We propose a scalable key management scheme for sensor networks consisting of a large-scale random deployment of commodity sensor nodes that are anonymous. The proposed scheme relies on a location-based virtual network infrastructure and is built upon a combinatorial formulation of the group key management problem. Primary features of our scheme include autonomously computing group keys, and dynamically computing, using a simple hash function, the mapping of nodes to group keys. The scheme scales well in the size of the network and supports dynamic setup and management of arbitrary structures of secure group communications in large-scale wireless sensor network.