Getting Started

This quick start guide assumes knowledge of the Double Ratchet algorithm.

Next to a few container classes and interfaces, the four major units of this library are DoubleRatchet, DiffieHellmanRatchet, SymmetricKeyRatchet and KDFChain. These classes are structured roughly the same:

• Instances can be created through factory methods (called e.g. create), NOT by calling the constructor/__init__.

• Instances can be serialized into JSON-friendly data structures.

• When creating a new instance or deserializing an old instance, a set of configuration options has to be passed. Note that it is your responsibility to pass the same configuration when deserializing as you passed when creating the instance.

• Some of the classes are abstract, requiring you to subclass them and to implement one or two abstract methods.

• For some of the interfaces and abstract classes, implementations using recommended cryptographic primitives are available in the doubleratchet.recommended package.

The DoubleRatchet class offers a thin and simple message en-/decryption API, using and combining all of the other classes under the hood. For details on the configuration, refer to the encrypt_initial_message() or decrypt_initial_message() methods of the DoubleRatchet class. Take a look at the Double Ratchet Chat example in the python-doubleratchet repository for an example of a full configuration, including the required subclassing and using some of the recommended implementations.

This library implements the core of the Double Ratchet specification and includes a few of the recommended algorithms. This library does currently not offer sophisticated decision mechanisms for the deletion of skipped message keys. Skipped message keys are only deleted when the maximum amount is reached and old keys are deleted from the storage in FIFO order. There is no time-based or event-based deletion.