aaronbassett/compact-core:privacy-disclosure

Privacy & Disclosure

Essential guidance for Midnight's privacy model: when disclosure is required, commitment patterns, and witness protection.

The Disclosure Rule

Witness values cannot flow to public outputs without explicit disclose().

The compiler tracks all witness-derived values through your code. If a value touches any of these "disclosure triggers", you must wrap it with disclose():

| Trigger | Why Disclosure Required | |---------|------------------------| | Ledger storage | Data on-chain is public | | Circuit return | Return values go to TypeScript | | External contract | Data leaves your contract boundary | | Comparison operations | Comparisons can leak information |

Quick Decision Tree

Is the value derived from a witness?
├── No → No disclosure needed
└── Yes → Does it flow to:
    ├── Ledger write → disclose() required
    ├── Circuit return → disclose() required
    ├── Comparison (==, <, >) → disclose() required
    └── Commitment (transientCommit/persistentCommit) → Safe, no disclosure

Safe vs Unsafe Operations

| Operation | Safe? | Why | |-----------|-------|-----| | persistentCommit(x) | ✅ Safe | Hides value with nonce | | transientCommit(x) | ✅ Safe | Hides value with nonce | | persistentHash(x) | ❌ Unsafe | No nonce, can be brute-forced | | transientHash(x) | ❌ Unsafe | No nonce, can be brute-forced |

Common Patterns

Commit-Reveal

witness get_secret(): Field;

// Phase 1: Commit
export circuit commit(): Bytes<32> {
    const secret = get_secret();
    return persistentCommit(secret);  // Safe: commitment hides secret
}

// Phase 2: Reveal (requires disclosure)
export circuit reveal(): Field {
    const secret = get_secret();
    return disclose(secret);  // Explicit: user knows this reveals
}

Nullifier Pattern

witness get_secret(): Field;

export circuit spend(commitment: Bytes<32>): Bytes<32> {
    const secret = get_secret();

    // Verify knowledge of secret
    assert persistentCommit(secret) == commitment, "Invalid commitment";

    // Generate nullifier (unique per secret, reveals nothing)
    const nullifier = persistentHash("nullifier", secret);
    return nullifier;
}

References

• Disclosure Rules - Complete decision tree
• Witness Protection - How the compiler tracks values
• Safe Operations - Safe vs unsafe stdlib functions