You can build secure and noncentralized! What you can't do is build secure and federated, where everyone lives in a shared, broadly reachable namespace comprise

You can build secure and noncentralized! What you can't do is build secure and federated, where everyone lives in a shared, broadly reachable namespace comprised of independently operated instances. I simply wouldn't build a secure group message system to begin with. It's a treacherously hard problem and the very few people who have done it well accomplished that with major UX compromises that put them at long-term disadvantage against schlock like Telegram, and survived mostly due to force-of-nature word of mouth. If you're going to try, and you want to be rigorous about it, I'd say you need to start by reading the whole history of cryptanalyses of secure messaging systems, even systems you don't care about. Read the papers carefully and work out the attacks for yourself. It's a little like math in that you're only going to figure it out by actually working the examples yourself.