Besides /keybase/public and /keybase/private, there should be `/keybase/dropbox`

Right now, if you want to share files with someone else you can put files in a private 'drop box' (by this I don't mean something like Dropbox the service, but a 'drop box' for another user) by putting them in /keybase/private/myusername,otherpersonsusername.

However, this can be a bit clunky, both because you'll end up with a bunch of such folders and you if you are sending files back and forth - it can be confusing as to who sent what.

I suggest that there should be a third top-level grouping within KBFS, which would essentially be set up to be a drop-box system. If you put something in /keybase/dropbox/otherpersonsusername it would be effectively the same as /keybase/private/myusername,otherpersonsusername except (a) it is directed at the other person, making it easy to tell that you sent the files and not the other way around, and (b) it would be easier to set up with this specific purpose in mind.

Start by tracing how KBFS resolves the existing /keybase/public and /keybase/private paths. Determine the design and implementation points needed for /keybase/dropbox/otherpersonsusername, then verify that it provides a directed drop-box distinct from the existing private shared folders.