Building Private Blockchains with FHEVM: A Developer's Guide

zama-ai/fhevm

From a software engineer's perspective, FHEVM is a game-changer for several reasons

Data Privacy
FHE allows you to perform computations on data without ever decrypting it. This is a massive leap forward for decentralized applications (dApps) that handle sensitive information, such as financial data, medical records, or user-specific metrics. You can now build dApps that process this data while ensuring it remains confidential, even from the smart contract itself.

Trustless Computation
FHEVM enables trustless on-chain computation. The EVM can execute smart contracts on encrypted data, and the result is also encrypted. The network can verify the computation without ever seeing the plaintext data. This is crucial for applications where the integrity of the computation is paramount but the data must remain private.

New Use Cases
FHEVM opens up a whole new realm of possibilities for dApps. Think about building a private voting system where votes are tallied without ever revealing individual choices, or a decentralized machine learning model where user data is used for training without being exposed. This technology is the key to creating more private and secure decentralized applications.

Getting started with FHEVM involves a few key steps. Since it's a full-stack framework, you'll be working with both front-end and back-end components.

Your smart contracts will need to use FHEVM-specific types and functions. You'll typically write these in Solidity, but they'll be compiled with an FHEVM-compatible compiler. The contracts will operate on encrypted values, often represented as euint (encrypted unsigned integer) or similar data types.

Example

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.1;

import "fhevm/lib/fhe.sol";

contract EncryptedMath {
    // Stores an encrypted value
    euint8 public encryptedValue;

    // A function that adds an encrypted value to the stored one
    function add(euint8 a, euint8 b) public returns (euint8) {
        // The addition happens on the encrypted values
        // without ever decrypting them.
        return a + b;
    }
}

On the front end, you'll use a library to handle the encryption of data before it's sent to the smart contract. The Zama FHEVM client library is designed for this. It handles the cryptographic heavy lifting, allowing you to focus on the application logic.

Example (using JavaScript with a hypothetical FHE client library)

import FheClient from '@fhevm/client';
import Web3 from 'web3';

// Assume you have a web3 provider and the contract address/ABI
const web3 = new Web3(window.ethereum);
const contract = new web3.eth.Contract(abi, contractAddress);
const fheClient = new FheClient();

// A function to interact with the encrypted smart contract
async function performEncryptedAddition(value1, value2) {
    // Encrypt the values before sending them to the blockchain
    const encryptedValue1 = await fheClient.encrypt(value1);
    const encryptedValue2 = await fheClient.encrypt(value2);

    // Call the smart contract function with the encrypted data
    const tx = await contract.methods.add(encryptedValue1, encryptedValue2).send({ from: accounts[0] });

    console.log("Transaction sent:", tx.transactionHash);
}

To deploy and test your FHEVM dApp, you'll need to use specific tools and networks. The Zama team provides a testnet and tooling that are compatible with their FHEVM implementation. You'll typically use a modified version of popular tools like Hardhat or Foundry that can compile and deploy your FHE-enabled contracts.

Let's walk through a complete, albeit simplified, example of a dApp for private voting.

The Goal
Allow users to vote "yes" or "no" on a proposal, but keep their individual votes private. The smart contract will only reveal the final, encrypted tally.

Smart Contract (EncryptedVoting.sol)

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.1;

import "fhevm/lib/fhe.sol";

contract EncryptedVoting {
    // The total encrypted tally of votes
    euint8 public encryptedTally;

    constructor() {
        // Initialize the tally to 0
        encryptedTally = fhe.zero();
    }

    // A voter submits their vote as an encrypted 0 (no) or 1 (yes)
    function vote(euint8 encryptedVote) public {
        // Add the encrypted vote to the tally
        // The smart contract never sees the plaintext vote
        encryptedTally = encryptedTally + encryptedVote;
    }

    // Function to get the encrypted final tally
    function getEncryptedTally() public view returns (euint8) {
        return encryptedTally;
    }
}

Front-End Logic (Simplified)

// ... (setup as before with Web3 and FheClient) ...

async function submitVote(userChoice) {
    // User's choice is 1 for 'yes', 0 for 'no'
    const encryptedChoice = await fheClient.encrypt(userChoice);

    await contract.methods.vote(encryptedChoice).send({ from: accounts[0] });
    console.log("Your vote has been submitted securely!");
}

async function getFinalTally() {
    const encryptedResult = await contract.methods.getEncryptedTally().call();
    
    // Decrypt the result. This can only be done by someone with the
    // decryption key (e.g., the owner of the dApp or a specific party).
    const decryptedResult = await fheClient.decrypt(encryptedResult);
    
    console.log("Final vote count:", decryptedResult);
}

// Call submitVote(1) to vote yes, submitVote(0) to vote no
// Then call getFinalTally() to see the final, decrypted count