  This paper investigates the utilization of simultaneously transmitting and
reflecting RIS (STAR-RIS) in supporting joint physical layer security (PLS) and
covert communications (CCs) in a multi-antenna millimeter wave (mmWave) system,
where the base station (BS) communicates with both covert and security users
while defeating eavesdropping by wardens with the help of a STAR-RIS.
Specifically, analytical derivations are performed to obtain the closed-form
expression of warden's minimum detection error probability (DEP). Furthermore,
the asymptotic result of the minimum DEP and the lower bound of the secure
rates are derived, considering the practical assumption that BS only knows the
statistical channel state information (CSI) between STAR-RIS and the wardens.
Subsequently, an optimization problem is formulated with the aim of maximizing
the average sum of the covert rate and the minimum secure rate while ensuring
the covert requirement and quality of service (QoS) for legal users by jointly
optimizing the active and passive beamformers. Due to the strong coupling among
variables, an iterative algorithm based on the alternating strategy and the
semi-definite relaxation (SDR) method is proposed to solve the non-convex
optimization problem. Simulation results indicate that the performance of the
proposed STAR-RIS-assisted scheme greatly surpasses that of the conventional
RIS scheme, which validates the superiority of STAR-RIS in simultaneously
implementing PLS and CCs.
