  Intelligent reflecting surfaces (IRSs) enable multiple-input multiple-output
(MIMO) transmitters to modify the communication channels between the
transmitters and receivers. In the presence of eavesdropping terminals, this
degree of freedom can be used to effectively suppress the information leakage
towards malicious terminals. This leads to significant potential secrecy gains
in IRS-aided MIMO systems. This work exploits these gains via a tractable joint
design of downlink beamformers and IRS phase-shifts. In this respect, we
consider a generic IRS-aided MIMO wiretap setting and invoke fractional
programming and alternating optimization techniques to iteratively find the
beamformers and phase-shifts which maximize the achievable weighted secrecy
sum-rate. Our design concludes two low-complexity algorithms for joint
beamforming and phase-shift tuning. Performance of the proposed algorithms are
numerically evaluated and compared to the benchmark. The results reveal that
integrating IRSs into MIMO systems not only boosts the secrecy performance of
the system, but also improves the robustness against passive eavesdropping.
