  In this paper, we consider a scenario where a source node wishes to broadcast
two confidential messages for two respective receivers via a Gaussian MIMO
broadcast channel. A wire-tapper also receives the transmitted signal via
another MIMO channel. It is assumed that the channels are degraded and the
wire-tapper has the worst channel. We establish the capacity region of this
scenario. Our achievability scheme is a combination of the superposition of
Gaussian codes and randomization within the layers which we will refer to as
Secret Superposition Coding. For the outerbound, we use the notion of enhanced
channel to show that the secret superposition of Gaussian codes is optimal. It
is shown that we only need to enhance the channels of the legitimate receivers,
and the channel of the eavesdropper remains unchanged.
