Encrypted Inputs

This document introduces the concept of encrypted inputs in the fhEVM, explaining their role, structure, validation process, and how developers can integrate them into smart contracts and applications.

Encrypted inputs are a core feature of fhEVM, enabling users to push encrypted data onto the blockchain while ensuring data confidentiality and integrity.

What are encrypted inputs?

Encrypted inputs are data values submitted by users in ciphertext form. These inputs allow sensitive information to remain confidential while still being processed by smart contracts. They are accompanied by Zero-Knowledge Proofs of Knowledge (ZKPoKs) to ensure the validity of the encrypted data without revealing the plaintext.

Key characteristics of encrypted inputs:

1. Confidentiality: Data is encrypted using the public FHE key, ensuring that only authorized parties can decrypt or process the values.

2. Validation via ZKPoKs: Each encrypted input is accompanied by a proof verifying that the user knows the plaintext value of the ciphertext, preventing replay attacks or misuse.

3. Efficient packing: All inputs for a transaction are packed into a single ciphertext in a user-defined order, optimizing the size and generation of the zero-knowledge proof.

Parameters in encrypted functions

When a function in a smart contract is called, it may accept two types of parameters for encrypted inputs:

1. einput: Refers to the index of the encrypted parameter, representing a specific encrypted input handle.

2. bytes: Contains the ciphertext and the associated zero-knowledge proof used for validation.

Here’s an example of a Solidity function accepting multiple encrypted parameters:

function myExample(
  address account,
  uint id,
  bool isAllowed,
  einput param1,
  einput param2,
  einput param3,
  bytes calldata inputProof
) public {
  // Function logic here
}

In this example, param1, param2, and param3 are encrypted inputs, while inputProof contains the corresponding ZKPoK to validate their authenticity.

Client-Side implementation

To interact with such a function, developers can use the fhevmjs library to create and manage encrypted inputs. Below is an example implementation:

import { createInstances } from "../instance";
import { getSigners, initSigners } from "../signers";

await initSigners(); // Initialize signers
const signers = await getSigners();

const instance = await createInstances(this.signers);
// Create encrypted inputs
const input = instance.createEncryptedInput(contractAddress, userAddress);
const inputs = input.add64(64).addBool(true).add8(4).encrypt(); // Encrypt the parameters

// Call the smart contract function with encrypted inputs
contract.myExample(
  "0xa5e1defb98EFe38EBb2D958CEe052410247F4c80", // Account address
  32, // Plaintext parameter
  true, // Plaintext boolean parameter
  inputs.handles[0], // Handle for the first parameter
  inputs.handles[1], // Handle for the second parameter
  inputs.handles[2], // Handle for the third parameter
  inputs.inputProof, // Proof to validate all encrypted inputs
);

In this example:

• add64, addBool, and add8: Specify the types and values of inputs to encrypt.

• encrypt: Generates the encrypted inputs and the zero-knowledge proof.

Validating encrypted inputs

Smart contracts process encrypted inputs by verifying them against the associated zero-knowledge proof. This is done using the TFHE.asEuintXX, TFHE.asEbool, or TFHE.asEaddress functions, which validate the input and convert it into the appropriate encrypted type.

Example validation that goes along the client-Side implementation

This example demonstrates a function that performs multiple encrypted operations, such as updating a user's encrypted balance and toggling an encrypted boolean flag:

  function myExample(
    einput encryptedAmount,
    einput encryptedToggle,
    bytes calldata inputProof
  ) public {
    // Validate and convert the encrypted inputs
    euint64 amount = TFHE.asEuint64(encryptedAmount, inputProof);
    ebool toggleFlag = TFHE.asEbool(encryptedToggle, inputProof);

    // Update the user's encrypted balance
    balances[msg.sender] = TFHE.add(balances[msg.sender], amount);

    // Toggle the user's encrypted flag
    userFlags[msg.sender] = TFHE.not(toggleFlag);
  }

  // Function to retrieve a user's encrypted balance
  function getEncryptedBalance() public view returns (euint64) {
    return balances[msg.sender];
  }

  // Function to retrieve a user's encrypted flag
  function getEncryptedFlag() public view returns (ebool) {
    return userFlags[msg.sender];
  }
}

Example validation in the encyrptedERC20.sol smart contract

Here’s an example of a smart contract function that verifies an encrypted input before proceeding:

function transfer(
  address to,
  einput encryptedAmount,
  bytes calldata inputProof
) public {
  // Verify the provided encrypted amount and convert it into an encrypted uint64
  euint64 amount = TFHE.asEuint64(encryptedAmount, inputProof);

  // Function logic here, such as transferring funds
  ...
}

How validation works

1. Input verification: The TFHE.asEuintXX function ensures that the input is a valid ciphertext with a corresponding ZKPoK.

2. Type conversion: The function transforms the einput into the appropriate encrypted type (euintXX, ebool, etc.) for further operations within the contract.

Best Practices

• Input packing: Minimize the size and complexity of zero-knowledge proofs by packing all encrypted inputs into a single ciphertext.

• Frontend encryption: Always encrypt inputs using the FHE public key on the client side to ensure data confidentiality.

• Proof management: Ensure that the correct zero-knowledge proof is associated with each encrypted input to avoid validation errors.

Encrypted inputs and their validation form the backbone of secure and private interactions in the fhEVM. By leveraging these tools, developers can create robust, privacy-preserving smart contracts without compromising functionality or scalability.

Upgrade of our Counter contract

Now that we have new knowledge on how to add encrypted inputs, let's upgrade our counter contract.

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

import "fhevm/lib/TFHE.sol";
import { SepoliaZamaFHEVMConfig } from "fhevm/config/ZamaFHEVMConfig.sol";

/// @title EncryptedCounter2
/// @notice A contract that maintains an encrypted counter and is meant for demonstrating how to add encrypted types
/// @dev Uses TFHE library for fully homomorphic encryption operations
/// @custom:experimental This contract is experimental and uses FHE technology
contract EncryptedCounter2 is SepoliaZamaFHEVMConfig {
  euint8 internal counter;

  constructor() {
    // Initialize counter with an encrypted zero value
    counter = TFHE.asEuint8(0);
    TFHE.allowThis(counter);
  }

  function incrementBy(einput amount, bytes calldata inputProof) public {
    // Convert input to euint8 and add to counter
    euint8 incrementAmount = TFHE.asEuint8(amount, inputProof);
    counter = TFHE.add(counter, incrementAmount);
    TFHE.allowThis(counter);
  }
}

Tests of for the Counter contract

import { createInstance } from "../instance";
import { getSigners, initSigners } from "../signers";
import { ethers } from "hardhat";

describe("EncryptedCounter2", function () {
  before(async function () {
    await initSigners(); // Initialize signers
    this.signers = await getSigners();
  });

  beforeEach(async function () {
    const CounterFactory = await ethers.getContractFactory("EncryptedCounter2");
    this.counterContract = await CounterFactory.connect(this.signers.alice).deploy();
    await this.counterContract.waitForDeployment();
    this.contractAddress = await this.counterContract.getAddress();
    this.instances = await createInstance(); // Set up instances for testing
  });

  it("should increment by arbitrary encrypted amount", async function () {
    // Create encrypted input for amount to increment by
    const input = this.instances.createEncryptedInput(this.contractAddress, this.signers.alice.address);
    input.add8(5);
    const encryptedAmount = await input.encrypt();

    // Call incrementBy with encrypted amount
    const tx = await this.counterContract.incrementBy(encryptedAmount.handles[0], encryptedAmount.inputProof);
    await tx.wait();
  });
});

How it works

The EncryptedCounter2 contract builds on the previous example by adding support for encrypted inputs. Here's how it works:

1. Encrypted state: Like before, the contract maintains an encrypted counter state variable of type euint8.

2. Encrypted input handling: The incrementBy function accepts two parameters:

   • einput amount: An encrypted input handle representing the increment value

   • bytes calldata inputProof: The zero-knowledge proof validating the encrypted input

3. Input processing: Inside incrementBy:

   • The encrypted input is converted to a euint8 using TFHE.asEuint8()

   • This conversion validates the proof and creates a usable encrypted value

   • The value is then added to the counter using homomorphic addition

Limitations

While we have resolved our problem with the Counter value visibility, there is still the problem with the Access Control for the counter. The counter is encrypted, but no access is granted to decrypt or view its value. Without proper ACL permissions, the counter remains inaccessible to users. To resolve this, refer to:

• Decryption

• Re-encryption