Sharing resource blocks in NOMA systems provides more opportunity to the internal users to overhear the messages of the other users. Therefore, some sort of secrecy against the internal users in addition to the external eavesdroppers must be provided. In this paper, we investigate the secrecy performance of a two-user NOMA system in existence of the external and internal passive eavesdroppers, where the far user acts as an internal eavesdropper and tries to overhear the message of the near user. Our system consists of a single antenna base station, two legitimate users and an external passive eavesdropper. We present the closed-forms for the ergodic secrecy rates of the users. Moreover, to derive the secrecy outage probability (SOP) of the system, we use Gaussian-Chebyshev quadrature method, which gives an approximation for the SOP. Numerical results show that this approximation is very close to the exact value of the SOP of the system. Finally, we eliminate the external eavesdropper and present the closed-forms for the ergodic rate of the far user, the ergodic secrecy rate of the near user and also the SOP of the system.