hax

hax is a tool for high assurance translations of a large subset of Rust into formal languages such as Lean, F* or Rocq.

Try out hax online now!

Supported Backends

General purpose proof assistants			Cryptography & protocols
(via Aeneas)	F*		ProVerif		EasyCrypt
🚀 active dev.	🟢 stable	🟠 experimental	🟠 experimental	🟠 experimental	🟠 experimental

Learn more

Here are some resources for learning more about hax:

• Manual (work in progress)
  • Quick start: Lean, F*
  • Tutorial: Lean, F*
• Examples: the examples directory contains a set of examples that show what hax can do for you.
• Other specifications of cryptographic protocols.

Questions? Join us on Zulip or open a GitHub Discussion. For bugs, file an Issue.

Usage

hax is a cargo subcommand. The command cargo hax accepts the following subcommands:

• into (cargo hax into BACKEND): translate a Rust crate to the backend BACKEND.
• json (cargo hax json): extract the typed AST of your crate as a JSON file.

Backends

Backend	Command	Description
Lean (via Aeneas)	cargo hax into lean	Recommended for Lean. Uses charon + aeneas.
Lean (legacy)	cargo hax into legacy-lean	Uses the hax engine directly. Prefer lean.
F*	cargo hax into fstar	Stable.
Rocq/Coq	cargo hax into coq	Experimental.
ProVerif	cargo hax into pro-verif	Experimental.
SSProve	cargo hax into ssprove	Experimental.
EasyCrypt	cargo hax into easycrypt	Experimental.

Use --help on any subcommand for options (e.g. cargo hax into fstar --z3rlimit 100).

Installation

hax is supported on Linux and macOS, on both x86_64 and aarch64. Windows is not supported; use WSL there.

Manual installation

1. Make sure to have the following installed on your system:

• opam
• rustup
• nodejs
• jq

2. Clone this repo: git clone git@github.com:cryspen/hax.git && cd hax
3. Create (or use an existing) opam switch by running opam switch create hax 5.1.1
4. Run the setup.sh script: ./setup.sh. This installs hax; aeneas and charon are downloaded on demand when first needed.
5. Run cargo-hax --help

Nix

This should work on Linux and MacOS.

Prerequisites: Nix package manager (with flakes enabled)

• Either using the Determinate Nix Installer, with the following bash one-liner:

  curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix | sh -s -- install

• or following those steps.

• Run hax on a crate directly to get Lean/F*/Coq/... (assuming you are in the crate's folder):

  • nix run github:cryspen/hax -- into fstar extracts F*.

• Install hax: nix profile install github:cryspen/hax, then run cargo hax --help anywhere

• Note: in any of the Nix commands above, replace github:cryspen/hax by ./dir to compile a local checkout of hax that lives in ./some-dir

• Setup binary cache: using Cachix, just cachix use hax

Note: the lean backend downloads its aeneas and charon binaries on demand (see Aeneas and Charon below); no extra install step is needed.

Using Docker

1. Clone this repo: git clone git@github.com:cryspen/hax.git && cd hax
2. Build the docker image: docker build -f .docker/Dockerfile . -t hax
3. Get a shell: docker run -it --rm -v /some/dir/with/a/crate:/work hax bash
4. You can now run cargo-hax --help (notice here we use cargo-hax instead of cargo hax)

Note: Please make sure that $HOME/.cargo/bin is in your $PATH, as that is where setup.sh will install hax.

Note: the lean backend downloads its aeneas and charon binaries on demand (see Aeneas and Charon below); no extra install step is needed.

Aeneas and Charon (standalone)

The lean backend (cargo hax into lean) uses the charon + aeneas pipeline instead of the hax engine. It requires the aeneas and charon binaries.

Their versions are managed by hax: pre-built binaries are downloaded on demand, verified against the version manifest shipped with the release, and cached under $XDG_CACHE_HOME/hax/tools/. To see the active versions or pre-populate the cache (e.g. in CI or before going offline), run, inside your project:

cargo hax tools show
cargo hax tools install

The default versions shipped with a hax release are tested together and are the recommended choice. Pinning versions yourself is an advanced option: combinations other than the defaults are untested, so establishing that one works is up to you. To pin versions for a project, declare them in a hax.toml at your project's root:

[tools]
aeneas = "nightly-2026.07.01"
charon = "nightly-2026.07.01"

You can also build or install aeneas and charon yourself (e.g. from source) and point to them with a path entry in hax.toml (e.g. charon = { path = "vendor/bin/charon" }).

See Managing tool versions in the manual for the full reference (cargo hax tools, the hax.toml schema, resolution order, and the hax-lib compatibility check).

Supported Subset of the Rust Language

hax intends to support full Rust, with the one exception, promoting a functional style: mutable references (aka &mut T) on return types or when aliasing (see #420) are forbidden.

Each unsupported Rust feature is documented as an issue labeled unsupported-rust. When the issue is labeled wontfix-v1, that means we don't plan on supporting that feature soon.

Quicklinks:

• 🔨 Rejected rust we want to support;
• 💭 Rejected rust we don't plan to support in v1.

Hacking on hax

The documentation of the internal crate of hax and its engine can be found here for the engine and here for the frontend.

Edit the sources (Nix)

Just clone & cd into the repo, then run nix develop .. You can also just use direnv, with editor integration.

The flake provides several dev shells:

Shell	Purpose
nix develop .	Hacking on hax itself: the toolchain to build the Rust CLI, the frontend and the OCaml engine. Provides no backend verifier.
nix develop .#fstar	The above plus F*, for the F* backend and the F* proof libraries. Used by CI to check the proof libraries.
nix develop .#examples	The above plus ProVerif and Lean (through elan), for running examples/ against a hax you build yourself.
nix develop .#ci-examples	Running examples/ against a hax built by the flake, rather than one you build from source. Used by CI.

The first three shells give you the toolchain to build hax, not a cargo-hax binary: run just build first (see below).

Structure of this repository

• frontend/: Rust library that hooks into the Rust compiler and extracts its internal typed abstract syntax tree THIR as JSON.
• engine/: the simplification and elaboration engine that translates programs from the Rust language to various backends (see engine/backends/). Written in OCaml.
• rust-engine/: an on-going rewrite of our engine from OCaml to Rust.
• cli/: the cargo hax subcommand and the custom rustc drivers it uses to run the frontend.
• hax-lib/: helper crate providing hax-specific macros (e.g. requires, ensures) for annotating Rust programs.
• hax-types/: types shared between the frontend, the CLI, and the engine.
• proof-libs/: a symlink to hax-lib/proof-libs/, the per-backend proof libraries that the extracted code builds against.
• examples/: examples showing what hax can do.
• tests/: integration tests.
• docs/: sources of the hax website, including the manual and the blog.

Compiling, formatting, and more

We use the just command runner. If you use Nix, the dev shell provides it automatically, if you don't use Nix, please install just on your system.

Anywhere within the repository, you can build and install in PATH (1) the Rust parts with just rust, (2) the OCaml parts with just ocaml or (3) both with just build. More commands (e.g. just fmt to format) are available, please run just or just --list to get all the commands.

Publications & Other material

• 📕 Tech report
• 📕 HACSpec: A gateway to high-assurance cryptography
• 📕 Original hacspec paper

Secondary literature, using hacspec:

• 📕 Last yard
• 📕 A Verified Pipeline from a Specification Language to Optimized, Safe Rust at CoqPL'22
• 📕 Hax - Enabling High Assurance Cryptographic Software at RustVerify24
• 📕 A formal security analysis of Blockchain voting at CoqPL'24
• 📕 Specifying Smart Contract with Hax and ConCert at CoqPL'24

Contributing

Before starting any work please join the Zulip chat, start a discussion on Github, or file an issue to discuss your contribution.

Acknowledgements

Zulip graciously provides the hacspec & hax community with a "Zulip Cloud Standard" tier.