Briar Reproducer

This is a tool you can use to verify that Briar was built exactly from the public source code and no modifications (such as backdoors) were made.

It works by building the app deterministically in a controlled standard environment and comparing the result to the official APK file.

More information about these so called reproducible builds is available at reproducible-builds.org.

The source code for this tool is available at https://code.briarproject.org/briar/briar-reproducer

How it works

We are using the Docker container technology to create a controlled and stable environment. Our image is based on the official Debian stable image and installs a minimum of software required to build Briar and to compare the result with the official APK files.

You can find all software that gets installed in the install*.sh files.

Then there are two Python scripts. reproduce.py is downloading the official APK and the source code from the official git repository. It builds a new APK from the source code and then hands over to verify-apk.py.

This script verifies that the official APK and the newly-built one are identical bit by bit. It does so by repacking both APKs in a deterministic way stripping both the Android v1 and v2 signature from the APK so that only the contents can be are compared.

Both repacked APKs are then hashed with SHA512. If their hashes are the same, the contents are identical and we know that the official APK was built from the exact same source. If the hashes differ, diffoscope is used to show where both versions differ.

How to use

Verify that you have docker installed:

docker --version

If this command does not work, please install Docker and continue once it is installed.

Using our pre-built image

If you trust that our pre-built Docker image was build exactly from its source, you can use it for faster verification. If not, you can read the next section to learn how to build the image yourself. Then you are only trusting the official debian:stable which is out of our control.

Otherwise, you can skip the next section and move directly to Run the verification.

Building your own image

Check out the source repository:

git clone https://code.briarproject.org/briar/briar-reproducer.git

Build our Docker image:

docker build -t briar/reproducer briar-reproducer

Run the verification

Currently, the verification needs disorderfs as a deterministic file-system. Therefore, please make sure that fuse is installed on your host system.

apt install fuse

To verify a specific version of Briar, run

docker run --cap-add SYS_ADMIN --device /dev/fuse briar/reproducer:latest ./reproduce.py [tag]

Where [tag] is the git tag (source code snapshot) that identifies the version you want to test, for example release-1.0.1.

You can find a list of tags in Briar's source code repository.

The SYS_ADMIN capability and the fuse device are required, so the container can build the app inside a disorderfs which shuffles the filesystem.

If the build fails with fuse: mount failed: Permission denied, you may need to add the argument --security-opt apparmor:unconfined to the Docker command.

Historical Changes

• Before version 1.4.2, Briar was build using Java 8. Newer release are built with Java 11 which produces different byte code breaking reproducibility An old version of briar-reproducer can be used to verify these releases.
• Before version 1.1.7, Briar needed a deterministic files system to work around an Android build system bug which broke reproducibility. An old version of briar-reproducer can be used to verify these releases.
• Before version 1.1.0, the path to the APK (gradle_apk_path in apps.json) was briar-android/build/outputs/apk/release/briar-android-release-unsigned.apk