Why do my parameters not comply with 128 bit security?

FHE Questions
1

Hello all,

I am trying to get a grasp on how model parameters play together with security levels. So I have a key generation main-file, with which I can play around. I am now curious why my parameters with

multDepth = 6
scalModsSize = 25
firstModSize = 30
ringDim = 8192

do not comply with 128 bit security. As it states here if I want to have that security with the desired ring dimension \log (q) should be lower than 220. But for this example it is:
\log (q) \approx 199.893
if I get q via the method context->GetModulus(). I also watched this talk where it says that Pallisade calculates q automatically, but how does it or rather OpenFHE now do it?
(edit) I am using CKKS

Thanks in advance and cheers

2

There are other auxiliary moduli used in specific settings. For example, if you use the Hybrid Key Switching (HKS) technique (that is parameters.SetKeySwitchTechnique(HYBRID)), the coefficient modulus will be ~\log{}(1/dnum*q), where dnum is a parameter of HKS - default 3. Hence, the overall modulus will be larger than the threshold you refer to above.

You can try a different key switching method (BV) that does not require additional auxiliary modules, but it will be slower.

You can also use FIXEDMANUAL/FIXEDAUTO scaling techniques (parameters.SetScalingTechnique(FIXEDMANUAL);) to reduce the moduli size a bit.