Hiding metadata is a difficult problem to solve, but it's vital. I expect you'll have to burn bandwidth. Broadcast "all" messages to "all" users, users can on

Hiding metadata is a difficult problem to solve, but it's vital. I expect you'll have to burn bandwidth. Broadcast "all" messages to "all" users, users can only decrypt the messages intended for them, discard the others. There are probably tricks you can do to reduce it to a fraction of "all" messages but I bet it will still be a substantial amount of bandwidth. Perhaps usenet will see a revival.