## 3 Blockchain  
Blockchain [54], a distributed ledger form, is a special data storage tool. Data are added to structures called blocks. Each block consists of two parts, the head and the body. The block number, nonce value, timestamp, the previous block's hash value, and the block's hash value are kept in the head section. The transactions/data are kept in the body section. The hash value of the previous block in the head section is not included in the genesis block called the starting block. The block number indicates the number of the block, while the Nonce value, \"Number Only Used Once\", refers to a number or value that can be used only once. The nonce is often used in authentication protocols and cryptographic hash functions. In blockchain technology, a nonce refers to a pseudo-random number used as a counter during the mining process. The timestamp indicates the time the block was generated. In addition, each block's hash values are stored for itself and for the previous block. In this way, each block carries information about the previous block. The fact that blocks are connected in the form of a chain characterizes the meaning of blockchain. Therefore, once data are stored in the blockchain, it is almost impossible to change or delete them. The basis of almost every blockchain is the mining process based on hash algorithms. It takes an input of any length and produces an output that will always be the same length. The output is called a \"hash\" and consists of 64 characters (256 bits). Because it is a one-way function, it is almost impossible to calculate the input from the output. A Merkle tree is a way of organizing and structuring large amounts of data. In the Merkle tree structure, when a transaction is made, it is hashed and then an equivalent hash value is given. After each transaction is hashed in the Merkle tree, the generated hashes are matched against another hash value and then hash again. This process of matching hash values is repeated until a final hash value is generated. The last hash value, the Merkleroot, provides a summary of all the transactions it contains. The Merkle root digest is then appended to the block header. The resulting output is the block hash and will serve as the identifier of the newly created block. SHA-256 is used as the hash. These situations are illustrated in Figure 1. In the study, information about the node and data in the network were kept in the blocks. While the information about the node consists of an ID, PVK, PBK, IDcard, and CN to which SN belongs, the data in the network consist of SN, Temp, and Hum data. Each SN datum is considered as a unit. Therefore, the capacity of each block is set to 100 units of data. After sufficient data are received, a new block is created. There are four types of blockchains. These are Public Blockchain, Private Blockchain, Hybrid Blockchain, and Consortium Blockchain. Public Blockchain is the ledger where records of all transactions are kept publicly. A public blockchain network is decentralized and does not require a trusted central authority to verify transactions. The word \"public\" means that anyone can join the blockchain network with read and write access permission. It is a permissionless platform where anyone can join and trade. A private blockchain is also known as a permission-based blockchain. An entity controls the block with read and write access. In this type of blockchain, access permissions are often restricted. Only authorized participants can access the chain network. The miners or validators are predefined in the private blockchain. The proposed blockchain mechanism is built on a private network that provides access to authorized users only. Hybrid Blockchain uses both the benefits of Private Blockchain's access control and the transparency features of Public Blockchain. It allows businesses to choose what information they want to keep private and what information they want to make public. Finally, Consortium Blockchain It is known as a semi-decentralized blockchain.