A wireless sensor network is a collection of spatially distributed sensor nodes for monitoring physical environmental conditions such as pressure, temperature or humidity. To decrease the energy consumption and to increase the lifetime of wireless sensor network a mechanism called data aggregation is introduced. Data aggregation is a process of aggregating the data from different sensor nodes so as to reduce the transmission overhead. However, this data aggregation creates new security challenges. In this paper, we are doing a survey on existing secure data aggregation protocols.