This document discusses secure data aggregation in wireless sensor networks. It presents three approaches to secure data aggregation: hop-by-hop encryption, end-to-end encryption, and privacy homomorphism. A general framework is also proposed that uses clustering to perform secure and energy-efficient data aggregation across sensor nodes. The framework applies end-to-end symmetric cryptography using privacy homomorphism to encrypt data before sending it to cluster heads. This helps prevent attackers from accessing plaintext sensor data during transmission and aggregation.