Paper 2022/515

MOSFHET: Optimized Software for FHE over the Torus

Abstract

Homomorphic encryption is one of the most secure solutions for processing sensitive information in untrusted environments, and there have been many recent advances toward its efficient implementation for the evaluation of approximated arithmetic as well as linear and arbitrary functions. The TFHE scheme [Chillotti et al., 2016] is the current state-of-the-art for the evaluation of arbitrary functions, and, in this work, we focus on improving its performance. We divide this paper into two parts. First, we review and implement the main techniques to improve performance or error behavior in TFHE proposed so far. Then, we introduce novel improvements to several of them and new approaches to implement some commonly used procedures. We also show which proposals can be suitably combined to achieve better results. We provide a single library containing all the reviewed techniques as well as our original contributions. Among the techniques we introduce, we highlight a new method for multi-value bootstrapping based on blind rotation unfolding and a faster-than-memory seed expansion, which introduces speedups of up to 2 times to basic arithmetic operations.