  We investigate how a classical private key can be used by two players,
connected by an insecure one-way quantum channel, to perform private
communication of quantum information. In particular we show that in order to
transmit n qubits privately, 2n bits of shared private key are necessary and
sufficient. This result may be viewed as the quantum analogue of the classical
one-time pad encryption scheme. From the point of view of the eavesdropper,
this encryption process can be seen as a randomization of the original state.
We thus also obtain strict bounds on the amount of entropy necessary for
randomizing n qubits.
