Efforts towards the realization of smart cities are gaining increasing attention to improve the efficiency and comfort of social activities. The smart cities are built on the IoT systems that gather and analyze various sensor data from different parts of the city. However, the wireless sensor networks that are essential for constructing the IoT systems have various vulnerabilities (e.g., data tampering, eavesdropping) to the integrity of the sensor data due to the openness of wireless communication. Our previous study proposes a new method to ensure the integrity of the sensor data by enabling nodes participating in different sensor networks to collaborate with the adjacent nodes to verify the data. However, when multiple sensor nodes from different sensor networks are collaborating, there is a risk of data being intercepted during communication between nodes. Therefore, in this study, we propose a method for achieving highly reliable sensor data exchange by applying encryption and digital signatures to data shared among nodes in heterogeneous sensor networks. Specifically, we propose an algorithm for mutual public key exchange among different wireless sensor networks to establish a reliable communication based on the public-key encryption method.