Overview

libsignal contains platform-agnostic APIs used by the official Signal clients and servers, exposed as a Java, Swift, or TypeScript library. The underlying implementations are written in Rust:

• libsignal-protocol: Implements the Signal protocol, including the Double Ratchet algorithm. A replacement for libsignal-protocol-java and libsignal-metadata-java.
• signal-crypto: Cryptographic primitives such as AES-GCM. We use RustCrypto's where we can but sometimes have differing needs.
• device-transfer: Support logic for Signal's device-to-device transfer feature.
• attest: Functionality for remote attestation of SGX enclaves and server-side HSMs.
• zkgroup: Functionality for zero-knowledge groups and related features available in Signal.
• zkcredential: An abstraction for the sort of zero-knowledge credentials used by zkgroup, based on the paper "The Signal Private Group System" by Chase, Perrin, and Zaverucha.
• poksho: Utilities for implementing zero-knowledge proofs (such as those used by zkgroup); stands for "proof-of-knowledge, stateful-hash-object".
• account-keys: Functionality for consistently using PINs as passwords in Signal's Secure Value Recovery system, as well as other account-wide key operations.
• usernames: Functionality for username generation, hashing, and proofs.
• media: Utilities for manipulating media.

This repository is used by the Signal client apps (Android, iOS, and Desktop) as well as server-side. Use outside of Signal is unsupported. In particular, the products of this repository are the Java, Swift, and TypeScript libraries that wrap the underlying Rust implementations. All APIs and implementations are subject to change without notice, as are the JNI, C, and Node add-on "bridge" layers. However, backwards-incompatible changes to the Java, Swift, TypeScript, and non-bridge Rust APIs will be reflected in the version number on a best-effort basis, including increases to the minimum supported tools versions.

Building

Toolchain Installation

To build anything in this repository you must have Rust installed, as well as recent versions of Clang, libclang, CMake, Make, protoc, Python (3.9+), and git.

Linux/Debian

On a Debian-like system, you can get these extra dependencies through apt:

$ apt-get install clang libclang-dev cmake make protobuf-compiler libprotobuf-dev python3 git

macOS

On macOS, we have a best-effort maintained script to set up the Rust toolchain you can run by:

$ bin/mac_setup.sh

Rust

First Build and Test

The build currently uses a specific version of the Rust stable compiler, which will be downloaded automatically by cargo. To build and test the basic protocol libraries:

$ cargo build
...
$ cargo test
...

Additional Rust Tools

The basic tools above should get you set up for most libsignal Rust development.

Eventually, you may find that you need some additional Rust tools like taplo for code formatting.

You should always install any Rust tools you need that may affect the build from cargo rather than from your system package manager (e.g. apt or brew). Package managers sometimes contain outdated versions of these tools that can break the build with incompatibility issues (especially cbindgen).

To install the main Rust extra dependencies matching the versions we use, you can run the following commands:

$ cargo +stable install --version "$(cat acknowledgments/cargo-about-version)" --locked cargo-about
$ cargo +stable install --version "$(cat .taplo-cli-version)" --locked taplo-cli
$ cargo +stable install cargo-fuzz

Java/Android

Toolchain Setup / Configuration

To build for Android you must install several additional packages including a JDK, the Android NDK/SDK, and add the Android targets to the Rust compiler, using

rustup target add armv7-linux-androideabi aarch64-linux-android i686-linux-android x86_64-linux-android

Our officially supported JDK version for Android builds is JDK 21, so be sure to install e.g. OpenJDK 21, and then point JAVA_HOME to it.

You can easily do this on macOS via:

export JAVA_HOME=$(/usr/libexec/java_home -v 21)

On Linux, the way you do this varies by distribution. For Debian based distributions like Ubuntu, you can use:

sudo update-alternatives --config java

We also check-in a .tools_version file for use with runtime version managers.

Building and Testing

To build the Java/Android jar and aar, and run the tests:

$ cd java
$ ./gradlew test
$ ./gradlew build # if you need AAR outputs

You can pass -P debugLevelLogs to Gradle to build without filtering out debug- and verbose-level logs from Rust, and -P jniTypeTagging to enable additional checks in the Rust JNI bridging code.

Alternately, a build system using Docker is available:

$ cd java
$ make

When exposing new APIs to Java, you will need to run rust/bridge/jni/bin/gen_java_decl.py in addition to rebuilding. This requires installing the cbindgen Rust tool, as detailed above.

Use as a library

Signal publishes Java packages for its own use, under the names org.signal:libsignal-server, org.signal:libsignal-client, and org.signal:libsignal-android. libsignal-client and libsignal-server contain native libraries for Debian-flavored x86_64 Linux as well as Windows (x86_64) and macOS (x86_64 and arm64). libsignal-android contains native libraries for armeabi-v7a, arm64-v8a, x86, and x86_64 Android. These are located in a Maven repository at https://build-artifacts.signal.org/libraries/maven/; for use from Gradle, add the following to your repositories block:

maven {
  name = "SignalBuildArtifacts"
  // The "uri()" part is only necessary for Kotlin Gradle; Groovy Gradle accepts a bare string here.
  url = uri("https://build-artifacts.signal.org/libraries/maven/")
}

Older builds were published to Maven Central instead.

When building for Android you need both libsignal-android and libsignal-client, but the Windows and macOS libraries in libsignal-client won't automatically be excluded from your final app. You can explicitly exclude them using packaging:

android {
  // ...
  packaging {
    resources {
      excludes += setOf("libsignal_jni*.dylib", "signal_jni*.dll")
    }
  }
  // ...
}

You can additionally exclude libsignal_jni_testing.so if you do not plan to use any of the APIs intended for client testing.

Testing a local build with Signal-Android