Olm

An implementation of the Double Ratchet cryptographic ratchet described by https://whispersystems.org/docs/specifications/doubleratchet/, written in C and C++11 and exposed as a C API.

The specification of the Olm ratchet can be found in docs/olm.md.

This library also includes an implementation of the Megolm cryptographic ratchet, as specified in docs/megolm.md.

Building

To build olm as a shared library run:

cmake . -Bbuild
cmake --build build

To run the tests, run:

cd build/tests
ctest .

To build olm as a static library (which still needs libstdc++ dynamically) run:

cmake . -Bbuild -DBUILD_SHARED_LIBS=NO
cmake --build build

The library can also be used as a dependency with CMake using:

find_package(Olm::Olm REQUIRED)
target_link_libraries(my_exe Olm::Olm)

Bindings

To build the JavaScript bindings, install emscripten from https://emscripten.org/ and then run:

make js

Note that if you run emscripten in a docker container, you need to pass through the EMCC_CLOSURE_ARGS environment variable.

To build the android project for Android bindings, run:

cd android
./gradlew clean assembleRelease

To build the Xcode workspace for Objective-C bindings, run:

cd xcode
pod install
open OLMKit.xcworkspace

To build the Python bindings, first build olm as a shared library as above, and then run:

cd python
make

to make both the Python 2 and Python 3 bindings. To make only one version, use make olm-python2 or make olm-python3 instead of just make.

Using make instead of cmake

WARNING: Using cmake is the preferred method for building the olm library; the Makefile may be removed in the future or have functionality removed. In addition, the Makefile may make certain assumptions about your system and is not as well tested.

To build olm as a dynamic library, run:

make

To run the tests, run:

make test

To build olm as a static library, run:

make static

Bindings

libolm can be used in different environments using bindings. In addition to the JavaScript, Python, Java (Android), and Objective-C bindings included in this repository, some bindings are (in alphabetical order):

• cl-megolm (MIT) Common Lisp bindings
• dart-olm (AGPLv3) Dart bindings
• Dhole/go-olm (Apache-2.0) Go bindings
• jOlm (Apache-2.0) Java bindings
• libQtOlm (GPLv3) Qt bindings
• matrix-kt (Apache-2.0) Kotlin library for Matrix, including Olm methods
• maunium.net/go/mautrix/crypto/olm (Apache-2.0) fork of Dhole/go-olm
• nim-olm (MIT) Nim bindings
• olm-sys (Apache-2.0) Rust bindings
• Trixnity (AGPLv3) Kotlin SDK for Matrix, including Olm bindings

Note that bindings may have a different license from libolm, and are not endorsed by the Matrix.org Foundation C.I.C.

Release process

First: bump version numbers in common.mk, CMakeLists.txt, javascript/package.json, python/olm/__version__.py, OLMKit.podspec, Package.swift, and android/gradle.properties.

Also, ensure the changelog is up to date, and that everything is committed to git.

It's probably sensible to do the above on a release branch (release-vx.y.z by convention), and merge back to master once the release is complete.

make clean

# build and test C library
make test

# build and test JS wrapper
make js
(cd javascript && \
     npm run test && \
     sha256sum olm.js olm_legacy.js olm.wasm > checksums.txt && \
     gpg -b -a -u F75FDC22C1DE8453 checksums.txt && \
     npm publish)

VERSION=x.y.z
git tag $VERSION -s
git push --tags

# OLMKit CocoaPod release
# Make sure the version OLMKit.podspec is the same as the git tag
# (this must be checked before git tagging)
pod spec lint OLMKit.podspec --use-libraries --allow-warnings
pod trunk push OLMKit.podspec --use-libraries --allow-warnings
# Check the pod has been successully published with:
pod search OLMKit

Python and JavaScript packages are published to the registry at https://gitlab.matrix.org/matrix-org/olm/-/packages. The GitLab documentation contains instructions on how to set up twine (Python) and npm (JavaScript) to upload to the registry.

To publish the Android library to MavenCentral (you will need some secrets), in the /android folder:

• Run the command ./gradlew clean build publish --no-daemon --no-parallel --stacktrace. The generated AAR must be approx 500 kb.
• Connect to https://s01.oss.sonatype.org
• Click on Staging Repositories and check the the files have been uploaded
• Click on close
• Wait (check Activity tab until step "Repository closed" is displayed)
• Click on release. The staging repository will disappear
• Check that the release is available in https://repo1.maven.org/maven2/org/matrix/android/olm-sdk/ (it can take a few minutes)

Design

Olm is designed to be easy port to different platforms and to be easy to write bindings for.

It was originally implemented in C++, with a plain-C layer providing the public API. As development has progressed, it has become clear that C++ gives little advantage, and new functionality is being added in C, with C++ parts being rewritten as the need ariases.

Error Handling

All C functions in the API for olm return olm_error() on error. This makes it easy to check for error conditions within the language bindings.

Random Numbers

Olm doesn't generate random numbers itself. Instead the caller must provide the random data. This makes it easier to port the library to different platforms since the caller can use whatever cryptographic random number generator their platform provides.

Memory

Olm avoids calling malloc or allocating memory on the heap itself. Instead the library calculates how much memory will be needed to hold the output and the caller supplies a buffer of the appropriate size.

Output Encoding

Binary output is encoded as base64 so that languages that prefer unicode strings will find it easier to handle the output.

Dependencies

Olm uses pure C implementations of the cryptographic primitives used by the ratchet. While this decreases the performance it makes it much easier to compile the library for different architectures.

Contributing

Please see CONTRIBUTING.md when making contributions to the library.

Security assessment

Olm 1.3.0 was independently assessed by NCC Group's Cryptography Services Practive in September 2016 to check for security issues: you can read all about it at https://www.nccgroup.com/globalassets/our-research/us/public-reports/2016/november/ncc_group_olm_cryptogrpahic_review_2016_11_01.pdf and https://matrix.org/blog/2016/11/21/matrixs-olm-end-to-end-encryption-security-assessment-released-and-implemented-cross-platform-on-riot-at-last/

Security issues

If you think you found a security issue in libolm, any of its bindings or the Olm/Megolm protocols, please follow our Security Disclosure Policy to report.

Bug reports

For non-sensitive bugs, please file bug reports at https://github.com/matrix-org/olm/issues.

What's an olm?

It's a really cool species of European troglodytic salamander. http://www.postojnska-jama.eu/en/come-and-visit-us/vivarium-proteus/

Legal Notice

The software may be subject to the U.S. export control laws and regulations and by downloading the software the user certifies that he/she/it is authorized to do so in accordance with those export control laws and regulations.