Efficient hierarchical key generation and key diffusion for sensor networks

Sensor networks are designed with the assumption that nodes are willing to collaborate. However, the open collabo- ration of nodes introduces privacy and security issues. Therefore, ensuring privacy in wireless sensor networks is a challenging task. Based on a multilevel security paradigm, in this paper we present a hierarchical key generation and distribution protocol for wireless sensor networks. We show by simulation results that our key generation scheme outperforms the existing hierarchical key generation schemes thus it is suitable for sensor networks with limited computation and energy capabilities. Furthermore, we present an energy efficient key diffusion protocol. We also discuss the possible security threats involved with the proposed protocol and provide suitable solutions to such threats. I. INTRODUCTION Sensor networks are characterized by a large number of wireless nodes distributed over a region for sensing and mon- itoring purposes. One of the main assumptions when designing sensor networks is that nodes are willing to collaborate with one another to process, aggregate and forward the collected data. However, the data collected by sensors varies in security, sensitivity and importance levels, thus the collaboration of nodes introduces several security and privacy challenges (1). For example, sensors located close to military locations pro- duce highly sensitive data while sensors located at less critical locations produce less sensitive data. The open collaboration among these sensors can cause privacy violations. Therefore, security techniques should be in place to prevent security breaches by managing the collaboration among the sensor nodes. In order to address this problem, we propose an approach based on a multi-level security model (2). The proposed approach assigns each sensor an access class from a partially ordered set of access classes, thus resulting in a hierarchical organization of sensors. The security class of each sensor is decided based on the contextual information of both the sensor and the collected data. The hierarchical access structure represents the sensor network security policy and controls the data flow among the sensor nodes. To implement the proposed approach, several issues have to be addressed. An important requirement is that hierarchical keys should be used for data encryption. This allows nodes at high access classes to decrypt data sent by lower access classes. The generation and distribution of the hierarchical key should be suitable for the sensor network environment. Other issues include the design of special data retrieval, data fusion and querying techniques that adhere to the hierarchical access control paradigm to prevent security and privacy breaches. In this paper, we propose a hierarchical key generation and distribution scheme that addresses the sensor network computational, energy and power limitations. The proposed key generation algorithm offers a novel, scalable and efficient technique for establishing hierarchical keys for sensor net- works with low computational and energy requirements. The proposed algorithm is based on low cost hashing functions that enable the efficient key generation. Moreover, we pro- pose an optimized version of the hierarchical key generation algorithm for tree shaped hierarchies. Consequently, a hybrid technique can be used to further optimize key generation for any hierarchical structure. We also propose a hierarchical key distribution protocol that enables the diffusion of the hierarchical keys in an efficient and systematic manner. The technique allows the diffusion of keys from high access levels to lower access levels and ensures the secure dissemination of the hierarchical keys in the network. Moreover, the structures generated for key distribution are used for data collection, data fusion and query propagation The rest of the paper is organized as follows. In Sections II and III we discuss the related work and the preliminaries. Our hierarchical key generation algorithm is discussed in Section IV. The optimized key generation technique for tree hierarchical structures and the hybrid technique for general hierarchical structures are presented in Section V. We then show simulation results comparing several key generation techniques to our proposed technique in Section VI. Our hierarchical key diffusion protocol is presented in Section VII. In Section VIII we discuss possible security threats and present suitable solutions to such threats. Concluding remarks and future work are added in Section IX.