**3. Blockchain technology**

Blockchain is a distributed database that allows the storage and transmission of information in a transparent and secure manner. It operates without a centralized control body because it is managed by a network of computers/users, also called nodes, on a peer-to-peer basis. This database is constituted by a growing list of digital records of validated transactions, known as blocks, which are chained to each other in chronological order through hashing function [4]. The validation of a new block of transactions involves all the relevant nodes. They execute algorithms to evaluate and verify the authenticity and accuracy of the transactions. If the majority of nodes agree that these transactions are valid, then the new block of transactions is accepted into the database. Each block is attributed with a unique hash number – a digital fingerprint of data, and it also carries the hash of the previous block. Once these blocks are chained to each other, they become immutable. **Figure 1** illustrates the main properties and operations mode of a blockchain.

Blockchain contains the following key features: immutability, automation, and security [6, 7]. First, transaction information stored on the blockchain can be seen by all participants and cannot be altered by any single node, as each node possesses a complete record of all the information within the blockchain. Data immutability ensures data accuracy, increases trust, and reduces fraud. This feature enables the tracking of the provenance of assets, which means that for any asset it is possible to tell where it is, where it has been, and what has happened throughout its lifetime [8].

#### **Figure 1.**

*The key properties of blockchain. Source: Queiroz & Wamba [5].*

Second, blockchain technology enables smart contracts, which is a consensus agreement based on a specific computer protocol [9]. The smart contract is executed automatically when conditions are proven to have been met [10]. In this way, digital assets could be operated without the need for third-party interventions, but by a program executing automatically certain rules defined by users [11, 12]. The trust issue in the network is resolved as everyone is playing by the rules and operation could be done more efficiently. Third, the distributed and encrypted nature of blockchain technology makes it resilient to different kinds of cyberattacks [13].

Blockchain has broadly been used according to one of the two models: public and private. Based on peer-to-peer network and consensus mechanisms, a public blockchain is a decentralized or distributed network that uses individual node to record transactions and distribute the data directly to each connected node. All the data and clusters of transactions are organized in a group, or "block." By adding a set of new blocks, the chain is hence formed. Every block is generated by a specific consensus algorithm to assure that all the participants involving in the chain agree upon a specific state of the system as its true state [14–16]. A public blockchain network is totally open to every creator of a block, which suggests that anyone can access to the blockchain and allow them to read its contents. The network typically has an encouraging mechanism to encourage more participants to join the network. Bitcoin is a typical public blockchain networks today [17, 18].

A private blockchain is similar as a public blockchain. They are both decentralized network connected by different blocks generated by a specific consensus mechanism. The only distinction between public and private blockchain is related to who is allowed to participate within the network, execute the consensus protocol, and maintain the shared ledger [18]. A private blockchain calls for invitation proof that should be validated by either the network starter or by a group of rules. Private blockchain is usually operated by one organization, which defines limited visibility rights to chosen participants within a permissioned network [13]. In another word, participants must obtain a permission to join the blockchain. The access control mechanism could vary according to organization's choice: existing participants could decide future entrants; a regulatory agency could issue licenses for participation or a consortium could make the choices instead [19].
