| name | compact-core:privacy-disclosure |
| description | Use when encountering "potential witness-value disclosure" compiler errors, implementing commit-reveal patterns, working with persistentCommit/transientCommit vs persistentHash/transientHash, or designing privacy-preserving circuits with proper witness protection. |
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
