Ensuring key management in Wireless Sensor Networks has a vital importance, especially when sensor nodes have to communicate in hostile environments. In this paper, we propose a new simple and resource-aware key management scheme. The scheme is based on the idea that the initial pre-distributed key is not pre-loaded in all nodes to improve its resilience to node compromising attacks. Each node has to store an initial key, a set of prime number groups and a pseudo-random function. This pre-distributed secret in- formation is used to establish pairwise keys between adja- cent nodes after their deployment. Moreover, the proposed scheme is dynamic since it allows a flexible key refresh.