Misleading exception message with scalingModSize

Library Questions
1

Hello,

I found an non-understandable exception message as following, and I figured out the reason. However, I want to suggest to correct this remaining misleading exception message related with scalingModSize.

Code Example & Output

Here’s the simple version of problematic code example (CKKS, v1.1.4):

int main(void)
{
  CCParams<CryptoContextCKKSRNS> parameters;
  parameters.SetRingDim(32768);
  parameters.SetMultiplicativeDepth(5);
  parameters.SetFirstModSize(37);
  parameters.SetScalingModSize(60);
  parameters.SetSecurityLevel(HEStd_256_classic);
  parameters.SetScalingTechnique(FIXEDMANUAL);

  CryptoContext<DCRTPoly> cc = GenCryptoContext(parameters);
  cc->Enable(PKE);
  cc->Enable(KEYSWITCH);
  cc->Enable(LEVELEDSHE);
  KeyPair<DCRTPoly> keyPair;
  keyPair = cc->KeyGen();
  cc->EvalMultKeyGen(keyPair.secretKey);
  size_t slots(11617);
  vector<complex<double>> tmp_vec_(11617);
  Plaintext tmp;

  Ciphertext<DCRTPoly> x, y;
  double yP;
  int c;
  vector<double> tmp_vec_1 = {1.0, 2.0};
  tmp = cc->MakeCKKSPackedPlaintext(tmp_vec_1);
  x = cc->Encrypt(keyPair.publicKey, tmp);
  yP = 3.0;
  vector<double> tmp_vec_2(slots, yP);
  tmp = cc->MakeCKKSPackedPlaintext(tmp_vec_2);
  x = cc->EvalMult(x, tmp);
  cc->Rescale(x);

  return 0;
}
  • I know that this one is not the typical case because the firstModSize (37) is smaller than scalingModSize (60). So if the code returns an exception message, I expected the message related with this.

Above code example, returns the exception message containing:
src/core/include/math/hal/intnat/mubintvecnat.h:l.315:SetModulus(): Requested modulus' size 61 is not supported. NativeVectorT supports only modulus size <= 60

  • At first, I couldn’t understand this message since none of mod sizes are above 60.
  • By the way, the exception was occurred during Rescale.

Reasoning

After I’ve searched for similar behavior reported before, I saw this issue #396. It was exactly same issue that I’ve experienced.

So from the replies, I noticed that following some internal logic, scalingModSize should not exceed 59. But the code above has scalingModSize as 60.

Problem & Suggestion

I’ve also checked created issue after that report: Correct the exception message for small CKKS scaling factor · Issue #339 · openfheorg/openfhe-development · GitHub

However, I found that it was fixed for only small scalingModSize (such as 10, 20).
It was why still the same exception message appeared for the case with scalingModSize = 60.

I think still this exception message is misleading.
Literally, NativeVectorT supports only modulus size <= 60 does not make sense in a situation where both mod sizes are <= 60.

Therefore, I want to suggest to consider correcting this message also for this case.
__
Thank you for your attention!

2

@hyerinpark Thank you! There is a new issue created Add a check: CKKS scalingModSize < 60 for NATIVEINT=64 · Issue #781 · openfheorg/openfhe-development · GitHub. The fix will be available for the next OpenFHE release.