Paper 2023/1223

Improved Circuit Synthesis with Amortized Bootstrapping for FHEW-like Schemes

Abstract

In recent years, the research community has made great progress in improving techniques for privacy-preserving computation, such as fully homomorphic encryption (FHE). Despite the progress, there remain open challenges, mainly in performance and usability, to further advance the adoption of these technologies. This work provides multiple contributions that improve the current state-of-the-art in both areas. More specifically, we significantly simplify the bootstrapping by Carpov, Izabachène, and Mollimard for Boolean-based FHE schemes such as FHEW or TFHE, making the concept usable in practice. Based on our simplifications, we implement an easy-to-use interface for amortized bootstrapping in the open-source library FHE-Deck, derive new parameter sets for multi-bit encryptions with state-of-the-art security, and build a toolset that translates high-level code to multi-bit operations on encrypted data using circuit synthesis. We propose the first non-trivial FHE-specific optimizations in synthesizing privacy-preserving circuits: look-up table (LUT) grouping and adder substitution. Using LUT grouping, we reduce the number of bootstrapping operations by almost 35% on average, while for adder substitution, we reduce the number of required bootstrappings by up to 80% for certain use cases. Overall, the execution time is up to 3.8× faster using our optimizations compared to previous state-of-the-art circuit synthesis.