  In this paper, we investigate how an illegitimate user can use a
reconfigurable intelligent surface (RIS) to maximally degrade the secrecy
performance of the legitimate system. We first consider the case when the
illegitimate user performs eavesdropping only. A problem is formulated to
minimize the system secrecy rate by optimizing the phase shifts of the RIS
elements. We also consider the case when the illegitimate user works with
full-duplex technology and can perform eavesdropping and jamming simultaneously
through the RIS. A problem is formulated to minimize the system secrecy rate by
optimizing the jamming power and the phase shifts of the RIS elements. Although
the two formulated problems are nonconvex, they are optimally solved in this
paper. Our results show that good eavesdropping performance can be achieved by
placing the RIS at an appropriate position. Further, jamming plus eavesdropping
can largely reduce the secrecy rate of the legitimate system.
