# HPoC: A Lightweight Blockchain Consensus Design for the IoT
## 2 Background and Related Work  
### Applying Blockchain to Solve Security Problems in the IoT  
The IoT has received greater attention and development in recent years than any other industry, due to its timeliness, interoperability, simplicity, and scalability, but the solution to its security challenges has not kept pace. By 2021, the cost of dealing with IoT information-security vulnerabilities rose to USD 119.9 billion. The features of blockchain technology, such as decentralization, self-organization, consensus mechanisms, smart contracts, and cryptography, are a natural fit with the IoT. In recent years, there has been a great deal of research to embed blockchain into the IoT to solve its information security issues. In general, blockchain can be applied in the IoT field to data privacy [19], data integrity, trusted accountability [20], trusted data-origin access control [21; 22], and network information sharing [23]. As a step closer to applying blockchain to the IoT, some scholars have investigated how to design lightweight blockchain adaptively embedded into IoT edge devices. Ref. [24] combined two design algorithms, LSB and FogBus, then proposed a hierarchical and domain-specific design idea to reduce latency and resist attacks, adopting the leader idea of using a leader to process, verify, and distribute transactions. The leader node was named as a broker, but there was no detailed discussion on how a node becomes a broker; it was simply assumed that those who are capable can act as a broker. Ref. [25] used a lightweight encryption algorithm and stored medical privacy data in the cloud through a combination of symmetric and asymmetric encryption to provide data security and privacy for remote-patient-monitoring systems in the IoT attributes, but there was no discussion of the application of blockchain itself in this IoT environment. Ref. [26] provided an idea of optimizing the storage volume from the ledger structure to reduce the storage pressure of common node injuries. Accordingly, research on lightweight blockchain is still in its infancy. The available work is limited, but can be broadly divided into the following three points:  
1. Partitioning or sub-domaining a blockchain to form numerous chains in parallel, with the ability to interact between parallel blockchains. 2. Separating the on-chain and off-chain storage of blockchain data to relieve the data pressure on the blockchain, or partitioning the data storage in Merkle Tree style and requesting data from other nodes when verification is required. 3. Dividing the roles of nodes in the blockchain and nominating more capable nodes to be responsible for more computation and storage work, so as to avoid all nodes participating in the block-generation or computation work. In short, the existing works describe the lightness of a blockchain in a broader sense, as if it were a plug-and-play module in the context of IoT applications. If an application is more granular, different blockchain types are discussed, such as using a public chain, a federated chain, or a private chain. Blockchain systems that are tailor-made for IoT edge-computing devices are unusual for this type of work. Instead of considering a lightweight concept in the true sense, some researchers believe that combining blockchain technology with the Internet of Things could simplify some of the security-authentication processes, thus achieving the goal of lightweight device security. In this work, we argue that running blockchain on IoT nodes will raise the computational load on these resource-constrained edge devices, especially when employing PoW and other consensus techniques that only involve mining, which is counter to the goal of lightweight devices. As a result, lightweight features should be used to describe the nature of blockchain, such as the time consumed by the blockchain consensus process and the data storage load in the blockchain.