You can not select more than 25 topics Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
Hubert Chathi c01164f001 add link to nim binding 6 days ago
.circleci the right dir 2 years ago
android bump version numbers and update changelog 3 months ago
cmake Add CMake support 2 years ago
docs DH ratchet sequence diagram 1 month ago
fuzzers python: Remove the python bindings. 2 years ago
include/olm SAS: add olm_sas_is_their_key_set 3 months ago
javascript Update index.d.ts; specify PRIVATE_KEY_LENGTH const export 3 months ago
lib OLMKit: Make the project build 4 years ago
python bump version numbers and update changelog 3 months ago
src Fix length calculation of fallback key json 1 month ago
tests fix memory leaks in tests 3 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. 5 years ago
xcode OLMKit: Make podspec point to new 1 year ago
.gitignore Merge remote-tracking branch 'origin/master' into olmkit 4 years ago
CHANGELOG.rst use the right version in the changelog 2 months ago
CMakeLists.txt Use current source directory in CMake. Thanks to Gorgurov Alexey. 2 months ago Convert CONTRIBUTING.rst to markdown 1 year ago
LICENSE Copyright notices and a license 5 years ago
Makefile simplify Makefile (olm_legacy.js) 3 months ago
OLMKit.podspec bump version numbers and update changelog 3 months ago add link to nim binding 6 days ago bump version numbers and update changelog 3 months ago fix build with emscripten 2.0.4 3 months ago python: Remove the python bindings. 2 years ago
version_script.ver Use a version script to restrict symbols in the .so 4 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 either:

cmake . -Bbuild
cmake --build build



Using cmake is the preferred method for building the shared library; the Makefile may be removed in the future.

To run the tests when using cmake, run:

cd build/tests
ctest .

To run the tests when using make, run:

make test

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 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

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.

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

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


make static

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

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


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.

Release process

First: bump version numbers in, CMakeLists.txt, javascript/package.json, python/olm/, OLMKit.podspec, and android/olm-sdk/build.gradle (versionCode, versionName and version).

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 && npm pack javascript)

gpg -b -a -u F75FDC22C1DE8453 javascript/olm-$VERSION.tgz
scp javascript/olm-$VERSION.tgz
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


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

Bug reports

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.