This paper investigates the physical layer security (PLS) of reconfigurable intelligent surface (RIS) aided wireless powered communication (WPC) system, where an energy-limited user first harvests energy from a hybrid access point (HAP) and then uses the harvested energy to transmit information in the presence of a passive eavesdropper. In particular, four phase shifting designs at RIS are considered, i.e., imperfect continuous phase shifting, perfect continuous phase shifting, imperfect discrete phase shifting, and perfect discrete phase shifting. To reveal the impact of four phase shifting designs on the secrecy performance of system, we analyze the ergodic secrecy capacity (ESC) and derive new closed-form expressions. Then, the asymptotic ESC is derived to gain useful insights. Both theoretical analysis and simulation results show that the imperfect continuous phase shifting has the same ESC performance with the imperfect discrete phase shifting, while the ESC of perfect continuous phase shifting is similar to that of perfect discrete phase shifting. Besides, the ESC firstly increases and then decreases with the increasing time switching factor, and each phase shifting design has a different optimal time switching factor. Moreover, RIS aided WPC system under four phase shifting designs outperforms the traditional WPC system without the aid of RIS under high transmission power, verifying the superiority of using RIS to enhance the secrecy performance.
