The Axolotl ratchet is a protocol (similar to OTR) that provides for perfect forward secrecy in (a)synchronous communications. It uses triple Diffie-Hellman for authentication and ECDHE for perfect forward secrecy. The protocol is lighter and more robust than the OTR protocol - providing better forward and future secrecy, as well as deniability.

The protocol was developed by Trevor Perrin and Moxie Marlinspike. Its chief use currently is in the Whisper Systems TextSecure SMS package.

A nice writeup of the protocol is on the Whisper Systems Blog. You can find the most recent specification of the protocol here.

Make sure that you have the following:

sudo apt-get install python-dev # If using Debian/Ubuntu
sudo yum install python-devel # If using Fedora

pyaxo also uses python-gnupg, curve25519-donna and passlib, but these packages will be downloaded and installed automatically by pip/setuptools.

If you use pip, install pyaxo with:

sudo pip install pyaxo

If you use setuptools, change to pyaxo's source folder and install with:

sudo python setup.py install

pyaxo will be ready for use!

If you do not use neither of those, you will have to manually install each dependency before running the previous command.

There are several examples showing usage. There are also encrypt_pipe() and decrypt_pipe() methods for use in certain applications. I haven't put together an example using them yet, but it should be straightforward.

pyaxo 0.4 was updated according to the latest (Oct 1, 2014) version of the protocol, which changed the order of the ratcheting. For that reason, old conversations (created with pyaxo < 0.4) might not work properly after the update. We suggest that users update pyaxo and restart their conversations.

Bugs, etc. should be reported to the pyaxo github issues page.