## 4 Findings
### Generic Approaches  
Multiple studies focused on proposing generic solutions to address certain features within IoT. An Ethereum-based approach was proposed to address layer-wise security issues and device authentication in IoT applications [91]. This work focused on specific aspects, including eliminating the idea of localizing authorization and authentication inside the IoT network and eliminating latency issues on the IoT network. Blockchain operation is only required when adding a new user or device is needed. Blockchain is the only entity that allows creating permissions regarding the scaling of IoT networks. In [92], the authors create a safe, fine-grained access control strategy for users, devices, and data. Then utilize smart contracts to implement the strategy. System design includes an access control strategy, user registration and authorization, device's safe insert, database, smart contract design, and transaction design. The access control strategy consists of three tables defining the access rights: the user access table, device resource table, and role table. The device's safe insert ensures that the device's hardware includes embedded identity information. The database scheme stores data collected by devices and data related to the behavior of the user, device, and gateway. Three types of smart contracts are designed in this scheme: user access, device insert, and log. The design of transactions describes the details that must be encapsulated in the message to be sent. Another approach focused on proposing a mechanism to address the access control management of how users deal with their data [93]. This model consists of the main blockchain network, off-chain storage, aggregators (publish), subscribers (subscribe), and vendors. Aggregators publish data and define how third-party can access the data. Third-party, known as subscribers, can access that data through transactions. Off-chain storage stores the data published by aggregators through a scheme called content-based addressing. Manufacturers of IoT devices are known as vendors, and they are responsible for distributing official firmware. In order to manage access permissions and update new firmware, the blockchain network is equipped with two smart contracts: FirmwareUpdate and AccessControl. The PTAS scheme [94] is proposed to ensure privacy using private information retrieval and security using (m-1)-private PTAS to protect against a collision of network nodes. This scheme allows thin clients to function normally as full-node users by hiding user identity in k indistinguishable identities. Security and functional comparisons are conducted to highlight this scheme's high level of security and comprehensive functionality compared to other schemes. However, PTAS improves safety while sacrificing little efficiency. IoTChain [95] is a scheme proposed to protect the security of IoT information based on blockchain technology characteristics and the AES encryption algorithm. The large-scale secure storage of IoT information data can be provided by IoTChain, which can authenticate and grant access to authorized users. As a result, the researchers in this study proposed efficient and secure authentication, privacy protection, and multi-signature conditional traceability solutions based on blockchain technology. Spacechain [96] is a blockchain architecture with a three-dimensional ledger that deals with the scalability and heterogeneity of IoT networks. They also proposed a consensus algorithm called 3D-GHOST to improve network performance and security. Macro-blocks are used to create Directed Acyclic Graph (DAG) to provide the system with the third dimension aspect. DAG consists of a vertex, edge, ack-edge, and ref-edge, which illustrates the operation of this foundation. In the data structure design, the validation process occurs in three steps: consensus algorithm validation and verification using PoW, header_hash validation, and timestamp validation.