  We derive a formal connection between quantum data hiding and quantum
privacy, confirming the intuition behind the construction of bound entangled
states from which secret bits can be extracted. We present three main results.
First, we show how to simplify the class of private states and related states
via reversible local operation and one-way communication. Second, we obtain a
bound on the one-way distillable entanglement of private states in terms of
restricted relative entropy measures, which is tight in many cases and shows
that protocols for one-way distillation of key out of states with low
distillable entanglement lead to the distillation of data hiding states. Third,
we consider the problem of extending the distance of quantum key distribution
with help of intermediate stations. In analogy to the quantum repeater, this
paradigm has been called the quantum key repeater. We show that when extending
private states with one-way communication, the resulting rate is bounded by the
one-way distillable entanglement. In order to swap perfect secrecy it is thus
essentially optimal to use entanglement swapping.
