  Wireless communications empowered by Reconfigurable Intelligent
(meta)Surfaces (RISs) are recently gaining remarkable research attention due to
the increased system design flexibility offered by RISs for diverse
functionalities. In this paper, we consider a Multiple Input Multiple Output
(MIMO) physical layer security system with multiple data streams including one
legitimate and one eavesdropping passive RISs, with the former being
transparent to the eavesdropper and the latter's presence being unknown at the
legitimate link. We first focus on the eavesdropping subsystem and present a
joint design framework for the eavesdropper's combining vector and the
reflection coefficients of the eavesdropping RIS. Then, focusing on the secrecy
rate maximization, we propose a physical layer security scheme that jointly
designs the legitimate precoding vector and the Artificial Noise (AN)
covariance matrix, as well as the legitimate combining vector and the
reflection coefficients of the legitimate RIS. Our simulation results reveal
that, in the absence of a legitimate RIS, transceiver spatial filtering and AN
are incapable of offering nonzero secrecy rates, even for eavesdropping RISs
with small numbers of elements. However, when a L-element legitimate RIS is
deployed, confidential communication can be safeguarded against cases with even
more than a 5L-element eavesdropping RIS.
