ZK Drop

ZK Drop unlocks regional airdrops and token presales using zero-knowledge location proofs — without revealing your exact coordinates.

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🔍 What It Does

ZK Drop allows users to prove that they are physically located within specific regions (like Argentina, Europe, or Asia) without sharing their coordinates.

This enables projects to offer:

• ✅ Regional airdrops (e.g., South America only)
• 🎯 Fair token presales by location
• 🔒 Private on-chain eligibility using ZK proofs

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🧪 How It Works

1. User generates a ZK proof on their mobile using GNSS data + region hash.
2. A public QR code provides the circuit’s public inputs (region_hash, challenge, nullifier).
3. The proof is verified on-chain by a smart contract before allowing access to airdrops or presales.

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🔧 Circuit Logic (Noir)

The circuit checks that:

• The user’s private GPS coordinates (lat, lon) are within a valid bounding box.
• The bounding box (min_lat, max_lat, min_lon, max_lon) matches the claimed region_hash.
• A challenge is included to generate a nullifier for session uniqueness or anti-replay protection.

use std::hash::poseidon;

fn main(
    lat: Field,
    lon: Field,
    min_lat: pub Field,
    max_lat: pub Field,
    min_lon: pub Field,
    max_lon: pub Field,
    region_hash: pub Field,
    challenge: pub Field,
    nullifier: pub Field,
) {
    // Cast to integers for comparison
    let lat_i32 = lat as i32;
    let lon_i32 = lon as i32;
    let min_lat_i32 = min_lat as i32;
    let max_lat_i32 = max_lat as i32;
    let min_lon_i32 = min_lon as i32;
    let max_lon_i32 = max_lon as i32;

    // Step 1: Verify location is within the region
    assert(lat_i32 >= min_lat_i32);
    assert(lat_i32 <= max_lat_i32);
    assert(lon_i32 >= min_lon_i32);
    assert(lon_i32 <= max_lon_i32);

    // Step 2: Verify region_hash is valid
    let computed_hash = poseidon::bn254::hash_4([min_lat, max_lat, min_lon, max_lon]);
    assert(computed_hash == region_hash);

    // Step 3: Prevent proof reuse with a nullifier
    let computed_nullifier = poseidon::bn254::hash_2([region_hash, challenge]);
    assert(computed_nullifier == nullifier);
}

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✅ On-Chain Verification

Once the user generates the zero-knowledge proof on their mobile device (after scanning the QR code), they send the result back to the frontend. The frontend then submits the proof to a smart contract which verifies the user’s region without revealing coordinates.

🔐 Flow Overview

1. Frontend shows QR with public inputs (region_hash, bounding box, challenge).
2. User scans it with a mobile app and generates a ZK proof locally using GNSS data.
3. Mobile app sends { proof, publicInputs } to the frontend (e.g. via QR, NFC, deep link, or server).
4. Frontend calls a Solidity smart contract that uses verifier.sol to verify the proof.
5. If valid, the contract lets the user:
   • Claim an airdrop
   • Purchase tokens during a regional presale
   • Access other regional features

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🧱 Solidity Verifier Integration

A typical interaction looks like this:

contract ZKDrop {
    IVerifier public verifier;
    mapping(bytes32 => bool) public usedNullifiers;

    constructor(address _verifier) {
        verifier = IVerifier(_verifier);
    }

    function claimAirdrop(bytes calldata proof, bytes32[] calldata publicInputs) external {
        require(verifier.verify(proof, publicInputs), "Invalid ZK proof");

        bytes32 nullifier = publicInputs[6];
        require(!usedNullifiers[nullifier], "Proof already used");
        usedNullifiers[nullifier] = true;
    }
}

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🛡️ Preventing Sybil Attacks (Optional zk-access Integration)

To strengthen resistance against Sybil attacks (users creating multiple wallets to farm airdrops), we are exploring an integration with zk-access — another project from the Noir Hackathon.

zk-access allows users to:

• Generate modular ZK credentials based on past proofs or identity claims.
• Selectively disclose facts like "I haven’t claimed this drop yet" or "I’m a unique person" — without revealing wallet addresses or identities.

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🤝 Credits & Acknowledgements

Built with ❤️ for the Noir Hackathon.

We believe privacy and fairness. ZK Drop is our step toward a more equitable and anonymous future. 🚀🌍