midnight-core-concepts:privacy-patterns

Privacy Patterns in Midnight

Fundamental rule: Anything passed to a ledger operation is publicly visible, except MerkleTree types. Use these patterns to keep data private.

Pattern Selection Guide

Pattern 1: Hashes

Use persistentHash to hide data while allowing later verification.

import { persistentHash } from "std/​compact/​hashes";

// Store hash of secret
ledger.stored_hash = persistentHash(secret_data);

// Later, prove knowledge by re-hashing
assert persistentHash(provided_data) == ledger.stored_hash;

Limitations

• Same input → same hash (correlatable)
• Small value sets vulnerable to brute-force

Use for: Data that won't repeat and has high entropy.

Pattern 2: Commitments

Use persistentCommit when same values must appear different.

import { persistentCommit } from "std/​compact/​commitments";

// Commitment with randomness prevents correlation
const commitment = persistentCommit(value, randomness);
ledger.stored_commitment = commitment;

Why Randomness Matters

Without randomness, commitments to "yes" always look identical. With randomness:

• commit("yes", random1) ≠ commit("yes", random2)
• Prevents correlation between identical values
• Critical for voting, bidding, any repeated values

// WRONG: Correlatable
const bad = persistentCommit(vote, Bytes<32>::zero());

// CORRECT: Unlinkable
const good = persistentCommit(vote, fresh_randomness);

Randomness Sources

// Option 1: Fresh randomness (ideal)
const r = generateRandomness();

// Option 2: Derived from secret key + counter (deterministic)
const r = persistentHash(secret_key, counter);

Use for: Any value that might repeat (votes, bids, choices).

Pattern 3: Merkle Trees

Use MerkleTree to prove set membership without revealing which element.

ledger authorized_keys: MerkleTree<32, Bytes<32>>;

export circuit authenticate(
  secret_key: Bytes<32>,
  merkle_path: MerkleTreePath<32, Bytes<32>>
): Void {
  // Prove key is in tree without revealing which key
  const public_key = persistentHash(secret_key);
  assert ledger.authorized_keys.member(public_key, merkle_path);
}

Tree Types

Use HistoricMerkleTree when: Elements added frequently and users need to prove membership against roots from when they obtained their path.

Pattern 4: Commitment + Nullifier

The most powerful pattern: single-use tokens with complete privacy.

How It Works

1. Commitment phase: Store commit(value, secret) in Merkle tree
2. Spend phase: Reveal nullifier = hash(commitment, secret)
3. Validation: Prove commitment exists, nullifier not seen before

ledger commitments: MerkleTree<32, Bytes<32>>;
ledger nullifiers: Set<Bytes<32>>;

export circuit spend(
  value: Field,
  secret: Bytes<32>,
  path: MerkleTreePath<32, Bytes<32>>
): Void {
  // Reconstruct commitment
  const commitment = persistentCommit(value, secret);

  // Prove commitment exists in tree
  assert ledger.commitments.member(commitment, path);

  // Compute and record nullifier
  const nullifier = persistentHash(commitment, secret);
  assert !ledger.nullifiers.member(nullifier);
  ledger.nullifiers.insert(nullifier);
}

Privacy Properties

• Commitment: Hides value and owner
• Nullifier: Prevents double-spend without linking to commitment
• Merkle proof: Proves existence without revealing which commitment

This is the foundation of Zerocash and Zswap's shielded UTXOs.

Authentication Pattern

Prove identity without revealing credentials:

export circuit authenticate(secret_key: Bytes<32>): Void {
  const public_key = persistentHash(secret_key);
  assert public_key == ledger.authorized_key;
  // Authorized! secret_key never revealed
}

Common Mistakes

Mistake 1: Reusing Randomness

// WRONG: Same randomness = correlatable
const r = fixed_value;
commit(vote1, r); commit(vote2, r);

// CORRECT: Fresh randomness each time
commit(vote1, random1); commit(vote2, random2);

Mistake 2: Small Value Sets Without Randomness

// WRONG: Only 2 possible values, easily brute-forced
hash(vote); // vote is "yes" or "no"

// CORRECT: Commitment with randomness
persistentCommit(vote, randomness);

Mistake 3: Forgetting What's Public

// WRONG: This exposes the secret!
ledger.public_field = secret_value;

// CORRECT: Store only commitment
ledger.public_field = persistentCommit(secret_value, randomness);

References

For detailed technical information:

• references/​commitment-schemes.md - Pedersen commitments, binding/​hiding properties
• references/​merkle-tree-usage.md - Tree operations, path generation, historic trees

Examples

Working patterns:

• examples/​private-voting.compact - Complete voting with commitment/​nullifier
• examples/​auth-patterns.compact - Authentication without revealing keys