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Top1. Introduction
Sensor based Internet of Things (IoT) have various applications such as healthcare applications, battlefield monitoring, street monitoring, disaster management, military, forest fire detection, unmanned vehicles and manufacturing industries (Ahuja et al. 2020; Rashid et al. 2016). Such IoT applications generate a huge amount of data that is usually stored at cloud to increase the usefulness of resources (Lin et al. 2019; Zhou et al. 2018). Sensor networks are integrated with cloud to improve the effectiveness of the application. This integration is termed as sensor cloud which is beneficial for both sensor networks and cloud. Various sensor networks store their sensed data at the cloud and cloud provides sensor as a service with help of virtualization to the multiple users according to their choice and demand. Any genuine end user can access the data of one or all authorized sensor networks just in one click with help of this integration (Dwivedi et al. 2018). Figure 1 presents a healthcare monitoring system where each human body behaves as a sensor network. Here, data from various wearable body sensors of many patients have been stored at cloud through base station such as mobile phone. Different types of authorized users viz., doctors, medical students, researchers can access the health records of the patients using their credentials. Doctors can provide medical support to the patients anytime and from anywhere. They can help the patients instantly if the emergency case is monitored. Cloud can provide sensor as a service to the authorized students and researchers too by providing them various types of data. Thus, legitimate end users can get data of one or more patients easily and quickly. Doctors, students, researchers and patients may belong to either same or different hospitals. In this way, everyone is benefitted with this sensor cloud integration.
Figure 1. Healthcare monitoring sensor cloud
There are several challenges to sensor cloud and security is one of them (Altaf et al. 2019; Díaz et al. 2016). Various security mechanisms have been devised to provide data security at the cloud (Park et al. 2011; Fernández-Alemán et al. 2013; Sangeetha et al.2017; Masood et al. 2018; Sun et al. 2018; Dwivedi et al. 2019). Some of them are less complex but have coarse grained access control. On the other hand, some schemes provide fine grained access control but they have some computational overheads. Hence, there is a need to provide an improved security mechanism having fine grained access control with reduced computational complexities. This paper focuses its work on sensor cloud of healthcare monitoring system. There are many medical cases in which the continuous monitoring of health conditions is required which allow doctors to know the health status of the patients regularly or when required (Ghoneim et al. 2018; Liu et al. 2019). Several computations may be involved before fetching the desired information (Goléa et al. 2019; Al-Ayyoub et al. 2018). IoT based healthcare system is very helpful in such situations which minimizes the healthcare treatment cost and allows the mobility of patients (Ko et al. 2010). In such systems, various body sensors are applied at the patients for the purpose of continuous monitoring of their health status. These body sensors are wearable devices worn by patients that can collect various body data such as blood pressure, body temperature and heart beat rate. Data collected by these sensors are sent to gateways via wireless communication medium and from gateways finally transferred to the cloud for storage and processing. Medical data of the patients collected by various body sensors are very crucial. Any alteration or loss in the medical data of the patients may result in negative health conditions or sometimes lead to very serious stages even death of the patient. Hence, it must be secured.