# Cybersecurity for Blockchain-Based IoT Systems: A Review
## 1 Introduction  
The Internet of Things (IoT) refers to the network of interconnected devices, sensors, and systems that communicate and exchange data to enable various applications and services [1]. The IoT has gained significant momentum in recent years and is being widely adopted across diverse domains, including smart homes, smart cities, healthcare, transportation, industrial automation, and agriculture, among others [2, 3]. However, the rapid proliferation of IoT devices has also raised concerns about their security and privacy [4]. IoT devices are often resource-constrained, with limited computational capabilities and storage capacities, making them vulnerable to security threats, such as unauthorized access, data breaches, tampering, and malware attacks [5, 6]. Additionally, the centralized architecture of many IoT systems presents a single point of failure, making them susceptible to systemic risks [7]. Blockchain technology, introduced by [PERSON] in 2008 as the underlying technology of Bitcoin, has emerged as a potential solution to enhance the security and privacy of IoT systems [5]. Blockchain is a distributed ledger technology that allows for secure, transparent, and tamper-proof transactions [3]. By leveraging blockchain technology, IoT systems can benefit from enhanced security and privacy [8]. Blockchain offers several advantages that contribute to the security of IoT systems:  
* Decentralization: Blockchain is a decentralized technology, which means that it is not controlled by any single entity [9]. This makes it difficult for hackers to target and attack. In a centralized system, all data is stored on a single server, making it a singlepoint of failure. In a decentralized system, data is stored on multiple nodes, which makes it more difficult for hackers to gain access to all of the data. * Transparency: Blockchain is a transparent technology, which means that all transactions are recorded on the blockchain and are visible to all participants [10]. This makes it difficult for hackers to commit fraud or tamper with data. In a centralized system, transactions are not always transparent, making it difficult to track down fraud or identify those responsible for tampering with data. * Consensus Mechanisms: Blockchain uses consensus mechanisms to validate transactions. Consensus mechanisms are algorithms that ensure that all participants in the network agree on the validity of a transaction [11]. This makes it difficult for hackers to manipulate the network and approve fraudulent transactions. In a centralized system, a single entity is responsible for validating transactions, making it easier for hackers to manipulate the system. * Cryptographic Techniques: Blockchain uses cryptographic techniques to secure data. Cryptographic techniques are algorithms that encrypt data and make it unreadable to unauthorized users [12]. This makes it difficult for hackers to steal data from the blockchain. In a centralized system, data is often stored in plain text, making it easier for hackers to steal data. * Smart Contracts: Blockchain can be used to create smart contracts, which are self-executing contracts stored on the blockchain. Smart contracts are tamper-proof and irreversible, enabling automation and enforcement of agreements between parties [13]. This can improve the efficiency and security of IoT systems. By incorporating these security features, blockchain-based IoT systems can provide a robust framework for ensuring the integrity, privacy, and reliability of IoT data and transactions. In the context of blockchain-based IoT systems, several layers can be identified to understand their architecture and security aspects [14]. Figure 1 illustrates these layers associated with IoT systems, as follows:  
* Device Layer: This layer consists of the IoT devices themselves, such as sensors, actuators, and embedded systems. These devices collect and generate data, which is then transmitted to the blockchain network for processing and storage.