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.rst.

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


To build olm as a shared library run:


To run the tests run:

make test

To build the javascript bindings, install emscripten from http://kripken.github.io/emscripten-site/ and then run:

make js

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

make static

Release process

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

Also, ensure the changelog is up to date, and that everyting 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

scp olm-$VERSION.tgz packages@ldc-prd-matrix-001:/sites/matrix/packages/npm/olm/
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 CONTRIBUTING.rst 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.trust/us/our-research/matrix-olm-cryptographic-review/ 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/

Bug reports

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/