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X3DH

This is a rudimentary implementation of the X3DH protocol used by the messaging app Signal. There are a few differences between the original implementation and this one, but for the most part this version follows the same cryptographic specifications as the ones outlined here.

The xeddsa library is adapted from the python-xeddsa package, and has been modified to use pynacl instead of libnacl. The xeddsa.ref10 library is copied from the same package, where it was adapted from the SUPERCOP library.

If you want to read my own summary of how X3DH works, you can do so on my blog. Here's the relevant post. While you read the Signal documentation, you might also be interested in reading about the XEdDSA Signature Scheme. This I will not document, but you can find the specification here.

Setting Up

After cloning this repository, there are a couple of things to do in order to get the required libraries set up. First, create a Python environment.

$ python3 -m venv env
$ source env/bin/activate

Install the required packages.

(env) $ pip install -r requirements.txt

Next, build the ref10 library. This is a low-level library that must be compiled on your machine, since it depends on your operating system and architecture.

(env) $ cd xeddsa && python ref10/build.py && cd ..

Running the Server

To start the server, run the following command.

(env) $ python server.py

Creating Clients

You need to specify a username for the client, which will be used to determine their UID. Pick one, and then start the client.

(env) $ python client.py <username>

Do this for at least two clients.

Performing an Exchange

To run the X3DH exchange between two clients, run the following request where <port> is the port the sending client is running on and <target> is the username of the receiving client.

(env) $ curl --location --request POST 'http://127.0.0.1:<port>/perform_handshake' \
--header 'Content-Type: application/json' \
--data-raw '{
    "send_to": "<target>"
}'

Alternatively, run this request using Python.

import requests

port, target = "<port>", "<target>"
url = f"http://127.0.0.1:{port}/perform_handshake"

payload=f"{{\n    \"send_to\": \"{target}\"\n}}"
headers = {
  'Content-Type': 'application/json'
}

response = requests.request("POST", url, headers=headers, data=payload)
print(response.text)

Viewing Data

The idea here is to create an exchange that is hard, if not impossible, to intercept. All data is therefore made visible in this experiment.

To see all the information stored by the server, visit http://127.0.0.1:5000/user_data. You will find each user's prekey bundle listed, along with the ports they're running on and their UID.

When you request a prekey bundle (done automatically by the client performing a handshake), the server will print out the bundle being returned as well. When the client performing the handshake sends information to the receiving client, the server will print the data being forwarded as well.

License

This experiment is licensed under the MIT license, though honestly you probably shouldn't use most of this in a production environment.

See LICENSE.