This guide demonstrates practical implementations of privacy-preserving applications using the Paillier cryptosystem on Gateway’s Shield testnet. Each example includes a complete smart contract implementation, usage patterns, and security considerations.

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1. Private Voting System

A decentralized voting system where individual votes remain encrypted while allowing for transparent vote tallying.

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Smart Contract Implementation

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import "./Paillier.sol";

contract PrivateVoting {
    Paillier public paillier;
    address public owner;
    BigNumber[] public encryptedVotes;

    constructor(address _paillier) {
        paillier = Paillier(_paillier);
        owner = msg.sender;
    }

    function submitVote(bytes calldata vote, bytes calldata publicKey) public {
        encryptedVotes.push(BigNumber(vote, false, BigNum.bitLength(vote)));
    }

    function tallyVotes(bytes calldata publicKey) public view returns (BigNumber memory) {
        BigNumber memory total = encryptedVotes[0];
        for (uint256 i = 1; i < encryptedVotes.length; i++) {
            total = paillier.add(total.toBytes(), encryptedVotes[i].toBytes(), publicKey);
        }
        return total;
    }
}

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Key Features

  • Individual votes remain encrypted
  • Homomorphic addition for vote tallying
  • No intermediary can determine individual votes
  • Final tally can only be decrypted by authorized parties

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Usage Example

async function runVotingSystem() {
  const voting = await PrivateVoting.deploy(paillierAddress);

  // Encrypt and submit votes
  const encryptedYesVote = await paillier.encrypt(1, randomness, publicKey);
  const encryptedNoVote = await paillier.encrypt(0, randomness, publicKey);

  await voting.submitVote(encryptedYesVote, publicKey);
  await voting.submitVote(encryptedNoVote, publicKey);

  // Get encrypted tally
  const encryptedTotal = await voting.tallyVotes(publicKey);

  // Decrypt total (off-chain)
  const finalCount = await paillier.decrypt(
    encryptedTotal,
    publicKey,
    privateKey
  );
}

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2. Privacy-Preserving Payroll

A system for managing employee salaries while keeping individual compensation private.

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Smart Contract Implementation

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import "./Paillier.sol";

contract PrivatePayroll {
    Paillier public paillier;
    address public owner;
    mapping(address => BigNumber) public encryptedSalaries;

    constructor(address _paillier) {
        paillier = Paillier(_paillier);
        owner = msg.sender;
    }

    function setSalary(address employee, bytes calldata encryptedSalary, bytes calldata publicKey) public {
        require(msg.sender == owner, "Only owner can set salary");
        encryptedSalaries[employee] = BigNumber(encryptedSalary, false, BigNum.bitLength(encryptedSalary));
    }

    function totalPayroll(bytes calldata publicKey) public view returns (BigNumber memory) {
        BigNumber memory total = BigNumber("0", false, 1);
        for (address employee : encryptedSalaries) {
            total = paillier.add(total.toBytes(), encryptedSalaries[employee].toBytes(), publicKey);
        }
        return total;
    }
}

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Key Features

  • Individual salaries remain confidential
  • Total payroll calculation without revealing individual amounts
  • Access control for salary management
  • Transparent budget tracking

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3. Anonymous Donations

A system enabling private charitable donations while maintaining transparency of total funds raised.

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Smart Contract Implementation

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import "./Paillier.sol";

contract AnonymousDonations {
    Paillier public paillier;
    address public owner;
    BigNumber[] public encryptedDonations;

    constructor(address _paillier) {
        paillier = Paillier(_paillier);
        owner = msg.sender;
    }

    function donate(bytes calldata encryptedAmount, bytes calldata publicKey) public {
        encryptedDonations.push(BigNumber(encryptedAmount, false, BigNum.bitLength(encryptedAmount)));
    }

    function totalDonations(bytes calldata publicKey) public view returns (BigNumber memory) {
        BigNumber memory total = encryptedDonations[0];
        for (uint256 i = 1; i < encryptedDonations.length; i++) {
            total = paillier.add(total.toBytes(), encryptedDonations[i].toBytes(), publicKey);
        }
        return total;
    }
}

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Key Features

  • Donor privacy protection
  • Transparent total donation tracking
  • No minimum donation requirement
  • Public verification of total funds

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4. Private Auction System

A sealed-bid auction system where bids remain private until the auction concludes.

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Smart Contract Implementation

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import "./Paillier.sol";

contract PrivateAuction {
    Paillier public paillier;
    address public owner;
    mapping(address => BigNumber) public encryptedBids;
    address public highestBidder;
    BigNumber public highestBid;

    constructor(address _paillier) {
        paillier = Paillier(_paillier);
        owner = msg.sender;
        highestBid = BigNumber("0", false, 1);
    }

    function placeBid(address bidder, bytes calldata encryptedBid, bytes calldata publicKey) public {
        BigNumber memory currentBid = BigNumber(encryptedBid, false, BigNum.bitLength(encryptedBid));
        encryptedBids[bidder] = currentBid;

        if (paillier.compare(currentBid.toBytes(), highestBid.toBytes(), publicKey) > 0) {
            highestBidder = bidder;
            highestBid = currentBid;
        }
    }

    function getHighestBid() public view returns (address, BigNumber memory) {
        return (highestBidder, highestBid);
    }
}

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Key Features

  • Private bid submissions
  • Automatic highest bid tracking
  • Fair price discovery
  • Bid privacy until auction conclusion

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5. Private Tax Calculation

A system for calculating taxes on private income data.

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Smart Contract Implementation

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import "./Paillier.sol";

contract PrivateTaxCalculation {
    Paillier public paillier;
    address public owner;
    mapping(address => BigNumber) public encryptedIncomes;
    uint256 public taxRate;

    constructor(address _paillier, uint256 _taxRate) {
        paillier = Paillier(_paillier);
        owner = msg.sender;
        taxRate = _taxRate;
    }

    function setIncome(address taxpayer, bytes calldata encryptedIncome, bytes calldata publicKey) public {
        require(msg.sender == owner, "Only owner can set income");
        encryptedIncomes[taxpayer] = BigNumber(encryptedIncome, false, BigNum.bitLength(encryptedIncome));
    }

    function totalTax(bytes calldata publicKey) public view returns (BigNumber memory) {
        BigNumber memory total = BigNumber("0", false, 1);
        for (address taxpayer : encryptedIncomes) {
            BigNumber memory tax = paillier.mul(encryptedIncomes[taxpayer].toBytes(), taxRate, publicKey);
            total = paillier.add(total.toBytes(), tax.toBytes(), publicKey);
        }
        return total;
    }
}

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Key Features

  • Private income reporting
  • Automated tax calculation
  • Transparent tax rate application
  • Total tax revenue tracking

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Security Considerations

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1. Key Management

  • Secure generation and storage of encryption keys
  • Regular key rotation
  • Access control for private keys

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2. Gas Optimization

  • Batch processing for multiple operations
  • Efficient storage patterns
  • Gas limit considerations

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3. Privacy Protection

  • Input validation
  • Prevention of data leakage
  • Secure random number generation

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4. Access Control

  • Role-based permissions
  • Owner-only functions
  • Emergency pause mechanisms

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Integration Resources

Integration Steps