What's involved in a global persistent secure data server?

Why hello there, brilliant members of the unity forum.

I'm looking into the possibility of creating some sort of messaging system similar to what Dark Souls has implemented - which means a globally accessible sever (or perhaps several, separated by region) that can access a database. and update clients with... let's say a list of messages for that given region, prioritized by upvotes, etc.

And since we're talking Dark Souls, I would also theoretically use this system to evaluate the number of players in a given area who are susceptible to invasion (that is, involuntary multiplayer attack), as well as "summon signs" that any given player could leave, which would be accessed by other players to initiate some kind of multiplayer connection.

I've done multiplayer games before, so I'm confident that I can more-or-less model what I'd need for the real-time multiplayer interaction, but this issue of having a persistent globally accessible server where all of the messages are stored (in a way that is actually secure) is another issue altogether.

Any of you have a good idea on where I can get started looking?