Comparison Operations

Encrypted comparisons (eq, ne, gt, lt, ge, le) and conditional selection. Demonstrates FHE.select for branching without information leakage, essential for "find maximum" or threshold logic.

To run this example correctly, make sure the files are placed in the following directories:

• .sol file → <your-project-root-dir>/contracts/

• .ts file → <your-project-root-dir>/test/

This ensures Hardhat can compile and test your contracts as expected.

🔐 FHE API Reference (13 items)

Types: ebool · euint32 · externalEuint32

Functions:

• FHE.allow() - Grants PERMANENT permission for address to decrypt/use value

• FHE.allowThis() - Grants contract permission to operate on ciphertext

• FHE.eq() - Encrypted equality: returns ebool(a == b)

• FHE.fromExternal() - Validates and converts external encrypted input using inputProof

• FHE.ge() - Encrypted greater-or-equal: returns ebool(a >= b)

• FHE.gt() - Encrypted greater-than: returns ebool(a > b)

• FHE.le() - Encrypted less-or-equal: returns ebool(a <= b)

• FHE.lt() - Encrypted less-than: returns ebool(a < b)

• FHE.ne() - Encrypted inequality: returns ebool(a != b)

• FHE.select() - Encrypted if-then-else: select(cond, a, b) → returns a if true, b if false

FHEComparison.sol

// SPDX-License-Identifier: BSD-3-Clause-Clear
pragma solidity ^0.8.24;

import {
    FHE,
    ebool,
    euint32,
    externalEuint32
} from "@fhevm/solidity/lib/FHE.sol";
import {ZamaEthereumConfig} from "@fhevm/solidity/config/ZamaConfig.sol";

/**
 * @notice Encrypted comparisons (eq, ne, gt, lt, ge, le) and conditional selection.
 *         Demonstrates FHE.select for branching without information leakage,
 *         essential for "find maximum" or threshold logic.
 *
 * @dev Comparisons return ebool; select avoids revealing values during branching.
 */
contract FHEComparison is ZamaEthereumConfig {
    euint32 private _a;
    euint32 private _b;
    ebool private _boolResult;
    euint32 private _selectedResult;

    function setA(externalEuint32 inputA, bytes calldata inputProof) external {
        _a = FHE.fromExternal(inputA, inputProof);
        FHE.allowThis(_a);
    }

    function setB(externalEuint32 inputB, bytes calldata inputProof) external {
        _b = FHE.fromExternal(inputB, inputProof);
        FHE.allowThis(_b);
    }

    function computeEq() external {
        _boolResult = FHE.eq(_a, _b);
        _grantBoolPermissions();
    }

    function computeNe() external {
        _boolResult = FHE.ne(_a, _b);
        _grantBoolPermissions();
    }

    function computeGt() external {
        _boolResult = FHE.gt(_a, _b);
        _grantBoolPermissions();
    }

    function computeLt() external {
        _boolResult = FHE.lt(_a, _b);
        _grantBoolPermissions();
    }

    function computeGe() external {
        _boolResult = FHE.ge(_a, _b);
        _grantBoolPermissions();
    }

    function computeLe() external {
        _boolResult = FHE.le(_a, _b);
        _grantBoolPermissions();
    }

    /// @notice Encrypted maximum using select: (a > b) ? a : b
    /// @dev 🔀 Why select? Using if(aGtB) would decrypt and leak the comparison!
    function computeMaxViaSelect() external {
        ebool aGtB = FHE.gt(_a, _b);
        // select evaluates both branches, picks one without revealing which
        _selectedResult = FHE.select(aGtB, _a, _b);
        FHE.allowThis(_selectedResult);
        FHE.allow(_selectedResult, msg.sender);
    }

    /// @notice Encrypted minimum using select: (a < b) ? a : b
    function computeMinViaSelect() external {
        ebool aLtB = FHE.lt(_a, _b);
        _selectedResult = FHE.select(aLtB, _a, _b);
        FHE.allowThis(_selectedResult);
        FHE.allow(_selectedResult, msg.sender);
    }

    function getBoolResult() public view returns (ebool) {
        return _boolResult;
    }

    function getSelectedResult() public view returns (euint32) {
        return _selectedResult;
    }

    function _grantBoolPermissions() internal {
        FHE.allowThis(_boolResult);
        FHE.allow(_boolResult, msg.sender);
    }
}

FHEComparison.ts