The revolutionary network that totally changed how the humankind life, the Internet, is now evolving into the Internet of things; which is being achieved by fetching the network connectivity to legacy industrial or commercial devices, as well as everyday objects. This connectivity may be an Internet connection, where the Internet protocol and related technologies are in use. But, there are also many constrained devices, like sensors, actuators, radio frequency identification (RFID) tags and smart cards etc., which cannot handle the full-stack. These devices usually form ad-hoc networks, in which the internal data communication is made via lighter protocols, while the Internet connection is provided by anchor/gateway nodes. This structure brings ease of use, saves power and allows development of new technologies. However, privacy and security are under severe threats, much more than ever. In this paper, we propose a key diversification method, inspired from RFID smart cards, to be used in symmetric encryption within these ad-hoc networks. Thus, lightweight devices will be able to communicate securely, without exhausting their processors and wasting energy.