Wireless sensor networks (WSNs) are energy constrained. Data aggregation is an important mechanism for achieving energy efficiency in such networks. The aggregation reduces redundancy in data transmission which results in improved energy usage. Several security issues are there in data aggregation, which includes data confidentiality, data integrity, availability, and freshness. Such issues become complex since WSN is deployed in hostile and unattended environment. Hence, the sensor nodes may fail and compromised by adversaries. Secure data aggregation (SDA) in sensor network is a topic of research. Many solutions are proposed for SDA, using different encryption methods. Homomorphic encryption is one of such techniques. In homomorphic encryption, all the nodes participate in the aggregation. Here, nodes cannot see any intermediate or final result, but the aggregation is efficient. In this paper, SDA methods are classified and the performance is compared in terms of integrity and confidentiality.