Since they haven't come up yet, and since the website obviously contains primarily marketing content, here are a few relevant links that might be of interest to the HN crowd:

- Cryptography whitepaper: https://threema.ch/press-files/2_documentation/cryptography_... Covers the technical aspects.

- Transparency report: https://threema.ch/en/transparencyreport Covers the legal aspects.

- Reverse engineering of the protocol (33c3): https://media.ccc.de/v/33c3-8062-a_look_into_the_mobile_mess...

- Third party open source re-implementation of Threema for the desktop (Qt): https://openmittsu.de/

- Source code of the web application: https://github.com/threema-ch/threema-web/

- Protocol developed to enable trustless WebRTC signaling between app and browser: https://saltyrtc.org/

Obligatory disclaimer: I'm working for Threema as a developer.