||Wireless sensor networks (WSNs) is an infrastructure comprised of measuring, computing and communication elements that gives an administrator the ability to instrument, observe and react to events. We present a new encryption mode of operation that allows nodes of a network to exchange messages securely without sharing a common key or using public key cryptography. We show that our proposal can be used in wireless sensor networks to send encrypted packets to very dynamic sets of nodes without having to establish and maintain group keys. The Security of the scheme is based on the indistinguishability property of Pseudo Random Function (PRF), a standard cryptographic primitive. We show that aggregation based on this scheme can be used to efficiently compute statistical values, such as mean, variance, and stand deviation of sensed data, while achieving significant bandwidth savings. As a result, a node can send encrypted data to all the nodes within a given geographical area, without having to identify the destination nodes in advance.