Random key-assignment for secure Wireless Sensor Networks

A distributed Wireless Sensor Network (WSN) is a collection of n sensors with limited hardware resources. Sensors can exchange messages via Radio Frequency (RF), whose range usually covers only a limited number of other sensors. An interesting problem is how to implement secure pair-wise communications among any pair of sensors in a WSN. A WSN requires completely distributed solutions which are particularly challenging due to the limited resources and the size of the network. Moreover, WSNs can be subject to several security threats, including the physical compromising of a sensor. Hence, any solution for secure pairwise communications should tolerate the collusion of a set of corrupted sensors. This paper describes a probabilistic model and two protocols to establish a secure pair-wise communication channel between any pair of sensors in the WSN, by assigning a small set of random keys to each sensor. We build, based on the first Direct Protocol, a second Co-operative Protocol. The Co-operative Protocol is adaptive: its security properties can be dynamically changed during the life-time of the WSN. Both protocols also guarantee implicit and probabilistic mutual authentication without any additional overhead and without the presence of a base station. The performance of the Direct Protocol is analytically characterized while, for the Co-operative Protocol, we provide both analytical evaluations and extensive simulations. For example, the results show that, assuming each sensor stores 120 keys, in a WSN composed of 1024 sensors with 32 corrupted sensors the probability of a channel corruption is negligible in the case of the Co-operative Protocol.