There may be a problem with the "prefix length" parameter. What the user will want to specify is not the proportion off all messages they want to download, but

There may be a problem with the "prefix length" parameter. What the user will want to specify is not the proportion off all messages they want to download, but the number or bandwidth. i.e. If subspace has 1,024 users, I might specify a prefix which lets me download ~128 users messages to find my own. When subspace becomes more popular and has 2^20 users, I don't want to download a thousand times more messages. The prefix needs to be adjustable, or auto-adjusted with regard to the total number of users.