This is a python port of libsignal-protocol-java originally written by Moxie Marlinspike

Overview from original author's:

This is a ratcheting forward secrecy protocol that works in synchronous and asynchronous messaging environments.

Installation

The library has some dependencies which are automatically pulled and installed if you use the instructions below for your OS

• protobuf 3.0+
• cryptography
• python-axolotl-curve25519

Linux

You need to have python headers installed, usually through installing a package called python-dev, then as superuser run:

python setup.py install

Windows

• Install mingw compiler
• Add mingw to your PATH
• In PYTHONPATH\Lib\distutils create a file called distutils.cfg and add these lines:

[build]
compiler=mingw32

• Install gcc: mingw-get.exe install gcc
• Install zlib
• python setup.py install

Usage

This python port is done in an almost 1:1 mapping to the original java code. Therefore any original documentation for the java code can be easily mapped and used with this python port.

Install time

At install time, a libaxolotl client needs to generate its identity keys, registration id, and prekeys.

    identityKeyPair = KeyHelper.generateIdentityKeyPair()
    registrationId  = KeyHelper.generateRegistrationId()
    preKeys         = KeyHelper.generatePreKeys(startId, 100)
    lastResortKey   = KeyHelper.generateLastResortKey()
    signedPreKey    = KeyHelper.generateSignedPreKey(identityKeyPair, 5)

    #Store identityKeyPair somewhere durable and safe.
    #Store registrationId somewhere durable and safe.

    #Store preKeys in PreKeyStore.
    #Store signed prekey in SignedPreKeyStore.

Building a session

A libaxolotl client needs to implement four interfaces: IdentityKeyStore, PreKeyStore, SignedPreKeyStore, and SessionStore. These will manage loading and storing of identity, prekeys, signed prekeys, and session state.

Once those are implemented, building a session is fairly straightforward:

sessionStore      = MySessionStore()
preKeyStore       = MyPreKeyStore()
signedPreKeyStore = MySignedPreKeyStore()
identityStore     = MyIdentityKeyStore()

# Instantiate a SessionBuilder for a remote recipientId + deviceId tuple.
sessionBuilder = SessionBuilder(sessionStore, preKeyStore, signedPreKeyStore,
                                                   identityStore, recipientId, deviceId)

# Build a session with a PreKey retrieved from the server.
sessionBuilder.process(retrievedPreKey)

sessionCipher = SessionCipher(sessionStore, recipientId, deviceId)
message       = sessionCipher.encrypt("Hello world!")

deliver(message.serialize())

Examples

python-axolotl is actively used in yowsup to support E2E encryption of WhatsApp

License

Licensed under the GPLv3: http://www.gnu.org/licenses/gpl-3.0.html