  European lawmakers have ruled that users on different platforms should be
able to exchange messages with each other. Yet messaging interoperability opens
up a Pandora's box of security and privacy challenges. While championed not
just as an anti-trust measure but as a means of providing a better experience
for the end user, interoperability runs the risk of making the user experience
worse if poorly executed. There are two fundamental questions: how to enable
the actual message exchange, and how to handle the numerous residual challenges
arising from encrypted messages passing from one service provider to another --
including but certainly not limited to content moderation, user authentication,
key management, and metadata sharing between providers. In this work, we
identify specific open questions and challenges around interoperable
communication in end-to-end encrypted messaging, and present high-level
suggestions for tackling these challenges.
