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Study – Interoperability between Messaging Services: Secure Implementation of Encryption (Bundesnetzagentur), Prof. Dr. Paul Rösler and Prof. Dr. Jörg Schwenk

Instant Messaging (IM) apps like WhatsApp, Signal, Threema, and Telegram are used by billions of people around the globe. Many of these apps offer End-to-End Encryption (E2EE), and people trust the security of these apps to communicate their most private thoughts. But what do we really know about the security of these apps?

In contrast to e-mail apps, IM apps are not interoperable. This means that it is impossible for a WhatsApp user to communicate with a Signal user. So similar to social networks, market mechanisms favor big IM applications – the more user an IM app has, the more attractive it is. This motivated the EU to issue the Digital Markets Act which is intended to enable interoperability, but without sacrificing security, and E2EE in particular.

In an effort to better understand the technical implications of the DMA, the German Bundesnetzagentur issued a call for tender on a technical study, which Hackmanit won. The study was written by two authors who had already published on IM security: Prof. Dr. Paul Rösler (now FAU Erlangen) and Prof. Dr. Jörg Schwenk (Ruhr University Bochum and Hackmanit GmbH). The study is public and can be downloaded here:


Cryptographic Background

To make the study self-contained, the first part of the study contains an introduction to modern cryptography, from simple building blocks like blockciphers to complex protocols like TLS. It covers block ciphers, hash functions, MACs, the Diffie-Hellman Key exchange, and the RSA and ElGamal public key encryption and signature algorithms. Authenticated key exchange protocols are used to establish keys between parties, and the given examples are TLS, X3DH and Noise. Sections on push protocols (SMS and e-mail) and trust establishment conclude this part.


Instant Messaging Specific Cryptography

For the public, this may be the most interesting part of the study, because the authors summarize the current knowledge in IM security. They describe how chat messages are protected and explain the novel concept of Ratcheting used by many IM apps. File transfer (image, audio and video files) is based to secure chat messages, by encrypting and uploading the files to a cloud server and including URL and key in a chat message. Different concepts of E2EE for groups of users are discussed, and a sketch how real-time communication (audio and video calls) are protected. How and where these concepts are used is described for Signal, WhatsApp, Facebook Messenger, Wire, Matrix, iMessage and Telegram.


Paths to Interoperability

The last part discusses paths to IM interoperability where E2EE security guarantees will be provided. It sketches the current state of standardization for IM, and explores an approach where cryptographic libraries will be exchanged between providers. This shows that interoperability can be achieved, without sacrificing security.

More details and information on the Federal Network Agency website:

Bundesnetzagentur: On­line-Kommunikations­dienste

Author and Chairman of Hackmanit

Prof. Dr. Jörg Schwenk



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