Protecting heirarchies

Hi. One thing I've been thinking about recently is security. I would
like to be able to assign different passwords to different levels of
a heirarchy, and to have the clearances be cumultive:

  <root>   Basic foo:bar
     |
     +----- <directory1>  Basic grok:baz
                |
                +----- <directory2> Basic foo:pax

where to get to directory2 via a URL you'd do something like:

   root/directory1/directory2

What I want is:

   1) To get to directory2, you need all the name+password pairs
   2) That when you move back up, you don't need to reauthenticate

This isn't directly related to HTTP, but is rather a server-side
issue. I was wondering is anyone has done this?