Program killed during generating bootstrapping key

Library Questions
1

Hello.
I want to use 4096 slots and also bootstrapping.
But my program is killed during generating evaluation key(bootstrapping key).
Could you please review my code to identify any issues?

Below is my code and its result. Thank you for reading.

#include "openfhe.h"

using namespace lbcrypto;

void BootstrapExample(uint32_t numSlots);

int main(int argc, char* argv[]) {
    BootstrapExample(4096);
}

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

    SecretKeyDist secretKeyDist = UNIFORM_TERNARY;
    parameters.SetSecretKeyDist(secretKeyDist);
    parameters.SetSecurityLevel(HEStd_NotSet);
    parameters.SetRingDim(1 << 17);
    parameters.SetNumLargeDigits(3);
    parameters.SetKeySwitchTechnique(HYBRID);

#if NATIVEINT == 128 && !defined(__EMSCRIPTEN__)
    // 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);

   
    std::vector<uint32_t> levelBudget = {4, 4};
    std::vector<uint32_t> bsgsDim = {0, 0};

    usint depth = 30;
    parameters.SetMultiplicativeDepth(depth);

    CryptoContext<DCRTPoly> cryptoContext = GenCryptoContext(parameters);

    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;

    cryptoContext->EvalBootstrapSetup(levelBudget, bsgsDim, numSlots);
    auto keyPair = cryptoContext->KeyGen();
    cryptoContext->EvalMultKeyGen(keyPair.secretKey);
    cryptoContext->EvalBootstrapKeyGen(keyPair.secretKey, numSlots);

    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));
    }

    Plaintext ptxt = cryptoContext->MakeCKKSPackedPlaintext(x, 1, depth - 1, nullptr, numSlots);
    ptxt->SetLength(numSlots);
    std::cout << "Input: " << ptxt << std::endl;

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

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

    auto ciphertextAfter = cryptoContext->EvalBootstrap(ciph);

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

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

image

2

I think you might have consumed all the RAM in your system, while the FHE scheme still needs more.
The ring dimension is too large, try to reduce it.
You can also reduce the multiplicative depth since you are using bootstrapping.

1 Like
3

Does the number of slots that i set also affect RAM consumption?

Or do only multiplicative depth and ring dimension cause such a problem?

4

I suggest this topic. The formula for the automorphism keys requires the depth of the circuit \ell and N. From my experience, automorphism keys are smaller with less slots (bootstrapping is indeed a little bit faster)

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5

So 1) monitor your memory consumption. How much ram do you have?
Run top or equivalent on linux systems to see if it is exhausted. On a linux system you can overcome the ram limitation by enabling swapping. I recommend at least 32GB ram.
2) # slots is nominally ringsize/2 for ckks, your ring dimension is 128k so the # slots is not the issue.
maybe reducing ring size will help.
3) you are requsting depth 30. ciphertext size scales almost linearly with # slots.
4) it looks like you are using/modifying the bootstrapping example. does the original run? Typically we suggest using default parameters. This example hardwires many of them.

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6

It does, but it is rather complicated to explain as it depends on several other parameters.
I would focus on optimizing the ring dimension and multiplicative depth.

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