FHEIfThenElse

Category: Basic | Difficulty: Beginner | Chapters: Arithmetic | Concept: Conditional selection on encrypted values using FHE.select

Use FHE.select to implement an encrypted if/else branch.

Why this example

This example focuses on Conditional selection on encrypted values using FHE.select. It is designed to be self-contained and easy to run locally.

Quick start

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

Dependencies

None

Contract and test

FHEIfThenElse.sol

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

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

/**
 * @title FHEIfThenElse
 * @author Gustavo Valverde
 * @notice Use FHE.select to implement an encrypted if/else branch.
 * @dev Example for fhEVM Examples - Basic Category
 *
 * @custom:category basic
 * @custom:chapter arithmetic
 * @custom:concept Conditional selection on encrypted values using FHE.select
 * @custom:difficulty beginner
 */
contract FHEIfThenElse is ZamaEthereumConfig {
    euint64 private lastResult;

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

    /**
     * @notice Pick between left/right depending on whether left <= threshold.
     * @dev All three encrypted inputs must be bound to the same proof.
     * @param left Encrypted left value
     * @param right Encrypted right value
     * @param threshold Encrypted threshold value
     * @param inputProof Proof for the encrypted inputs
     */
    function choose(
        externalEuint64 left,
        externalEuint64 right,
        externalEuint64 threshold,
        bytes calldata inputProof
    ) external {
        euint64 leftValue = FHE.fromExternal(left, inputProof);
        euint64 rightValue = FHE.fromExternal(right, inputProof);
        euint64 thresholdValue = FHE.fromExternal(threshold, inputProof);

        ebool takeLeft = FHE.le(leftValue, thresholdValue);
        lastResult = FHE.select(takeLeft, leftValue, rightValue);

        FHE.allowThis(lastResult);
        FHE.allow(lastResult, msg.sender);
    }
}

FHEIfThenElse.test.ts

Pitfalls to avoid

No pitfalls are highlighted in the tests for this example.

API Reference

Overview

Use FHE.select to implement an encrypted if/else branch.

Developer Notes

Example for fhEVM Examples - Basic Category

getLastResult

function getLastResult() external view returns (euint64)

Return the last encrypted result.

Return Values

Name	Type	Description
[0]	euint64	The last encrypted selection result.

choose

function choose(externalEuint64 left, externalEuint64 right, externalEuint64 threshold, bytes inputProof) external

Pick between left/right depending on whether left <= threshold.

All three encrypted inputs must be bound to the same proof.

Parameters

Name	Type	Description
left	externalEuint64	Encrypted left value
right	externalEuint64	Encrypted right value
threshold	externalEuint64	Encrypted threshold value
inputProof	bytes	Proof for the encrypted inputs