Wrong level after bootstrap

FHE Questions
1

Hello, I’m trying to choose the right parameters for my application (in ckks). I am using

security = 128 bit
ring = 2^16
full packing
levelBudget = {3, 3}
approxBootstrapDepth = 4
levelsUsedBeforeBootstrap = 11

So the depth of the circuit is 15+11=26.

When i bootstrap though, the result is at level 20. Is that right or am i missing something? I think it should be at 15.
Thank you

2

I am not sure I follow your calculations. Why is approxBootstrapDepth set to 4, and where do you get 15?

I suggest looking at the simple CKKS bootstrapping example. Below is the relevant code.

    std::vector<uint32_t> levelBudget = {4, 4};
    uint32_t approxBootstrapDepth     = 8;

    uint32_t levelsUsedBeforeBootstrap = 10;
    usint depth =
        levelsUsedBeforeBootstrap + FHECKKSRNS::GetBootstrapDepth(approxBootstrapDepth, levelBudget, secretKeyDist);
    parameters.SetMultiplicativeDepth(depth);
3

15 is approxBootstrapDepth + levelBudget[0] + levelBudget[1] + 5 (just like is computed by FHECKKSRNS::GetBootstrapDepth)

I thought approxBootstrapDepth can be chosen by the user, so 8 is a fixed value?

4

8 is a fixed value. We will simplify the bootstrapping interface in a later version where approxBootstrapDepth is automatically chosen by OpenFHE.

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