Nowadays, smart cities are becoming an emerging area of research for upgrading and modifying our existing society by adopting the latest and the most trending technologies in the market. Though the number of IoT based applications is constantly increasing, with new products being launched every 6 months, many organizations are afraid of an early adoption of such products because of their security issues. In particular, the transmission and storage of online information causes a lot of cybersecurity issues while ensuring a secure communication mechanism. The aim of this paper is thus to present an efficient and effective communicating mechanism for smart cities using two decision-making models based on the SMART and Subjective approaches. The SMART approach is used to make an intelligent and ideal decision when communicating in the network. In addition, the continuous surveillance of the communicating entities can be done by computing their trust values through a subjective mechanism. The devices having a higher trust value are thus considered as more trustworthy devices. The proposed mechanism is simulated and verified for various security metrics, being compared to the state-of-art approaches. In addition, the proposed mechanism is simulated and out-performed against existing approaches by showing a 97% improvement in terms of accuracy, utility value, delay and threat metrics.