We propose a key management scheme for two-tire hierarchical mobile network using triple key distribution. Sensor nodes are divided into clusters managed by cluster heads which collect and aggregate data and send to base stations. The use of both symmetric and public key techniques makes the network highly resilient to node compromise. The nodes are resource constrained and thus communicate securely using symmetric triple key distribution technique. The cluster heads are powerful and use asymmetric key distribution techniques. The anonymity of cluster heads while forwarding messages is also preserved in our protocol. To the best of our knowledge, this is the first key management scheme which uses both symmetric and asymmetric triple key distribution mechanism. The analysis shows that our scheme is highly secure, preserves privacy of cluster heads and can support efficient addition and deletion of nodes.