Authors: Christoph Döberl, Wolfgang Eibner, Simon Gärtner, Manuela Kos, Florian Kutschera, Sebastian Ramacher

With recent advances on the capabilities of quantum computers and the seminal work by Shor on the prime factorization of integers with quantum algorithms, the threat of quantum computers is looming over public-key based cryptographic systems. Two distinct areas of research – post-quantum and quantum cryptography – provide the technical means to secure cryptographic systems against potentially powerful enough quantum computers. As both technologies are now becoming readily available, integration in a wide range of applications to understand their characteristics and trade-offs and the associated evaluation is of importance.

In this work, we investigate the integration of quantum-resistant cryptography into secure messaging applications. Specifically, we target Delta Chat which is an end-to-end secure and open source messaging application. We integrate both post-quantum secure digital signature schemes and public-key encryption schemes to keep the end-to-end secure messaging properties of the system, whereas the server-to-server communication is additionally secured via secret keys provided by a metropolitan quantum key distribution network. Since Delta Chat transports messages via the users’ email accounts, we thus also obtain a quantum-resistant email infrastructure. As such we also analyze mechanisms commonly employed to secure email communication and the required implementation steps to ensure quantum-system for users of S/MIME and OpenPGP.