  We consider the problem of secure key distribution among $n$ trustful agents:
the goal is to distribute an identical random bit-string among the $n$ agents
over a noisy channel such that eavesdroppers learn little about it. We study
the general situation where the only resources required are secure bipartite
key distribution and authenticated classical communication. Accordingly,
multipartite quantum key distribution can be proven unconditionally secure by
reducing the problem to the biparitite case and invoking the proof of security
of bipartite quantum key distribution.
