Bug: multiply_plain[_inplace] - incorrect product if the plaintext holds negative encoded values

[s0l0ist]

Hello all,

When using evaluator::multiply_plain[_inplace], the decrypted output is not correct when passing in a Plaintext created by encoding a signed vector (vector<int64>) holding negative values.

Note: I have not tested this for gsl::span, but it should be affected.

Example C++ code:

int main()
{
    EncryptionParameters parms(scheme_type::BFV);
    size_t poly_modulus_degree = 4096;
    parms.set_poly_modulus_degree(poly_modulus_degree);
    parms.set_coeff_modulus(CoeffModulus::BFVDefault(poly_modulus_degree));
    parms.set_plain_modulus(786433);
    auto context = SEALContext::Create(parms);
    KeyGenerator keygen(context);
    PublicKey public_key = keygen.public_key();
    SecretKey secret_key = keygen.secret_key();
    BatchEncoder batch_encoder(context);
    Encryptor encryptor(context, public_key);
    Evaluator evaluator(context);
    Decryptor decryptor(context, secret_key);

    size_t slot_count = batch_encoder.slot_count();
    vector<int64_t> vect(slot_count, -3); // Filling with *negative* values

    Plaintext plain;
    batch_encoder.encode(vect, plain);

    Ciphertext cipher;
    encryptor.encrypt(plain, cipher);

    // Successful: multiplying ciphers together results in the proper output
    // evaluator.multiply_inplace(cipher, cipher); // Output [9,9,9,...]

    // Silent fail: produces an incorrect decryption (same with multiply_plain).
    evaluator.multiply_plain_inplace(cipher, plain); // Output [-258029, -233196, 291981,...]

    Plaintext plain_result;
    decryptor.decrypt(cipher, plain_result);

    vector<int64_t> vect_result;
    batch_encoder.decode(plain_result, vect_result);

    for (auto const& i: vect_result) {
        std::cout << i << ", " << std::endl;
    }
    return 0;
}

Build settings:

cmake \
  -DSEAL_USE_CXX17=ON \
  -DSEAL_USE_INTRIN=OFF \
  -DSEAL_USE_ZLIB=ON \
  -DSEAL_USE_MSGSL=ON \
  -DSEAL_BUILD_EXAMPLES=OFF \
  -DSEAL_BUILD_TESTS=OFF \
  -DBUILD_SHARED_LIBS=OFF \
  -DCMAKE_BUILD_TYPE=Release \
  .

[WeiDaiWD]

It's fixed. v3.5.1 is coming really soon. Thanks a lot for capturing this. Our unit tests should certainly be improved.