Implement CKKS- coefficient encoding multiplication

Library Questions
1

I want to implement an inner product using CKKS coefficient encoding (i.e., encode vectors as polynomial coefficients, multiply polynomials, and extract the relevant coefficient).

Furthermore, I want to implement them in noise-flooding methods. I need to compute a polynomial like f(1/X) (reverse coefficients) and then multiply ciphertexts to obtain the inner product coefficient.

As a first step, I wrote the following test code to multiply two ciphertexts using coefficient encoding:

#include <iostream>
#include <vector>
#include "openfhe.h"
#include <cmath>
#include <algorithm>

using namespace lbcrypto;
using namespace std;

CryptoContext<DCRTPoly> generate_parameter(CCParams<CryptoContextCKKSRNS> &params){
    params.SetSecretKeyDist(UNIFORM_TERNARY);
    params.SetSecurityLevel(HEStd_128_classic);
    params.SetScalingModSize(50);
    params.SetFirstModSize(60);
    params.SetBatchSize(8);
    params.SetKeySwitchTechnique(HYBRID);
    params.SetExecutionMode(EXEC_EVALUATION);
    params.SetNoiseEstimate(19.0);
    params.SetRingDim(8192);
    params.SetDecryptionNoiseMode(NOISE_FLOODING_DECRYPT);
    params.SetMultiplicativeDepth(1);
    params.SetDesiredPrecision(20);
    params.SetScalingTechnique(FIXEDAUTO);
    
    CryptoContext<DCRTPoly> cc = GenCryptoContext(params);
    cc->Enable(PKE);
    cc->Enable(KEYSWITCH);
    cc->Enable(LEVELEDSHE);
    cc->Enable(ADVANCEDSHE);

    return cc;
}

void keySetting(KeyPair<DCRTPoly> &keys, CryptoContext<DCRTPoly> &cc){
    cc->EvalMultKeyGen(keys.secretKey);
}

int main() {
    CCParams<CryptoContextCKKSRNS> parameters;
    CryptoContext<DCRTPoly> cc = generate_parameter(parameters);
    auto keyPair = cc->KeyGen();
    keySetting(keyPair, cc);

    std::vector<int64_t> coef = {1,2,3,4,5,6,7,8};
    Plaintext pt = cc->MakeCoefPackedPlaintext(coef);

    auto ct = cc->Encrypt(keyPair.publicKey, pt);

    ct *= ct;

    Plaintext decrypted;
    cc->Decrypt(keyPair.secretKey, ct, &decrypted); 

    std::cout << "Decrypted (as plaintext): " << decrypted << std::endl;

    return 0;
}

However, when I build and run it, I get the following error:

terminate called after throwing an instance of 'lbcrypto::OpenFHEException'
  what():  /home/sandia/plaintext/openfhe-development/src/pke/lib/scheme/ckksrns/ckksrns-pke.cpp:l.67:Decrypt(): sizeQl 2!= 1. If sizeQl = 0, consider increasing the depth. If sizeQl > 1, check parameters (this is unsupported for NativePoly).
Aborted (core dumped)

My question is

  1. How can I fix this sizeQl 2 != 1 error in this context?
  2. Does the openfhe-development library include a function to compute a polynomial like f(1/X) (i.e., reversing the coefficients of a plaintext or ciphertext)?
2

MakeCoefPackedPlaintext is specifically designed for the finite-field case, i.e., where the coefficients are \bmod t, where t is the plaintext modulus. This encodng is used for BGV and BFV. When we deal with the coefficient encoding of CKKS, we use the notion of RLWE schemelet (which was only added in v1.4 to support functional CKKS bootstrapping from General Functional Bootstrapping using CKKS). In summary, this feature is not supported out of the box by OpenFHE.