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Commit-Reveal Randomness for High-Stakes Raffles

Overview

Commit-reveal prevents oracle front-running by requiring the oracle to commit to randomness before the raffle ends, then reveal after draw is triggered.

How It Works

Commit Phase (Before end_time)

  1. Oracle generates random secret and nonce
  2. Computes commitment = SHA-256(secret || nonce)
  3. Submits commitment to contract via commit_randomness(raffleId, commitment)
  4. Stores secret/nonce locally for later reveal

Reveal Phase (After draw triggered)

  1. Oracle retrieves stored secret/nonce
  2. Submits to contract via reveal_randomness(raffleId, secret, nonce)
  3. Contract verifies: SHA-256(secret || nonce) == stored_commitment
  4. If valid, uses secret as randomness seed for winner selection

Security Properties

  • Front-running prevention: Oracle cannot observe ticket purchases after committing
  • Unpredictability: Secret is cryptographically random
  • Verifiability: Anyone can verify the reveal matches the commitment
  • Non-manipulability: Oracle cannot change commitment after submission

Usage

// Commit phase (before raffle ends)
await commitRevealWorker.processCommit({
  raffleId: 1,
  endTime: Date.now() + 86400000,
});

// Reveal phase (after draw triggered)
await commitRevealWorker.processReveal({
  raffleId: 1,
  requestId: 'req-123',
});

Contract Interface

The contract must implement:

// Commit phase
pub fn commit_randomness(env: Env, raffle_id: u32, commitment: BytesN<32>)

// Reveal phase
pub fn reveal_randomness(env: Env, raffle_id: u32, secret: BytesN<32>, nonce: BytesN<16>)

Contract verification:

let computed = sha256(secret || nonce);
assert_eq!(computed, stored_commitment);

Implementation Status

✅ CommitmentService - Generate and store commitments
✅ CommitRevealWorker - Process commit and reveal phases
✅ Unit tests for commitment verification
⏳ Contract integration (waiting for contract support)
⏳ TxSubmitter methods for commit/reveal transactions

Configuration

Set threshold for commit-reveal vs direct randomness:

// In worker configuration
const USE_COMMIT_REVEAL = prizeAmount >= 500; // XLM

if (USE_COMMIT_REVEAL) {
  await commitRevealWorker.processCommit(...);
  // Later...
  await commitRevealWorker.processReveal(...);
} else {
  await randomnessWorker.processRequest(...);
}

Testing

npm test commitment.service.spec.ts

Notes

  • Commitment storage is in-memory; consider persistent storage for production
  • Ensure commit happens before end_time to prevent front-running
  • Monitor for failed reveals and implement retry logic