Consequences of Complexity in Group Instant Messaging using the Example of WhatsApp and Signal

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Presented at RuhrSecBochum 2018 by

Paul Rösler

Group instant messaging is a complex primitive – due to the number of involved users and dynamic modifications to groups – that at the same time needs to provide high efficiency – for providing instant delivery of messages. As we show in our paper (Roesler, Mainka, Schwenk EuroS&P '18), most widespread messengers do not reach expected and required security guarantees for this primitive. This talk aims to provide an overview on the underlying reasons for this lack of security as well as on approaches how this issue can be solved, both on the constructive side and for the developers' view. After presenting the most severe attacks on WhatsApp and Signal, we aim to shed a light on the topic in a more general way. Thereby we want to motivate the reasons for end-to-end encryption more intuitively, provide an overview on what future secrecy means and how ratcheting can be used to reach this property. Of course the talk will include the protocol descriptions of the analyzed protocols and the respective attacks, but the focus will be more constructive. The talk will conclude with outlook questions (and answers): What are the expectable problems of intensive key protocols? How might they be solved by protocol and software developers? Is there a sensible threshold on which security guarantees should be achieved and which attacks can be disregarded when designing a protocol for instant messaging?