Prompt file imported from DaJo-Code/midnight-dev-skills (.github/prompts/implement-privacy-feature.prompt.md). Fill in {{featureName}}, {{privacyGoal}}, {{userFlow}} before use. Copyright stays with the author.

Implement Privacy Feature

Implement a complete privacy-preserving feature across both Compact contract and TypeScript dApp.

Input Variables

• Feature Name: {{featureName}}
• Privacy Goal: {{privacyGoal}}
• User Flow: {{userFlow}}

Implementation Steps

1. Contract Layer (Compact)

Design the privacy-preserving circuit:

• Identify public vs private inputs
• Use witness for ZK-verified data
• Use secret for off-chain-only data
• Implement commitment/nullifier patterns

2. TypeScript Integration

Create the dApp integration:

• Set up provider configuration
• Implement contract interaction functions
• Handle private state management
• Build user interface components

3. Privacy Verification

Ensure privacy guarantees:

• Witnesses are never exposed
• Commitments hide values
• Nullifiers prevent replay
• Error messages don't leak info

Example: Private Voting

Contract

pragma language_version 0.18;

import CompactStandardLibrary;

ledger {
  commitments: Set<Field>,
  nullifiers: Set<Field>,
  yesCounts: Counter,
  noCounts: Counter
}

export circuit registerVote(witness choice: Boolean, witness salt: Field): Field {
  const commitment = hash2(choice, salt);
  ledger.commitments.insert(commitment);
  return commitment;
}

export circuit castVote(
  witness choice: Boolean,
  witness salt: Field,
  commitment: Field
): [] {
  assert(ledger.commitments.member(commitment), "Not registered");
  const nullifier = hash(salt);
  assert(!ledger.nullifiers.member(nullifier), "Already voted");
  ledger.nullifiers.insert(nullifier);
  if (choice) {
    ledger.yesCounts.increment(1n);
  } else {
    ledger.noCounts.increment(1n);
  }
}

TypeScript

async function vote(choice: boolean) {
  const salt = generateRandomField();
  const commitment = await contract.call.registerVote({ choice, salt });

  // Store salt securely in private state
  await privateState.set('voteSalt', salt);

  // Cast the vote
  await contract.call.castVote({ choice, salt, commitment });
}

Output Format

Provide:

1. Complete Compact contract code
2. TypeScript integration code
3. Privacy analysis (what is hidden, what is revealed)
4. Security considerations
5. Test cases for privacy properties

Use #tool:search to find relevant patterns. Use #tool:edit/editFiles to implement the feature.