Fheanor

A toolkit library to build Homomorphic Encryption

Building on feanor-math, this library provides efficient implementations of various building blocks for Homomorphic Encryption (HE). The focus is on implementations of the ring R_q = Z[X]/(Phi_m(X), q) as required for second-generation HE schemes (like BGV, BFV), but also contains many other components and schemes.

The goal of this library is not to provide an easy-to-use implementation of homomorphic encryptions for use in applications - there are many good libraries for that already. Instead, the goal is to provide a toolkit for researchers that simplifies implementing variants of existing HE schemes, as well as new HE schemes.

Note Fheanor was previously called "HE-Ring".

Features

In short, Fheanor contains the following:

• Multiple efficient implementations of arithmetic in the ring R_q, which provide different performance characteristics (supporting arbitrary m)
• An implementation of the isomorphism R/(p^e) = GR(p, e, d) x ... x GR(p, e, d) via "hypercube structures" (compare "Bootstrapping for HElib" by Halevi and Shoup, https://ia.cr/2014/873)
• An implementation of "gadget products", i.e. the certain kind of inner product that is used in HE schemes to multiply ciphertexts with lower noise growth
• Fast "RNS-conversions", i.e. non-algebraic operations like rounding, implemented directly on the values x mod p_1, ..., x mod p_r for different primes p_1, ..., p_r
• Implementations of the BFV, BGV and CLPX encryption schemes
• Bootstrapping for BFV and BGV
• Tools for arithmetization, including modelling of arithmetic circuits, polynomial-to-circuit conversion via Paterson-Stockmeyer and HElib-style linear transforms

The following features are available partially, and/or WIP:

• Noise estimation and optional automated modulus-switching for BGV

Examples

In addition to the API documentation, detailed guides and examples to some parts of Fheanor can be found in [crate::examples].

Notation (comparison with HElib)

We sometimes use notation differently from the way it is used in HElib, and follow instead most modern HE literature. In particular, we use the following letters:

• m was previously called n in Fheanor, with n being referred to only as phi(n). For consistency with previous work, this has been renamed.

SemVer and Backward Compatibility

As indicated by the 0.x.y SemVer version, Fheanor is still in an alpha phase, and every new release may contain breaking changes. In fact, most of the version I published recently contained many significant breaking changes. Hence, it is recommended to fix the used version of Fheanor, using version = "=0.x.y" in your Cargo.toml, and only upgrade when you are willing to adjust to the new API changes. This will be different once we reach version 1.0.0 (although I will probably use the stability crate to mark unstable APIs, as already done for feanor-math).

Performance

When optimizing for performance, please use the Intel HEXL library (by enabling the feature use_hexl and providing a build of HEXL, as described in more detail in the documentation of feanor-math-hexl), since the default NTT does not provide SOTA performance (this is being worked on, and improving). Also note that Fheanor is currently single-threaded (as usual, type implementing [std::marker::Sync] are thread-safe, which includes basically all types in Fheanor).

Note that while this library is already quite optimized, it may not be fully competitive with other HE libraries that have existed for longer and thus received more optimization effort. Also, our goal of providing a modular toolkit of building blocks makes some kinds of optimizations more difficult, since components cannot always make as many assumptions on the input as they could if they only support a single HE scheme.

Profiling

Fheanor is instrumented using the framework defined by the Rust library tracing. Hence, running any Fheanor functions with an active tracing subscriber will generate corresponding tracing events that the subscriber can use for profiling purposes. There are various crates that implement tracing subscribers with profiling functionality.

For tests within this crate, we use tracing-chrome which generates Perfetto json trace files (can be displayed by Google Chrome without requiring plugins). In particular, if you enable ignored tests and run one of the measure_time_-prefixed test in this crate, this will generate a trace file. Of course, this is only included on test builds, in library builds, the parent application is free to configure tracing as desired.

Disclaimer

This library has been designed for research on homomorphic encryption. I did not have practical considerations (like side-channel resistance) in mind, and advise against using using it in production.

How to cite Fheanor

Please use the following bibtex entry to cite Fheanor:

@misc{cryptoeprint:2025/864,
    author = {Hiroki Okada and Rachel Player and Simon Pohmann},
    title = {Fheanor: a new, modular {FHE} library for designing and optimising schemes},
    howpublished = {Cryptology {ePrint} Archive, Paper 2025/864},
    year = {2025},
    url = {https://eprint.iacr.org/2025/864}
}

License

Fheanor is licensed under the MIT license.