Imagine a user A who wishes to download an mp3 file with title T. Server B has the file. Our goal is to design a system that enables A to download the file from B so that no single network entity knows the identity of both A and B. At your disposal is a mixed network (e.g. TOR) as well as a directory service.
Note: you need not hide the fact that someone downloaded a copy of title T. In particular, the directory service will know that a download of T took place. We are only protecting the identity of the end points.

describe how B can register anonymously at the directory service (letting the service know that it has title T) such that A can use the directory service to obtain the file from B. Note that the directory service does NOT take part in the file transfer.