:Wireless Sensor and Actor Networks (WSANs) consist of three types of nodes with different capabilities and functions. Because of these heterogeneities, current network security mechanisms would be hard to satisfy the security/cost requirements of all types of nodes. In this paper, we propose an integrated security approach, called Hybrid Security Scheme (HSS). It splits a WSAN into two layers, the sink-actor layer and the actor-sensor layer, according to nodes' functionalities and capacities. Different security mechanisms are provided to achieve the security services at corresponding levels. In the sink-actor layer, a mix-key cryptography technique is used to prevent most possible attacks, including passive attacks and active attacks. On the other hand, in the actor-sensor layer, HSS presents a light-weight key management method based on symmetric cryptography to reduce the number of keys stored in sensor nodes and simplify the agreement and update process of secret keys. An ad...