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A look into the Mobile Messaging Black Box

Roland Schilling, Frieder Steinmetz

Despite its ubiquitous application and widespread acceptance, mobile instant messaging remains a complex matter and is often not understood by its users. Easy-to-use apps and security assurances by their developers suggest users a safe and private environment for conversation. At the same time, more and more apps flood the market and it is becoming increasingly difficult, even for technically-educated users, to keep track of both technological development and their own security and privacy requirements. We want to present a talk that sheds some light into technical aspects of mobile instant messaging and presents an overview of techniques and design decisions by different mobile instant messaging app developers. We aim at both technically-educated and casual users alike, trying to present simple analogies and break down complex details into understandable components. After an introduction to the mobile instant messaging world, we will dissect one of the most popular mobile instant messaging apps in Germany: Threema. It is closed-source and only superficially documented, yet widely used. We picked it for a particular design decision in its protocol, the lack of which we consider the most important flaw in competitor protocols like Signal: the use of discardable IDs in favor of phone numbers. Another interesting aspect about Threema’s protocol is its use of the NaCl library for end-to-end encryption. We have fully reverse-engineered the Threema app and can therefore analyze and present its protocol and our analysis of it in detail.

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