Is there a way to find non negative values in BFV?

Library Questions
1

Hi,

I’m using BFV for a project. After some homomorphic operations, i have a result ciphertext, similar to the simple integers example.

Is there anyway for me to find the indexes of the positive values in the equivalent plaintext without decrypting it?

Thanks in advance

2

Unfortunately that would require an encrypted comparison function, which is not currently supported in BFV. We are working on adding scheme switching, where one can convert from BFV to say BinFHE, where comparison operations can be done with functional bootstrapping but 1) it will be very slow because each element would need to be converted separately – there is no SIMD in BinFHE, and 2) we do not expect BFV<->BinFHE conversion to be ready until next year sometime. Others may be able to add some suggestions.

3

I believe one alternative approach is to perform the comparison operation in BFV directly. This can be done via evaluating interpolation polynomials. If you can decompose the values to be compared in the plaintext before encryption, you can use the method in this paper. Alternatively, if you want to compare the values directly, check this paper.