Radio frequency identification (RFID) is a technology for the automated identification of physical entities using radio frequency transmissions. In the past ten years, RFID systems have gained popularity in many applications, such as supply chain management, library systems, e-passports, contactless cards, identification systems, and human implantation. RFID is one of the most promising technologies in the field of ubiquitous and pervasive computing. Many new applications can be created by embedding an object with RFID tags. However, the rapid development of RFID systems raises serious privacy and security concerns that could prevent the benefits of RFID technology from being fully utilized. In this talk, first I will give an overview for the proposed methods in the literature for authentications in RFID systems, then I will present a lightweight WG stream cipher for securing RFID systems, and provide the security analysis and efficient implementation of an instance of WG-8 on microcontroller.