Unexpected Behaviors with auto scaling techniques in BFV Scheme

Library Questions
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1

Hello OpenFHE Team,

I apologize for posting frequently, but I’ve encountered another issue that I believe is worth reporting.

Below is a minimal snippet of the code that reproduces the issue:

#include "openfhe.h"

using namespace lbcrypto;
using namespace std;

int main(void)
{
  CCParams<CryptoContextBFVRNS> parameters;
  parameters.SetPlaintextModulus(65537);
  parameters.SetScalingTechnique(FIXEDAUTO); // or, FLEXIBLEAUTO, FLEXIBLEAUTOEXT

  CryptoContext<DCRTPoly> cc = GenCryptoContext(parameters);
  cc->Enable(PKE);
  cc->Enable(KEYSWITCH);
  cc->Enable(LEVELEDSHE);
  KeyPair<DCRTPoly> keyPair;
  keyPair = cc->KeyGen();

  Plaintext tmp;
  Ciphertext<DCRTPoly> x, y;

  tmp = cc->MakePackedPlaintext({1});
  x = cc->Encrypt(keyPair.publicKey, tmp);
  y = cc->Encrypt(keyPair.publicKey, tmp);
  x = cc->EvalAdd(x, y);
  return 0;
}

When executed on the latest version (v1.1.4), this code throws an exception with a quite ambiguous message:

terminate called after throwing an instance of 'lbcrypto::OpenFHEException'
  what():  OpenFHE-v1.1.4/src/pke/include/schemebase/base-leveledshe.h:l.764:AdjustLevelsAndDepthInPlace(): Mutable Operations are not supported for this scheme

Upon inspecting the OpenFHE codes a bit, it seems to me that the BFV scheme is supposed to use either the FIXEDMANUAL or NORESCALE technique. Is this understanding correct? If so, I would like to suggest, for clarity and to prevent future errors, maybe prohibiting the use of automatic scaling techniques in the BFV scheme could be considered.

Thank you!

2

Thank you for pointing this out. For BFV, only the NORESCALE scaling method should be used. BFV supports different multiplication techniques, specified in the OpenFHE design document https://eprint.iacr.org/2022/915.pdf page 6. Currently, cryptocontext parameters do not check which scheme they are called for. I opened an issue Check that the parameters input to the cryptocontext are scheme-specific · Issue #713 · openfheorg/openfhe-development · GitHub.

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