advanced-ckks-bootstrapping.cpp

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/*

Example for CKKS bootstrapping with sparse packing

*/

#define PROFILE

#include "openfhe.h"

using namespace lbcrypto;

void BootstrapExample(uint32_t numSlots);

int main(int argc, char* argv[]) {
    // We run the example with 8 slots and ring dimension 4096 to illustrate how to run bootstrapping with a sparse plaintext.
    // Using a sparse plaintext and specifying the smaller number of slots gives a performance improvement (typically up to 3x).
    BootstrapExample(8);
}

void BootstrapExample(uint32_t numSlots) {
    // Step 1: Set CryptoContext
    CCParams<CryptoContextCKKSRNS> parameters;

    // A. Specify main parameters
    /*  A1) Secret key distribution
    * The secret key distribution for CKKS should either be SPARSE_TERNARY or UNIFORM_TERNARY.
    * The SPARSE_TERNARY distribution was used in the original CKKS paper,
    * but in this example, we use UNIFORM_TERNARY because this is included in the homomorphic
    * encryption standard.
    */
    SecretKeyDist secretKeyDist = UNIFORM_TERNARY;
    parameters.SetSecretKeyDist(secretKeyDist);

    /*  A2) Desired security level based on FHE standards.
    * In this example, we use the "NotSet" option, so the example can run more quickly with
    * a smaller ring dimension. Note that this should be used only in
    * non-production environments, or by experts who understand the security
    * implications of their choices. In production-like environments, we recommend using
    * HEStd_128_classic, HEStd_192_classic, or HEStd_256_classic for 128-bit, 192-bit,
    * or 256-bit security, respectively. If you choose one of these as your security level,
    * you do not need to set the ring dimension.
    */
    parameters.SetSecurityLevel(HEStd_NotSet);
    parameters.SetRingDim(1 << 12);

    /*  A3) Key switching parameters.
    * By default, we use HYBRID key switching with a digit size of 3.
    * Choosing a larger digit size can reduce complexity, but the size of keys will increase.
    * Note that you can leave these lines of code out completely, since these are the default values.
    */
    parameters.SetNumLargeDigits(3);
    parameters.SetKeySwitchTechnique(HYBRID);

    /*  A4) Scaling parameters.
    * By default, we set the modulus sizes and rescaling technique to the following values
    * to obtain a good precision and performance tradeoff. We recommend keeping the parameters
    * below unless you are an FHE expert.
    */
#if NATIVEINT == 128
    // Currently, only FIXEDMANUAL and FIXEDAUTO modes are supported for 128-bit CKKS bootstrapping.
    ScalingTechnique rescaleTech = FIXEDAUTO;
    usint dcrtBits               = 78;
    usint firstMod               = 89;
#else
    // All modes are supported for 64-bit CKKS bootstrapping.
    ScalingTechnique rescaleTech = FLEXIBLEAUTO;
    usint dcrtBits               = 59;
    usint firstMod               = 60;
#endif

    parameters.SetScalingModSize(dcrtBits);
    parameters.SetScalingTechnique(rescaleTech);
    parameters.SetFirstModSize(firstMod);

    /*  A4) Bootstrapping parameters.
    * We set a budget for the number of levels we can consume in bootstrapping for encoding and decoding, respectively.
    * Using larger numbers of levels reduces the complexity and number of rotation keys,
    * but increases the depth required for bootstrapping.
	* We must choose values smaller than ceil(log2(slots)). A level budget of {4, 4} is good for higher ring
    * dimensions (65536 and higher).
    */
    std::vector<uint32_t> levelBudget = {3, 3};

    /* We give the user the option of configuring values for an optimization algorithm in bootstrapping.
    * Here, we specify the giant step for the baby-step-giant-step algorithm in linear transforms
    * for encoding and decoding, respectively. Either choose this to be a power of 2
    * or an exact divisor of the number of slots. Setting it to have the default value of {0, 0} allows OpenFHE to choose
    * the values automatically.
    */
    std::vector<uint32_t> bsgsDim = {0, 0};

    /*  A5) Multiplicative depth.
    * The goal of bootstrapping is to increase the number of available levels we have, or in other words,
    * to dynamically increase the multiplicative depth. However, the bootstrapping procedure itself
    * needs to consume a few levels to run. We compute the number of bootstrapping levels required
    * using GetBootstrapDepth, and add it to levelsAvailableAfterBootstrap to set our initial multiplicative
    * depth.
    */
    uint32_t levelsAvailableAfterBootstrap = 10;
    usint depth = levelsAvailableAfterBootstrap + FHECKKSRNS::GetBootstrapDepth(levelBudget, secretKeyDist);
    parameters.SetMultiplicativeDepth(depth);

    // Generate crypto context.
    CryptoContext<DCRTPoly> cryptoContext = GenCryptoContext(parameters);

    // Enable features that you wish to use. Note, we must enable FHE to use bootstrapping.
    cryptoContext->Enable(PKE);
    cryptoContext->Enable(KEYSWITCH);
    cryptoContext->Enable(LEVELEDSHE);
    cryptoContext->Enable(ADVANCEDSHE);
    cryptoContext->Enable(FHE);

    usint ringDim = cryptoContext->GetRingDimension();
    std::cout << "CKKS scheme is using ring dimension " << ringDim << std::endl << std::endl;

    // Step 2: Precomputations for bootstrapping
    cryptoContext->EvalBootstrapSetup(levelBudget, bsgsDim, numSlots);

    // Step 3: Key Generation
    auto keyPair = cryptoContext->KeyGen();
    cryptoContext->EvalMultKeyGen(keyPair.secretKey);
    // Generate bootstrapping keys.
    cryptoContext->EvalBootstrapKeyGen(keyPair.secretKey, numSlots);

    // Step 4: Encoding and encryption of inputs
    // Generate random input
    std::vector<double> x;
    std::random_device rd;
    std::mt19937 gen(rd());
    std::uniform_real_distribution<> dis(0.0, 1.0);
    for (size_t i = 0; i < numSlots; i++) {
        x.push_back(dis(gen));
    }

    // Encoding as plaintexts
    // We specify the number of slots as numSlots to achieve a performance improvement.
    // We use the other default values of depth 1, levels 0, and no params.
    // Alternatively, you can also set batch size as a parameter in the CryptoContext as follows:
    // parameters.SetBatchSize(numSlots);
    // Here, we assume all ciphertexts in the cryptoContext will have numSlots slots.
    // We start with a depleted ciphertext that has used up all of its levels.
    Plaintext ptxt = cryptoContext->MakeCKKSPackedPlaintext(x, 1, depth - 1, nullptr, numSlots);
    ptxt->SetLength(numSlots);
    std::cout << "Input: " << ptxt << std::endl;

    // Encrypt the encoded vectors
    Ciphertext<DCRTPoly> ciph = cryptoContext->Encrypt(keyPair.publicKey, ptxt);

    std::cout << "Initial number of levels remaining: " << depth - ciph->GetLevel() << std::endl;

    // Step 5: Perform the bootstrapping operation. The goal is to increase the number of levels remaining
    // for HE computation.
    auto ciphertextAfter = cryptoContext->EvalBootstrap(ciph);

    std::cout << "Number of levels remaining after bootstrapping: " << depth - ciphertextAfter->GetLevel() << std::endl
              << std::endl;

    // Step 7: Decryption and output
    Plaintext result;
    cryptoContext->Decrypt(keyPair.secretKey, ciphertextAfter, &result);
    result->SetLength(numSlots);
    std::cout << "Output after bootstrapping \n\t" << result << std::endl;
}