## 2 Preliminaries
### Blockchain  
Blockchain is a continuously expanding sequence of blocks interconnected through cryptographic techniques [61]. The structure of a blockchain network is depicted as an organized chain of blocks, as illustrated in Figure 5, where each block references its predecessor, forming a linked chain. Once a block is created and added to the blockchain, the transactions within that block become immutable and irreversible.  
Figure 5: Nodes on blockchain network.  
At the heart of blockchain lies its consensus mechanism, which ensures that all consensus nodes in the network agree on a unified global state of the blockchain. A blockchain network generally comprises data producers, consensus nodes, and a data pool. When data producers wish to record information on the blockchain, they first submit it to the data pool, as depicted in Figure 6. Consensus nodes then retrieve this data from the pool, verify its authenticity, and execute the consensus protocol to designate a bookkeeping node. The bookkeeping node is tasked with writing the verified data onto the blockchain, ensuring its integrity and consistency.  
The character of blockchain can be as follows:  
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* Decentralization: Blockchain operates without a central authority. Any participant node can validate transactions via the proof of work (PoW) mechanism, avoiding bottlenecks and single points of failure.
* Anonymity: Participants use public/private keys, concealing their real identities.
* Autonomy: Nodes communicate directly without server involvement.
* Security: Smart contracts ensure secure communication, preventing unauthorized access and loss of data by encrypting data with public/private keys.
* Non-repudiation: Transactions, once validated and added to the chain, are irreversible and easily verified.
* Resiliency: Each node maintains a copy of the ledger, eliminating single points of failure.
* Smart Contracts: Self-executing programs define agreements among participants, ensuring efficiency, transparency, and trust. Each contract is uniquely addressed on the chain and invoked via transactions.
* Auditing: Blockchain records ownership changes, updates, and maintenance details, ensuring traceability throughout a device's lifecycle.
* Immutability: Once verified, transactions cannot be altered, ensuring data integrity and accountability.
* Capacity: Blockchain leverages the resources of participant devices, storing data securely until needed.
* Cryptography: Advanced algorithms ensure data privacy and prevent unauthorized access, even if nodes gain fake chain access.
* Publicity: All devices have a ledger copy, enabling visibility into transactions while protecting content via private keys.
* Speed: Transactions are validated and distributed across the network in minutes.
* Cost-Efficiency: Decentralized architecture reduces costs by sharing resources for storing and transmitting large datasets.  
Figure 6: Operational process of a blockchain network.  
The blockchain is classified, as shown in Table 3, and its applications vary across different domains. By distributing data across a network of nodes, blockchain eliminates the risk of a single point of failure and reduces the likelihood of unauthorized tampering. Additionally, each transaction is encrypted and linked to the previous one, creating an immutable chain that ensures data integrity [62].