These days, many chat services allow users to use multiple devices in parallel. Therefore, one might assume that such a multi-device functionality must be easy enough to implement. Provided that security and privacy protection are not a major priority, this notion isn’t far off. If, however, the multi-device protocol is required to meet Threema standards, things get complicated.
In order to fulfill Threema’s requirements, a multi-device solution must, of course, provide full end-to-end encryption. That’s a given. But on top of that, it also has to rule out any possibility for the server to alter key material, and the amount of transferred metadata must be kept to the absolute minimum.
Even though these requirements pose a true technological challenge, we believe to have found a capable and elegant solution, which is currently in development and will become available in the course of 2021.
For technically inclined readers, we outline our approach after the jump…