Abstract-Providing forward and backward secrecy is still a big challenge in Unattended Wireless Sensor Networks (UWSNs), though some storage schemes have been proposed. Additionally, high storage requirement needs efficient storage techniques. In this paper, we propose a novel homomorphic encryption and key-evolution based scheme for efficient and secure data storage, which supports both forward and backward secrecy in UWSNs. We show that the stored data based on our scheme can be used to efficiently compute statistic values, e.g., expected value and variance of the sensed data, and at the same time the storage cost is significantly reduced using our scheme. Detailed analysis has been conducted to evaluate the scheme in terms of efficiency and security.