This is coming along well @Hendrik Peter. FYI the "where are the keys?" question is a really hot / dangerous topic. At the moment from your most recent diagram (and my guessing), you have all your keys in the hub.. because otherwise how is that pub serving private messages to you ... it wouldn't know which ones are for you without your private key.

What I mean by "hot / dangerous topic" is that some people will get very upset if you build a system which holds private keys on an external server. The problem is that this is a thin end of the centralisation + control wedge, and could lead to some massive security problems. This is something Rabble and verse have been through. It's also a hard problem because you want to be able to do private messages in a light way but doing it with a light client is VERY HARD... the only way with the way private messages are currently written is if you include the hub in the PM, or if you send every message to the phone and ask it if it's a private message for it , then maybe the hub could store a list of keys which it knows are you PMs, without storing the content.... this could be made lighter by not checking privates messages you publish, and immediately telling the hub it's a message "for you"... and only checking private message of people you follow to see if they're "for you" ... hmmm hard.

Anyway. I hope that comes across ok. I'm not upset at all, I'm just trying to be very clear that this is dangerous territory because I want you to have a good time and succeed.