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