Airdrop Example

Let’s take a closer look at how to integrate our smart contract by using a concrete example. In this case, we will demonstrate how to integrate our Airdrop contract.

Importing Required Files

In the Airdrop contract, we override the verifySelfProof function to add custom functionality. For this purpose, import the following files:

import {SelfVerificationRoot} from "@selfxyz/contracts/contracts//abstract/SelfVerificationRoot.sol";
import {ISelfVerificationRoot} from "@selfxyz/contracts/contracts//interfaces/ISelfVerificationRoot.sol";

Verification Requirements for the Airdrop

For the Airdrop use case, the following verification checks are required:

• Scope Verification: Confirm that the proof was generated specifically for the Airdrop application by checking the scope used in the proof.

• Attestation ID Verification: Ensure the proof was generated using the correct attestation ID corresponding to the document type intended for the Airdrop.

• Nullifier Registration and Verification: Prevent double claims by registering and verifying a nullifier. Although the nullifier does not reveal any document details, it is uniquely tied to the document.

• User Identifier Registration: Verify that the proof includes a valid user identifier (in this case, the address that will receive the Airdrop).

• Proof Verification by IdentityVerificationHub: Validate the proof itself using our IdentityVerificationHub, which also performs additional checks (e.g., olderThan, forbiddenCountries, and OFAC validations) as configured for the Airdrop.

State Variables for Nullifier and User Identifier

Within the Airdrop contract, mappings are declared to keep track of used nullifiers and registered user identifiers:

mapping(uint256 => uint256) internal _nullifiers;
mapping(uint256 => bool) internal _registeredUserIdentifiers;

Overriding the verifySelfProof Function

The verifySelfProof function is overridden as follows to include all necessary checks:

function verifySelfProof(
    IVcAndDiscloseCircuitVerifier.VcAndDiscloseProof memory proof
) 
    public 
    override 
{
    if (!isRegistrationOpen) {
        revert RegistrationNotOpen();
    }

    if (_nullifiers[proof.pubSignals[NULLIFIER_INDEX]] != 0) {
        revert RegisteredNullifier();
    }
    
    if (proof.pubSignals[USER_IDENTIFIER_INDEX] == 0) {
        revert InvalidUserIdentifier();
    }

    super.verifySelfProof(proof);

    _nullifiers[proof.pubSignals[NULLIFIER_INDEX]] = proof.pubSignals[USER_IDENTIFIER_INDEX];
    _registeredUserIdentifiers[proof.pubSignals[USER_IDENTIFIER_INDEX]] = true;

    emit UserIdentifierRegistered(proof.pubSignals[USER_IDENTIFIER_INDEX], proof.pubSignals[NULLIFIER_INDEX]);
}