# A Lightweight Hash-Based Blockchain Architecture for Industrial IoT
## 1 Introduction  
Blockchain is a technology that can ensure data integrity by consisting of the immutable ledger in distributed network [1; 2]. The data blocks are connected with the hash value of each previous block, and the ledger, including all of the information about the transactions, which is stored in each node in the distributed network. The distributed ledger is updated by making a consensus using defined consensus protocol such as Proof of Work (PoW), Proof of Stake (PoS), Proof of Property (PoP), it helps entire sharing of the same data blocks for all nodes of the network [3]. These features are the key features of the data integrity in the network [4]. Besides, blockchain can be a potential solution for IoT, such as expanding of address space, identify things, governance, data authentication, authorization, and privacy [5]. For these reasons, blockchain-based IoT networks are gaining attention. As the IoT technology extends its area to the various field, many researchers are trying to apply the blockchain-based IoT technology to their interest fields. Notably, in the industrial field, IIoT brings to advantages (e.g., operation automatically, cost reduction, boost productivity, etc.) [6; 7]. Additionally, if the blockchain combines IIoT, it can also serve useful services such as on-demand manufacturing,smart diagnostics, machine maintenance, traceability, supply chain tracking, etc. [8]. As a result, many researchers have shown interest in blockchain-based IIoT. However, the IoT network consists of many heterogeneous devices with resource-constrained environment, and the availability issue on blockchain-based IoT network remains an open challenge [9]. In the industrial field, the availability issue is more critical compared to the general blockchain-based IoT network. The use of IIoT networks in the industrial domain, such as Smart Factory, Industrial Control Systems (ICS), and Supervisory Control And Data Acquisition (SCADA), can cause a disaster or loss of money if they do not initiate their functions at the right time [10; 11]. It means that the availability of the industrial network is the primary concern. To solve this challenge, there are some existing results for constructing the blockchain network more lightly. In these results, either a new architecture of the blockchain-based IoT network was designed, or the block structure and consensus protocol of blockchain was modified. However, the selection of cryptographic algorithms have not been considered deeply in the results. In the blockchain network, the mining process has mainly a performance issue because it requires high computational power for searching a hash value that satisfies the condition of validating. However, IoT devices that usually consist of the resource-constrained devices are compactly designed to perform their specific purpose. Therefore, they do not have sufficient memory, power, for calculating or area for implementing a hardware module, because the device can perform a specific purpose. For this reason, the hash functions which are generally used may cannot be applied or show improper performance. This limitation can make a latency during the mining process in the blockchain. If it is delayed, the updating block process is also delayed. This means that the performance of the hash function can affect the period of updating block, thereby it cannot serve the sufficient availability of the network. In reality, even though the target updating time is \(\sim\)10 min in the case of Bitcoin, there are some cases that the updating block time is \(\sim\)40 min. If the block updating process is delayed in blockchain-based IIoT, it can result in a severe impact. Therefore, the selection of hash algorithms has to be conducted very carefully according to available resources. Recently, many lightweight hash functions have been proposed for use on resource-constrained devices, and it can be a proper solution to select the hash function in the industrial field.