2. Player 2 (Commit): Also commits their own seed in a similar fashion.
3. Reveal Phase: Both players reveal their seeds and nonces.
4. Outcome Determination: The combined seeds from both players are XORed and hashed to generate a fair random outcome (heads or tails).

Because both players commit before revealing, neither can change their seed based on the other’s choice, guaranteeing fairness.

Technical Considerations and Best Practices

• Use secure hash functions: SHA-256 or Keccak-256 are common choices to create commitment hashes.
• Include a nonce: This prevents dictionary attacks where someone tries to guess the committed value.
• Timeouts and dispute resolution: Games must handle cases where a player refuses to reveal after committing, often penalizing non-revealing parties.
• On-chain vs off-chain: Commitments are typically stored on-chain for transparency, but reveals can be off-chain with verification proofs to reduce gas costs.

Implementing commit-reveal correctly requires careful smart contract design to handle edge cases and potential exploits.

Alternatives and Enhancements

While commit-reveal is effective, it relies on honest reveals and can introduce latency (players must wait for the reveal phase). Some projects explore alternatives:

• VRF (Verifiable Random Functions): Chainlink VRF provides unpredictable randomness that can be verified on-chain, removing the need for commit-reveal.
• Multi-party computation: Several participants jointly generate randomness without revealing their individual contributions.
• Zero-knowledge proofs: Advanced cryptography to prove fairness without revealing secrets.

However, commit-reveal remains one of the simplest and most accessible techniques for provably fair gaming.

Conclusion

Provably fair gaming hinges on mechanisms that prevent cheating and manipulation, especially in trustless environments. Commit-reveal schemes provide a straightforward, cryptographically sound method to ensure fairness by forcing players to commit to their moves before revealing them.

By understanding and implementing commit-reveal schemes, blockchain game developers can build transparent and trustworthy gaming experiences. My work on yoss.gg demonstrates how this technique can be applied to create a fair, zero-rake coin flip game that puts players first.

If you’re building games or random outcome mechanisms in crypto, I encourage you to explore commit-reveal schemes as a fundamental building block for fairness.

Happy building!