The Double Ratchet Algorithm 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 Open Whisper Systems Signal package.
A nice writeup of the protocol is on the Open Whisper Systems Blog. You can find the most recent specification of the protocol here.
Make sure that you have the following:
# If using Debian/Ubuntu sudo apt-get install gcc libffi-dev libsodium-dev python-dev # If using Fedora sudo yum install gcc libffi-devel libsodium-devel python-devel redhat-rpm-config
pyaxo also uses pynacl 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 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.