  There have been recently many studies demonstrating that the performance of
wireless communication systems can be significantly improved by a
reconfigurable intelligent surface (RIS), which is an attractive technology due
to its low power requirement and low complexity. This paper presents a
measurement-based characterization of RISs for providing physical layer
security, where the transmitter (Alice), the intended user (Bob), and the
eavesdropper (Eve) are deployed in an indoor environment. Each user is equipped
with a software-defined radio connected to a horn antenna. The phase shifts of
reflecting elements are software controlled to collaboratively determine the
amount of received signal power at the locations of Bob and Eve in such a way
that the secrecy capacity is aimed to be maximized. An iterative method is
utilized to configure a Greenerwave RIS prototype consisting of 76 passive
reflecting elements. Computer simulation and measurement results demonstrate
that an RIS can be an effective tool to significantly increase the secrecy
capacity between Bob and Eve.
