# Blockchain Applications in Forestry: A Systematic Literature Review
## 2 Overview of Blockchain  
Blockchain was initially described as a 'chain of blocks' by [PERSON] [1] in 2008, the technology behind bitcoin. [PERSON] [1] introduced bitcoin as a peer-to-peer version of electronic cash. Blockchain is a decentralized digital database of timestamp transactions (distributed ledger) by hashing these transactions into a chain of hash-based proof-of-work [1,15]. Figure 1 demonstrates the blockchain data structure. As shown in Figure 1, the blockchain system records the data of timestamped transactions into a 'block' that is cryptographically linked like a chain of blocks [16]. The chain of blocks is constantly growing while each new block is added, and each block (block n) holds hashed code of the previous block (hash of block n \(-\) 1) [16; 17; 18]. The blocks in blockchain connect to the genesis block (the first block) in chronological order [17]. Blockchain technology has been rapidly developed in the last decades. The evolutions of blockchain can be summarized into four stages [19]:  
* Blockchain 1.0 was initially introduced in 2008 [1]. The first stage mainly focused on digital currency such as bitcoin. Blockchain technology was used for cryptocurrencies and payment systems to reduce transaction costs by removing intermediaries [19; 20];
* Blockchain 2.0 was introduced for smart contracts in 2013. A smart contract is a piece of code executed while a transaction is performed [18]. The code in smart contracts automatically executes when all conditions are met [21]. Smart contracts can be used as agreements between all parties in transactions. All parties involved will be forced to perform the transaction once conditions are met. In other words, the transactions with smart contracts do not need to be monitored by a trusted third party which makes the transactions irreversible, secure, and decentralized [21]. Smart contracts are transparent and autonomous to eliminate manipulation and human error [19]. One famous example of platforms to implement smart contracts is Ethereum proposed in 2013 [22]. Ethereum is a platform for blockchain and allows everyone to program smart contracts by creating their ownership rules and transaction formats [22];
* Blockchain 3.0 was introduced in 2015 and mainly focused on the development of decentralized applications and computing [23]. In this stage, decentralized applications are formed by back-end codes running on the open-source platform [24]. Blockchain 3.0 is integrated with cryptographic tokens. Tokens are digital assets that can represent any value [25]. The token economy became the front-end face in this era;
* Blockchain 4.0 integrated artificial intelligence (AI) with blockchain technology to make better decisions without the need of direct human intervention in 2018 [19; 23]. The combination of AI and blockchain can help solve complex issues worldwide. In general, AI predicts outcomes using algorithms based on probability. The outcomes of AI constantly change since the algorithm can 'learn' from new data. On the contrary, blockchain technology hash data and the outcomes are permanent and unchangeable. The characteristics of blockchain make data accurate which is helpful and useful to input it into AI systems. Moreover, blockchain could record and secure the outcomes of AI systems [19]. Based on the characteristics of blockchain technology, the types of blockchain technology are categorized into two types: public and private [15; 18; 23]. A public blockchain is also called a permission-less blockchain [23] or open blockchain [15]. The public blockchain allows everyone to join or check the transactions in the blockchain network with no privacy. All participants in the blockchain network are pseudo-anonymous, which means the parties in a transaction are anonymous prior to the transaction. Bitcoin, Litecoin [26], and Ethereum are examples of public blockchain [15]. In contrast, a privacy blockchain provides more privacy to transactions in blockchains.