## 1 Introduction  
Blockchain is a distributed database which provides decentralization and immutability of the transactions or records in a peer-to-peer network. On the other hand, IoT consists of physical devices that are connected to the internet to collect and share data. Combination of blockchain and IoT i.e. blockchain of IoT (BCIoT) will be beneficial to the world in terms of decentralization and immutability but challenges are also present with opportunities. The volume of data collected is currently tremendous and grows due to the growing number of IoT devices. Large number of IoT devices and data platforms open the door to new applications and use cases. However, IoT data security is a major concern that has slowed the technology's adoption as they are a prime target for a range of attacks. Scalability is another issue with today's IoT networks. When the number of devices linked through an IoT network rises, current centralized techniques for authenticating and linking sensor nodes in a network will become a bottleneck. If the server that handles the vast volume of data exchange goes down, the entire network can go down. Blockchain is considered to be a game-changing technology that has the potential to address IoT security and scalability concerns [1, 2]. Blockchain is a distributed ledger technology that can be used to distribute and access data in a secure and decentralized manner [3]. Every transaction may be authenticated to avoid conflicts and build trust among all network participants. However, the integration of IoT and blockchain gives rise to new challenges (Figure 1), such as scalability, big data of IoT devices, security and privacy that need to be resolved [4]. It is also necessary for blockchain and IoT to implement an encryption algorithm but both ecosystems have different computing capabilities; so, processing time and power [5] could be a great difficulty. Blockchain provides decentralization, stores transaction on every node's ledger and its size is continuously increasing with time. IoT devices have very low storage capacity [6], as this is beyond the capabilities of sensors. IoT is an open nature network and so in BCIoT, security and privacy could be a potential threat. An efficient authentication scheme by message authentication for Internet of Vehicles (EASSAIV) is proposed in [7] to resolve security and privacy concerns. In blockchain, the consensus is a process of achieving agreement on a single value in a distributed computing environment. The two most popular consensus algorithms are proof of work (PoW) and proof of stake (PoS). However, these algorithms cannot be directly used for IoT scenarios due to their scalability issues [8]. Lightweight consensus algorithms are required for IoT to adopt blockchain because current blockchain platforms such as Bitcoin and Ethereum blockchains are computationally expensive, having high bandwidth overheads and delays for IoT [9]. So, it must be ensured to solve this issue by making a suitable consensus protocol by consulting IoT problems, such as lack of security, different standards of devices, less memory of devices, and a large amount of data. Security and privacy is major concern of the IoT field but if blockchain is involved, then it has to be addressed. A novel blockchain-based approach reported in [10] is used for IoT and suggested that distributed blockchain can solve security concerns for IoT. A decentralized identity management system based on blockchain was presented by [11] which provides security and privacy of the patients for remote healthcare. An idea proposed in [12] is to preserve privacy using blockchain for medical IoT. The storage issue was addressed in [13] for industrial IoT by providing hierarchical blockchain storage structure (ChainSplitter) in which most of the blockchain is stored on the clouds. Blockchain of IoT needs lightweight algorithms to overcome the power and processing time challenges. A lightweight consensus  
Figure 1: Blockchain IoT challenges.