Zero-Knowledge Age Verification on Midnight: How Compact Enables Private On-Chain Data

How do you verify user eligibility on-chain without revealing sensitive personal identity data?

Traditional KYC processes force an unwanted trade-off: transparency versus privacy. Exposing dates of birth or identity credentials directly on a public ledger introduces severe privacy and security risks.

Midnight Network solves this dilemma through a clean separation of Public vs. Private State:

• Private State: The user's sensitive data (e.g., birthdate) stays on-device in their local environment/wallet.

• Public State: Only the verification status (e.g., a boolean true/false) is posted to the ledger.

How Compact Structures ZK Predicate Logic

Midnight’s domain-specific language, Compact, allows developers to write zero-knowledge logic without drowning in complex cryptographic circuits.

Here is how a simple ZK predicate for age verification looks in Compact:

contract AgeVerification {
    state {
        is_verified: Map<Address, bool>;
    }

    transition verify_age(birth_year: Uint, current_year: Uint) {
        // Executed inside the ZK circuit
        assert (current_year - birth_year) >= 18;

        // Public ledger only learns the boolean output
        next(is_verified[caller]) = true;
    }
}

How It Works Under the Hood

1. Local Input: The user inputs their birth_year locally inside their wallet.

2. ZK Proof Generation: The wallet executes the circuit locally and generates a Zero-Knowledge Proof confirming (current_year - birth_year) >= 18.

3. Off-Chain Privacy: The actual birth_year raw data never leaves the off-chain environment.

4. On-Chain Verification: Validators only verify the validity of the mathematical proof—never the raw user inputs.

Why This Matters

This architecture replaces raw data exposure with verifiable cryptographic proofs. From selective KYC to private DeFi credit scoring, Midnight abstracts complex ZK mathematics into simple, readable logic.

Building privacy-first dApps on Midnight changes how we handle data ownership on-chain.