Yes, but this still requires me, Bob, to *tell* Alice whom I don't want to see my posts.
This is why a trusted third party, who *does* know the shape of the graph, but does not tell any of the participants, is essential to allow the sort of restrictions on distribution that we've become accustomed to on, say, Facebook.
The sort of *problem* we're trying to fix here, of course, is the one I have right now. At 12:26EDT Friday, I bought an ad on FB, to promote an FB event. Apparently, this is Suspicious Behavior; my account was locked out the same minute, and I'm told their "upload us a picture of your government ID" process to unlock it can take *three to 15 days*.
Oh: yes, they accepted the ad, 3 hours later, and they are, presumably, happily charging me for it, though I couldn't log in to cancel it if I wanted to. So, clearly, I encourage thought about how to get *around* this category of problem, as unfruitful as I think it might be...