Function Evaluation and CMUX

Library Questions
1

Hi all,

using FHEW/TFHE I wanted to calculate a simple score

a = b* (x) + (b-1)* (x+1) ; for x = {1,..,5} and b = {0,1}

I figured I could use EvalFunc() for computing x+1 and use the CMUX gate to choose either x or x+1 depending on b. But this does not seem to be the case. The result I get is neither x nor x+1. What can I do to fix this?

2

It would be hard for us to help you without examining your code. Please share your code or provide more details so we can help you better.

3

Most of the code is originally from the eval_func example. Here is the code:

 auto cc = BinFHEContext();
    cc.GenerateBinFHEContext(STD128, true, 12);
    auto sk = cc.KeyGen();
    cc.BTKeyGen(sk);
    int p = cc.GetMaxPlaintextSpace().ConvertToInt();  // Obtain the maximum plaintext space
    std::cout << "Maximum plaintext space " << p << "." << std::endl;
    auto xEnc = cc.Encrypt(sk, xIn % p, SMALL_DIM, p);    
    // Initialize Function f(x) = x + 1 % p
    auto fp = [](NativeInteger m, NativeInteger p1) -> NativeInteger {
        if (m < p1)
            return (m + 1) % p1;
        else
            return ((m +1 - p1 / 2)) % p1;
    };

    // Generate LUT from function f(x)    
    LWEPlaintext result;    
    auto lut = cc.GenerateLUTviaFunction(fp, p);
    auto x_plus_one = cc.EvalFunc(xEnc, lut);
   
    for (int i = 0; i < 2; i++){
        std::vector<LWECiphertext> cmux_test;                
        cmux_test.push_back(xEnc);
        cmux_test.push_back(x_plus_one);
        cmux_test.push_back(cc.Encrypt(sk, i, SMALL_DIM, p));
        auto test_score = cc.EvalBinGate(CMUX, cmux_test);
        cc.Decrypt(sk, test_score, &result, p);
        std::cout << "Input x: " << xIn << "b: " << i << ". score: " << result << std::endl;
    }
4

Hi @mgall,

The CMUX gate is only supported for the mode where all inputs are Boolean, which is not the case for x.

A simpler way would be to to use BGV or BFV for this (w/o any bootstrapping). You could simply use the homomorphic multiplication for CMUX. If b comes from an encrypted comparison, then you could do comparison using EvalFunc and run CMUX (multiplication) in CKKS. In other words, you could look into scheme switching examples for this.