## 2 Overview
### IoT
#### 2.1.3 Challenges  
Although IoT has numerous benefits, it introduced many challenges that must be addressed, such as interoperability, scalability, heterogeneity, security, and privacy [35]. Many researchers have proposed various measures to enhance interoperability [36; 37; 38; 39; 40]. Interoperability describes the capacity of a system component's technical requirements to work together effectively, regardless of how different they are. Scalability is introduced due to the fact that IoT is facing a tremendous issue in dealing with the rapid growth in the number of devices. It describes the system's ability to handle future growth without negatively impacting its performance. Hence, when more devices are connected, scalability must be examined to see how the system can handle it. Refs. [41; 42; 43] are examples of studies on the scalability issue. Since the IoT network consists of a huge number of devices, it is a prominent illustration of the heterogeneity issue. When it comes to IoT, the primary goal is to provide a standard abstraction approach and maximize the functionality of connected devices. Due to the rapid expansion of IoT, there is a wide range of hardware and software configurations that the developers are striving to create an application that can work on top of them. Some examples of prior work to address the issue of heterogeneity are provided in [44]. New security vulnerabilities of system-of-systems appear due to the growth of IoT, which are caused by heterogeneity, decentralization, and individual vulnerabilities of IoT systems [45; 46]. The complexity of deploying security mechanisms in resource-constrained IoT networks [47] resulted in difficulty in implementing traditional security techniques such as encryption, authentication, and authorization which might not be appropriate anymore. Furthermore, a complex cyber-physical system-of-systems may require autonomous approaches to handle security and safety issues [48; 49]. Additionally, IoT devices are susceptible to malware activity due to the inability of security firmware to be updated on a timely basis [50]. In addition to security, it is difficult to maintain data privacy. There is a growing tendency to combine IoT with cloud computing, which provides IoT with additional storage power and computing abilities. However, data may be compromised if uploaded to third-party cloud servers, which are prone to privacy breaches [51]. Security and privacy aspects are the main focus of this paper because IoT has many issues within this area, and the research on integrating blockchain technology with IoT ismainly conducted to enhance these solution aspects. Managing security and privacy risks should be a top goal for increasing consumer acceptance of IoT applications. In addition, as IoT devices and related apps grow increasingly common in people's daily lives, they must be completely secure. Security and privacy aspects in IoT may raise serious concerns due to a lack of proper authentication and authorization procedures. IoT protocols operate at different layers, which are a favorite target for hackers who always strive to identify new methods to intercept IoT connections even when proper authentication tools are used [28]. For example, possible attacks on each protocol in a specific layer include slowloris, cross-site scripting, HTTP flooding, DDoS, and repudiation attacks that target the application layer. The data processing layer is targeted with exhaustion attacks and targeted malware. The networking layer is further vulnerable to injection, smurf, SYN flooding, opt-ack, Sybil, sinkhole, wormhole, and other attacks. Further resource consumption, byzantine, and IP address spoofing attacks are in sight regarding the actual blockchain network. Finally, there physical damage or destruction, access control, and the disconnection of physical links are attacks toward the sensors and actuators layer [52]. Some attacks mainly target IoT layers based on the system, such as Wireless Sensor Networks (WSN) and Radio Frequency Identification (RFID).