Bug: multiply_plain[_inplace] - incorrect product if the plaintext holds negative encoded values #166
Description
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 \
.