A network authentication system comprises user equipment (UE), a service network (SN) and a home network (HN). The HN generates an expected user response (XRES) based on an identifier of the UE and generate an indicator, and sends the part of XRES and the indicator to the SN. The SN receives the part of XRES and indicator, and receives a user response (RES) from the UE. The SN then compares the RES with the XRES base on the indicator, and sends a confirmation message to the HN when the comparison succeeds.