Proofs

Anon Aadhaar Proofs

The proof generation is handled by the SDK, you can either work with the @anon-aadhaar/react package (see quick-setup) that makes abstraction of this process or by using the methods of the @anon-aadhaar/core package like so:

Install @anon-aadhaar/core:

yarn add @anon-aadhaar/core

Then:

import { init, prove, InitArgs, artifactUrls } from "@anon-aadhaar/core";

// Change prod to test if you want to verify the test Aadhaar data
const anonAadhaarInitArgs: InitArgs = {
  wasmURL: artifactsUrls.prod.wasm,
  zkeyURL: artifactsUrls.prod.zkey,
  vkeyURL: artifactsUrls.prod.vk,
  isWebEnv: true,
};

// Initialize the core package
await init(anonAadhaarInitArgs);

const nullifierSeed = 1234;

// QRData: the string read from the QR code
// certificate: x509 certificate containing the public key
// it can be downloaded from: https://www.uidai.gov.in/en/916-developer-section/data-and-downloads-section/11349-uidai-certificate-details.html
const args = await generateArgs({
  QRData,
  certificateFile,
  nullifierSeed,
});

const anonAadhaarCore = await prove(args);

Once the proof is generated the output will be an AnonAadhaarCore object, it's composed of:

• type: anon-aadhaar
• id: Not relevant and should not be used for any operations.
• claim:
  • pubKey: The RSA public key of the issuer.
  • signalHash: Hash of the signal.
• proof: anonAadhaarProof

Here are the details of the anonAadhaarProof:

• nullifier: Hash(nullifierSeed, photo bytes).
• timestamp: Timestamp of the QR code signature, could be used to ensure that the proof was generated x hours ago.
• pubkeyHash: Hash of the public key.
• nullifierSeed: Let you check that the nullifier was indeed computed with your seed.
• signalHash: Hash of the signal.
• ageAbove18: "0", if false, "1" if true.
• gender: "77" if M, "70" if F.
• state: State name in string format.
• pincode: Pincode in string format.
• groth16Proof: The groth16 SNARK proof.

Verify a proof off-chain

Again if using the react SDK like in the quick-setup the proof is verified once the user generated it, and set the status of the user to "logged-in".

But if you are integrating with the @anon-aadhaar/core package you'll simply need to use the verify method and pass it the AnonAadhaarCore object. Also at this step you might want to verify the timestamp to ensure that the QR data was generated recently and that the user have access to his UIDAI portal.

Verify a proof on-chain

To verify a proof on-chain you will need to use the AnonAadhaar.sol contract, that you can import with from @anon-aadhaar/contracts.

Install the package in your hardhat project:

yarn add @anon-aadhaar/contracts

Then you'll be able to import the IAnonAadhaar.sol interface like so:

import "@anon-aadhaar/contracts/interfaces/IAnonAadhaar.sol";

Then you can call the verifier, by sending these parameters:

• nullifierSeed
• nullifier
• timestamp
• signal
• revealArray: Array of the values used as input for the proof generation (equal to [0, 0, 0, 0] if no field reveal where asked)

IAnonAadhaar(anonAadhaarVerifierAddr).verifyAnonAadhaarProof(
  nullifierSeed, // nulifier seed
  nullifier,
  timestamp,
  signal,
  revealArray,
  groth16Proof
);

Here you can find an example Vote contract integration:

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.19;

import "@anon-aadhaar/contracts/interfaces/IAnonAadhaar.sol";
import "@anon-aadhaar/contracts/interfaces/IAnonAadhaarVote.sol";

contract AnonAadhaarVote is IAnonAadhaarVote {
    string public votingQuestion;
    address public anonAadhaarVerifierAddr;

    // List of proposals
    Proposal[] public proposals;

    // Mapping to track if a userNullifier has already voted
    mapping(uint256 => bool) public hasVoted;

    // Constructor to initialize proposals
    constructor(string memory _votingQuestion, string[] memory proposalDescriptions, address _verifierAddr) {
        anonAadhaarVerifierAddr = _verifierAddr;
        votingQuestion = _votingQuestion;
        for (uint256 i = 0; i < proposalDescriptions.length; i++) {
            proposals.push(Proposal(proposalDescriptions[i], 0));
        }
    }

    /// @dev Convert an address to uint256, used to check against signal.
    /// @param _addr: msg.sender address.
    /// @return Address msg.sender's address in uint256
    function addressToUint256(address _addr) private pure returns (uint256) {
        return uint256(uint160(_addr));
    }

    /// @dev Check if the timestamp is more recent than (current time - 3 hours)
    /// @param timestamp: msg.sender address.
    /// @return bool
    function isLessThan3HoursAgo(uint timestamp) public view returns (bool) {
        return timestamp > (block.timestamp - 3 * 60 * 60);
    }

    /// @dev Register a vote in the contract.
    /// @param proposalIndex: Index of the proposal you want to vote for.
    /// @param nullifierSeed: Nullifier Seed used while generating the proof.
    /// @param nullifier: Nullifier for the user's Aadhaar data.
    /// @param timestamp: Timestamp of when the QR code was signed.
    /// @param signal: signal used while generating the proof, should be equal to msg.sender.
    /// @param revealArray: Array of the values used as input for the proof generation (equal to [0, 0, 0, 0] if no field reveal where asked).
    /// @param groth16Proof: SNARK Groth16 proof.
    function voteForProposal(
        uint256 proposalIndex,
        uint nullifierSeed,
        uint nullifier,
        uint timestamp,
        uint signal,
        uint[4] memory revealArray,
        uint[8] memory groth16Proof
    ) public {
        require(
            proposalIndex < proposals.length,
            '[AnonAadhaarVote]: Invalid proposal index'
        );
        require(
            addressToUint256(msg.sender) == signal,
            '[AnonAadhaarVote]: wrong user signal sent.'
        );
        require(
            isLessThan3HoursAgo(timestamp) == true,
            '[AnonAadhaarVote]: Proof must be generated with Aadhaar data generated less than 3 hours ago.'
        );
        require(
            IAnonAadhaar(anonAadhaarVerifierAddr).verifyAnonAadhaarProof(
                nullifierSeed, // nulifier seed
                nullifier,
                timestamp,
                signal,
                revealArray,
                groth16Proof
            ) == true,
            '[AnonAadhaarVote]: proof sent is not valid.'
        );
        // Check that user hasn't already voted
        require(
            !hasVoted[nullifier],
            '[AnonAadhaarVote]: User has already voted'
        );

        proposals[proposalIndex].voteCount++;
        hasVoted[nullifier] = true;

        emit Voted(msg.sender, proposalIndex);
    }
}

There is some some important details to notice here.

• signal, is used as way to prevent from the proof being front-runned, while the proof will be generated the user will commit to his address as a signal and the contract will check that the msg.sender is corresponding to the signal.
• timestamp, is used as a way to verify that the proof was generated from a QR code created less than # hours ago.
• nullifier, is used to nullify the user contract interaction, letting him voting only once as the nullifier will remain unique for each identity.
• nullifierSeed, to prevent double spending, letting your app know what was the seed to the nullifier.