Paper 2025/864

Fheanor: a new, modular FHE library for designing and optimising schemes

Abstract

Implementations of modern FHE schemes are available in various highly-optimized libraries. Many of these libraries are designed to allow developers who may not have deep expertise in FHE to build fast and secure privacy-preserving applications. To support such users, the API of these libraries often hides the internals of the schemes in question from the user. However, this design choice makes it hard for users of these libraries to modify existing schemes, or implement new ones; work that is often valuable when conducting research on FHE schemes. We present our new Rust library Fheanor, which aims to facilitate such research on FHE schemes. The core target user is an FHE researcher, rather than an application developer. Most importantly, the design of Fheanor is very modular, and mirrors the mathematical structure of the available FHE schemes. By exposing the mathematical structure, but still hiding implementation details, it is easy to modify or extend the functionality of FHE schemes implemented in the library and still preserve high performance. Indeed, Fheanor demonstrates performance that is close to that of HElib or SEAL, with the potential for optimizations in the future. Fheanor implements several features that have not, or have only rarely, been implemented in previous libraries. These include non-power-of-two cyclotomic rings, single-RNS based ring arithmetic, the CLPX/GBFV scheme, and bootstrapping for BFV and BGV. In addition, this paper presents new theoretical contributions that are also implemented in Fheanor. The first is an extension of optimal digit extraction circuits, used in BFV/BGV bootstrapping, to the case 2^23. The second is a more efficient algorithm for computing the trace in the non-power-of-two cyclotomic setting.