FHESub

Category: Basic | Difficulty: Beginner | Chapters: Arithmetic | Concept: Subtract two encrypted values with FHE.sub

Encrypted subtraction example for two values.

Why this example

This example focuses on Subtract two encrypted values with FHE.sub. It is designed to be self-contained and easy to run locally.

Quick start

npm install
npm run test:mocked -- test/basic/FHESub.test.ts

Dependencies

None

Contract and test

FHESub.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.27;

import {FHE, euint64, externalEuint64} from "@fhevm/solidity/lib/FHE.sol";
import {ZamaEthereumConfig} from "@fhevm/solidity/config/ZamaConfig.sol";

/**
 * @title FHESub
 * @author Gustavo Valverde
 * @notice Encrypted subtraction example for two values.
 * @dev Example for fhEVM Examples - Basic Category
 *
 * @custom:category basic
 * @custom:chapter arithmetic
 * @custom:concept Subtract two encrypted values with FHE.sub
 * @custom:difficulty beginner
 */
contract FHESub is ZamaEthereumConfig {
    euint64 private lastResult;

    /// @notice Returns the last encrypted difference.
    /// @return The last encrypted difference
    function getLastResult() external view returns (euint64) {
        return lastResult;
    }

    /// @notice Subtract two encrypted inputs (a - b) and store the encrypted result.
    /// @param encA First encrypted value handle (minuend)
    /// @param encB Second encrypted value handle (subtrahend)
    /// @param inputProof Proof for the encrypted inputs
    function subValues(
        externalEuint64 encA,
        externalEuint64 encB,
        bytes calldata inputProof
    ) external {
        euint64 a = FHE.fromExternal(encA, inputProof);
        euint64 b = FHE.fromExternal(encB, inputProof);
        lastResult = FHE.sub(a, b);
        FHE.allowThis(lastResult);
        FHE.allow(lastResult, msg.sender);
    }
}

FHESub.test.ts

Pitfalls to avoid

No pitfalls are highlighted in the tests for this example.

API Reference

Overview

Encrypted subtraction example for two values.

Developer Notes

Example for fhEVM Examples - Basic Category

getLastResult

function getLastResult() external view returns (euint64)

Returns the last encrypted difference.

Return Values

Name	Type	Description
[0]	euint64	The last encrypted difference

subValues

function subValues(externalEuint64 encA, externalEuint64 encB, bytes inputProof) external

Subtract two encrypted inputs (a - b) and store the encrypted result.

Parameters

Name	Type	Description
encA	externalEuint64	First encrypted value handle (minuend)
encB	externalEuint64	Second encrypted value handle (subtrahend)
inputProof	bytes	Proof for the encrypted inputs