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This is the compiler for the Flexible and Constant Time cryptographic programming language. FaCT is a domain-specific language that aids you in writing constant-time code for cryptographic routines that need to be free from timing side channels.

Useful links:


To build the compiler, you can either build from source or download a pre-built release. We recommend building from source if possible.

Virtual machine image

You can download a VM image pre-configured for building the FaCT compiler and case studies. The file fact.ova should have a SHA256 sum of 089398c85c5074d911c2f2b67ca22df453235e8733f1eb283c71717cf70f714c.

Using Docker

If you have docker installed, you can load our docker image with the build environment already installed:

cd docker/

Once inside the docker shell, run the following to finish setting up the environment:

cd FaCT/
eval $(opam config env)

Setting up the build environment

FaCT is developed using OCaml 4.06.0 and LLVM 6.0.

Using a local environment

Building FaCT has been tested on Ubuntu 16.04, 18.04, and macOS.

1. Install System Dependencies

Ubuntu (16.04 & 18.04)

sudo apt install llvm-6.0 clang-6.0 cmake libgmp-dev m4 pkg-config


These instructions require Homebrew

brew install cmake gmp m4 pkg-config
brew install llvm@6 --with-toolchain

Note: This does not put the proper version of clang on your PATH. You will need to run:

export PATH="$(brew --prefix llvm@6)/bin:$PATH"

2. Install OCaml + packages

We recommend installing the opam package manager to manage OCaml and package dependencies:

sh <(curl -sL

Then, install OCaml and the libraries:

opam init
eval $(opam config env)
opam switch create 4.06.0
eval $(opam config env)
opam switch import ocamlswitch.txt

Finally, make sure the Z3 lib is available to the OCaml compiler:

export LD_LIBRARY_PATH="$HOME/.opam/4.06.0/lib/z3:$LD_LIBRARY_PATH"

3. Configure Paths

The FaCT compiler depends on the LLVM 6.0 toolchain at runtime, and expects binaries with -6.0 suffixes. Ensure that clang-6.0 is in your PATH:

clang-6.0 --version

Compiling FaCT

You can now build the compiler:

oasis setup

This will produce the factc executable.


Basic Usage

Run ./factc <source files> to compile a FaCT program.

Link to a C library

FaCT is designed to be called from C code. Compiling FaCT source files will output an object file, which can then be linked to a C file. As an example:

cd example/
../factc -generate-header example.fact
clang-6.0 -c main.c
clang-6.0 -o final main.o example.o

You can then run the executable:



Many debugging options and intermediate data structures are available. Run ./factc -help for all options.


We thank the anonymous PLDI and PLDI AEC reviewers for their suggestions and insightful comments.