In many applications of Wireless Sensor Networks a Sink is interested in aggregated data instead of exact values from all sensors. To send aggregated data, it is also helpful to reduce the amount of data to be transmitted and thereby conserve energy. Indeed current in-network aggregation schemes are helpful to conserve energy but they are designed without considering possible security issues related to data privacy. Often we find wireless sensor networks designed with neighbouring nodes sharing keys or with decryption at aggregator nodes. In either situation the potential for aggregator nodes to be physically compromised means data privacy is at high risk. Therefore secure data aggregation is desirable where data can be aggregated without the need for decryption at aggregator nodes. Aggregation becomes especially challenging if end-to-end privacy between a source and a destination (sink or group leader) is required. In this paper we extend our Structure and Density Independent Group Based Key Management Protocol with the additional feature of secure data aggregation to provide better data privacy to every single node in a large scale Wireless Sensor Network.