Study on BGV-BFV evaluation for HE-friendly ciphers

Abstract

We propose a BGV-BFV hybrid evaluation method for HE-friendly ciphers in the transciphering framework for the BFV scheme. Since the keystream evaluation part of the transciphering framework computes a predetermined circuit for the HE-friendly cipher, we can improve the keystream evaluation by using optimized BGV evaluation and converting the resulting BGV-ciphertexts to BFV-ciphertexts. Applying the BGV-BFV hybrid evaluation to the RtF transciphering framework, which supports transciphering of approximate data using BFV-encrypted keystream, we improve the homomorphic keystream evaluation of HERA and Rubato of 128-bit security by up to 34% and 42%, respectively.