  We illustrate using a quantum system the principle of a cryptographic switch,
in which a third party (Charlie) can control to a continuously varying degree
the amount of information the receiver (Bob) receives, after the sender (Alice)
has sent her information. Suppose Charlie transmits a Bell state to Alice and
Bob. Alice uses dense coding to transmit two bits to Bob. Only if the 2-bit
information corresponding to choice of Bell state is made available by Charlie
to Bob can the latter recover Alice's information. By varying the information
he gives, Charlie can continuously vary the information recovered by Bob. The
performance of the protocol subjected to the squeezed generalized amplitude
damping channel is considered. We also present a number of practical situations
where a cryptographic switch would be of use.
