This document proposes a hierarchical key agreement protocol for wireless sensor networks that uses both symmetric and asymmetric cryptographic techniques. Specifically, it uses probabilistic key pre-distribution for sensor nodes within clusters to establish secure communication with low computational overhead. It uses identity-based asymmetric key distribution between cluster heads and the base station to achieve secure communication with low communication overhead. The goal is to balance security, resilience, and resource overhead of the key management protocol for the constrained sensor nodes. It provides mathematical background on bilinear pairings and elliptic curves needed to implement the identity-based cryptographic techniques.