Two attacks on a sensor network key distribution scheme of Cheng and Agrawal

A sensor network key distribution scheme for hierarchical sensor networks was recently proposed by Cheng and Agrawal. A feature of their scheme is that pairwise keys exist between any pair of high-level nodes (which are called cluster heads) and between any (low-level) sensor node and the nearest cluster head. We present two attacks on their scheme. The first attack can be applied for certain parameter sets. If it is applicable, then this attack can result in the compromise of most if not all of the sensor node keys after a small number of cluster heads are compromised. The second attack can always be applied, though it is weaker.