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David Robertson 983e78dc53 Fix dead link to e2ee guide in the README 2 weeks ago
.circleci the right dir 4 years ago
android release 3.2.12 1 month ago
cmake Add CMake support 4 years ago
docs Describe the session export format. 1 month ago
fuzzing Fix unpickling error handling. 11 months ago
include improve handling of olm_session_describe when buffer is too short 7 months ago
javascript release 3.2.12 1 month ago
lib switch to doctest for unit testing 7 months ago
python Fix dead link to e2ee guide in the README 2 weeks ago
src improve handling of olm_session_describe when buffer is too short 7 months ago
tests switch to doctest for unit testing 7 months ago
tracing switch from /usr/bin/python to /usr/bin/env python. this doesn't help folks whose python path points at python3 (e.g. Arch linux) though, but I see no choice than they have to change the shebangs, as we do on Synapse. For instance, OSX doesn't have a python2 symlink, otherwise we'd use /usr/bin/env python2 shebang. 7 years ago
xcode exposed olm_sas_calculate_mac_fixed_base64 in the bindings 2 months ago
.editorconfig Add .editorconfig. 1 year ago
.gitignore Fix type hints on the PkDecryption class 2 months ago
.gitlab-ci.yml keep testing logs 7 months ago
CHANGELOG.rst release 3.2.12 1 month ago
CMakeLists.txt release 3.2.12 1 month ago Convert CONTRIBUTING.rst to markdown 3 years ago
LICENSE Copyright notices and a license 7 years ago
Makefile allow memory to grow in wasm 2 months ago
OLMKit.podspec release 3.2.12 1 month ago
Package.swift release 3.2.12 1 month ago minor documentation fixes 1 month ago
Windows64.cmake add support file for cross-compiling Windows library 1 year ago release 3.2.12 1 month ago fix JavaScript build 1 year ago
flake.lock let pinning of nixos-unstable commit happen in flake.lock rather than flake.nix 2 months ago
flake.nix Make sure checks actually run 2 months ago
gitlab-math.lua fix doc building. Thanks to Jonas Smedegaard. 6 months ago python: Remove the python bindings. 4 years ago Add for printing list of exported functions. 12 months ago
libolm.version make functions const where possible 1 year ago create and install a pkg-config file on Unix-like systems 1 year ago
version_script.ver Use a version script to restrict symbols in the .so 6 years ago


An implementation of the Double Ratchet cryptographic ratchet described by, written in C and C++11 and exposed as a C API.

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

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


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)


To build the JavaScript bindings, install emscripten from 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 build

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

cd xcode
pod install
open OLMKit.xcworkspace

To build the Python 3 bindings, first build olm as a shared library as above, and then run:

cd python

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:


To run the tests, run:

make test

To build olm as a static library, run:

make static


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):

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

Release process

First: bump version numbers in, CMakeLists.txt, javascript/package.json, python/olm/, OLMKit.podspec, Package.swift, and android/

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)

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 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
  • 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 (it can take a few minutes)


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.


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.


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.


Please see 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 and

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

What's an olm?

It's a really cool species of European troglodytic salamander.

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.