In this paper, we present two practical ARQ-Based security schemes for Wi-Fi and RFID networks. Our proposed schemes enhance the confidentiality and authenticity functions of these networks, respectively. Both schemes build on the same idea; by exploiting the statistical independence between the multipath fading experienced by the legitimate nodes and potential adversaries, secret keys are established and then are continuously updated. The continuous key update property of both schemes makes them capable of defending against all of the passive eavesdropping attacks and most of the currently-known active attacks against either Wi-Fi or RFID networks. However, each scheme is tailored to best suit the requirements of its respective paradigm. In Wi-Fi networks, we overlay, rather than completely replace, the current Wi-Fi security protocols. Thus, our Wi-Fi scheme can be readily implemented via only minor modifications over the IEEE 802.11 standards. On the other hand, the proposed RFID scheme introduces the first provably secure low cost RFID authentication protocol. The proposed schemes impose a throughput-security tradeoff that is shown, through our analytical and experimental results, to be practically acceptable.