Olm

An implementation of the Double Ratchet cryptographic ratchet described by https://github.com/trevp/double_ratchet/wiki, 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 or https://matrix.org/docs/spec/olm.html

Building

To build olm as a shared library run:

make

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

Release process

# Bump version numbers in Makefile and javascript/package.json # Prepare changelog # git commit # make test # make js # npm pack javascript # scp olm-x.y.z.tgz packages@ldc-prd-matrix-001:/sites/matrix/packages/npm/olm/ # git tag x.y.z # git push --tags

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

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.

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.