  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. First we 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. We
show that we only need to enhance the channels of the legitimate receivers, and
the channel of the eavesdropper remains unchanged. Then we extend the result of
the degraded case to non-degraded case. We show that the secret superposition
of Gaussian codes along with successive decoding cannot work when the channels
are not degraded. we develop a Secret Dirty Paper Coding (SDPC) scheme and show
that SDPC is optimal for this channel. Finally, we investigate practical
characterizations for the specific scenario in which the transmitter and the
eavesdropper have multiple antennas, while both intended receivers have a
single antenna. We characterize the secrecy capacity region in terms of
generalized eigenvalues of the receivers channel and the eavesdropper channel.
We refer to this configuration as the MISOME case. In high SNR we show that the
capacity region is a convex closure of two rectangular regions.
