Program Listing for File bgvrns-multiparty.cpp

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/*
Description:

This code implements RNS variants of the Cheon-Kim-Kim-Song scheme.

The CKKS scheme is introduced in the following paper:
- Jung Hee Cheon, Andrey Kim, Miran Kim, and Yongsoo Song. Homomorphic
encryption for arithmetic of approximate numbers. Cryptology ePrint Archive,
Report 2016/421, 2016. https://eprint.iacr.org/2016/421.

 Our implementation builds from the designs here:
 - Marcelo Blatt, Alexander Gusev, Yuriy Polyakov, Kurt Rohloff, and Vinod
Vaikuntanathan. Optimized homomorphic encryption solution for secure genomewide
association studies. Cryptology ePrint Archive, Report 2019/223, 2019.
https://eprint.iacr.org/2019/223.
 - Andrey Kim, Antonis Papadimitriou, and Yuriy Polyakov. Approximate
homomorphic encryption with reduced approximation error. Cryptology ePrint
Archive, Report 2020/1118, 2020. https://eprint.iacr.org/2020/
1118.
 */

#define PROFILE

#include "scheme/bgvrns/bgvrns-multiparty.h"

#include "scheme/bgvrns/bgvrns-cryptoparameters.h"
#include "ciphertext.h"

namespace lbcrypto {

DecryptResult MultipartyBGVRNS::MultipartyDecryptFusion(const std::vector<Ciphertext<DCRTPoly>>& ciphertextVec,
                                                        NativePoly* plaintext) const {
    const auto cryptoParams =
        std::dynamic_pointer_cast<CryptoParametersBGVRNS>(ciphertextVec[0]->GetCryptoParameters());

    const std::vector<DCRTPoly>& cv0 = ciphertextVec[0]->GetElements();
    DCRTPoly b                       = cv0[0];
    for (size_t i = 1; i < ciphertextVec.size(); i++) {
        const std::vector<DCRTPoly>& cvi = ciphertextVec[i]->GetElements();
        b += cvi[0];
    }
    b.SetFormat(Format::COEFFICIENT);

    size_t sizeQl = b.GetNumOfElements();

    NativeInteger scalingFactorInt = ciphertextVec[0]->GetScalingFactorInt();
    if (sizeQl > 0) {
        for (size_t i = sizeQl - 1; i > 0; --i) {
            b.ModReduce(cryptoParams->GetPlaintextModulus(), cryptoParams->GettModqPrecon(),
                        cryptoParams->GetNegtInvModq(i), cryptoParams->GetNegtInvModqPrecon(i),
                        cryptoParams->GetqlInvModq(i), cryptoParams->GetqlInvModqPrecon(i));
        }
        if (cryptoParams->GetScalingTechnique() == FLEXIBLEAUTO ||
            cryptoParams->GetScalingTechnique() == FLEXIBLEAUTOEXT) {
            for (size_t i = 0; i < sizeQl - 1; ++i) {
                NativeInteger modReduceFactor    = cryptoParams->GetModReduceFactorInt(sizeQl - 1 - i);
                NativeInteger modReduceFactorInv = modReduceFactor.ModInverse(cryptoParams->GetPlaintextModulus());
                scalingFactorInt = scalingFactorInt.ModMul(modReduceFactorInv, cryptoParams->GetPlaintextModulus());
            }
        }
    }

    *plaintext = b.GetElementAtIndex(0).DecryptionCRTInterpolate(cryptoParams->GetPlaintextModulus());

    return DecryptResult(plaintext->GetLength(), scalingFactorInt);
}

DecryptResult MultipartyBGVRNS::MultipartyDecryptFusion(const std::vector<Ciphertext<DCRTPoly>>& ciphertextVec,
                                                        Poly* plaintext) const {
    const auto cryptoParams =
        std::dynamic_pointer_cast<CryptoParametersBGVRNS>(ciphertextVec[0]->GetCryptoParameters());

    const std::vector<DCRTPoly>& cv0 = ciphertextVec[0]->GetElements();

    DCRTPoly b = cv0[0];
    for (size_t i = 1; i < ciphertextVec.size(); i++) {
        const std::vector<DCRTPoly>& cvi = ciphertextVec[i]->GetElements();
        b += cvi[0];
    }
    b.SetFormat(Format::COEFFICIENT);

    *plaintext = b.CRTInterpolate().Mod(cryptoParams->GetPlaintextModulus());

    return DecryptResult(plaintext->GetLength());
}

}  // namespace lbcrypto