Multiparty sceheme switching non probibalistic?

Library Questions
1

Hello, my question is regarding whether there exists a method or set of parameters in this library that allow a client and server to compare two values and return true or false.

The use case is a follows. A server holds biometric data in an encrypted template. The client trying to authenticate a user will take some biometric data, make a template and encrypt it.
Thus the server holds these encrypted values and find their similarity through threshold CKKS with MULTIPARTY and SCHEMESWITCH enabled. It is this similarity that is compared with a threshold value, i.e. subtracted from each other. Then we switch from CKKS to FHEW to evaluate the sign of the result and return True or False to the Client and completing the authentication.

As it stands now, i have been able to obtain the ciphertexts, compare them and scheme switch, but it is right with some probability.

I have used both the scheme switching and threshold CKKS examples that work well on their own, but when combining them it suddenly becomes wrong.

Comparing 5 and 1
5 is smaller than 1
Comparing 5 and 2
5 is larger than 2
Comparing 5 and 3
5 is smaller than 3
Comparing 5 and 4
5 is smaller than 4
Comparing 5 and 5
5 is smaller than 5
Comparing 5 and 6
5 is larger than 6
Comparing 5 and 7
5 is larger than 7
Comparing 5 and 8
5 is larger than 8
Comparing 5 and 9
5 is smaller than 9

So, is this a parameter issue, or do i need to change my approach to keygen?

Thanks!

Edit: for added context

2

Thank you for pointing this out. The SCHEME_SWITCHING_CAPABILITY.md should be updated to mention this capability is currently supported for single-key CKKS and FHEW.

The current multiparty key generation in OpenFHE introduces more noise than the single-key version. The current parameters and operations in scheme switching were only tested for the single-key version. You can try increasing some of the moduli, but the effect is not guaranteed (more internal changes would have to be made).

It’s worth mentioning that for more complex key generation procedures such as https://eprint.iacr.org/2025/2057.pdf and the references within, the noise-increase effect from the multiparty keygen ca be removed.