Mod switch on fresh ciphertext

FHE Questions
1

Hello there,

I am trying to study the BGV scheme as described in your implementation, in particular I tried to implement a toy modulus switching. Given a ciphertext with an error bound B^2, the idea is to scale it down by a factor B.

Nevertheless, while playing around, I was thinking about this: is there something preventing me to perform a mod switch on a fresh ciphertext? Theoretically is should scale the error B by a factor 1/B, almost removing the original error.

What am I missing? Thank you!!

2

Hi @narger,

This is already done in the FLEXIBLEAUTOEXT mode of BGV by default. The error B is reduced to the modulus switching error in this case. Please see Section 4 of https://eprint.iacr.org/2021/204 for more details.

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