  We analyse a central broadcast continuous variable quantum key distribution
protocol in which a beam produced by a thermal source is used to create a
secret key between two parties, Alice and Bob. A beam splitter divides the
initial beam into a pair of output beams, which are sent to Alice and Bob, with
Eve intercepting Bob's beam. We investigate the protocol in detail, calculating
mutual informations through a pair of analytic methods and comparing the
results to the outputs of a Monte Carlo simulation of the protocol. In a
lossless system, we find that a lower bound on the key rate remains positive in
the protocol under a beam splitter attack, provided Bob receives a nonzero
proportion of the beam initially sent to him. This suggests that the thermal
state protocol could be used experimentally to produce secure keys.
