**1. Introduction**

Blockchain is now one of the most important technologies to have emerged in recent years. Many experts believe that this technology has the potential to change the world over the next two decades. Although it is still in its infancy, corporate giants are interested in its applications in several areas. So far, venture capitalists have invested billions of dollars in this field, with several applications [1]

Indeed, the applications of blockchain seem close to infinite [2]. While one immediately thinks of its financial applications—international payments, money transfers, complex financial products—blockchain can also solve problems and create new opportunities in healthcare, defense, management, supply chains, luxury, and other industries. At more advanced stages, blockchain could give rise to what Gartner calls the "programmable economy" [3], powered by entirely new business models that eliminate all kinds of middlemen. Given the importance of blockchain in the technological evolution of society, including across industries, especially the financial sector, researchers have undertaken considerable work to further strengthen the security level of this technology.

Indeed, work on the methodologies for encrypting the data preceding and following each block has made the data of the blockchain virtually unbreakable. In addition, the security of the blockchain is also due to the fact that several computers called nodes to store the blockchain. In addition to that, to modify the ledger, one would have to take control of at least 50% of the nodes in the network and their computing power in order to modify the data in the blockchain [4]. This is a difficult feat to accomplish, especially for a public blockchain such as the one behind bitcoin. With the advent of the quantum machine, this unparalleled security within the blockchain may be challenged in the future. Therefore, blockchain monitoring [1] could add an important layer of security to the blockchain. However, our work will consist in discussing and studying this topic while proposing, at the end of this study, an efficient and exploitable blockchain monitoring methodology [5] in order to allow a good understanding of the topic.

### **1.1 Blockchain technology**

A blockchain is a technology that allows information to be stored and transmitted without a control body [6]. Technically, it is a distributed database in which the information sent by users is verified and grouped at regular intervals into blocks, thus forming a chain. The whole is secured by cryptography. By extension, a blockchain is a distributed database that manages a list of records that are protected against alteration or modification by storage nodes [7]. Not all blockchains work in the same way. For example, they may differ in their consensus mechanisms, whose rules prevail depending on the technology that updates the ledger [1]. But fundamentally, a blockchain is a distributed and secure record of all transactions made since the beginning of the distributed system [8]. By extension, a blockchain constitutes a database that contains the history of all exchanges made between its users since its creation as shown in **Figure 1** [9].

However, there are public blockchains, open to all, and private blockchains, whose access and use are limited to a certain number of actors. A public blockchain can therefore be likened to a public, anonymous, and unforgeable accounting ledger. As the mathematician Jean-Paul Delahaye writes, one must imagine "a very large notebook, which everyone can read freely and for free, on which everyone can write, but which is impossible to erase and indestructible" [10], which is well illustrated in the figure above. Today, blockchains cover several aspects of computer security because of the numerous researches made around this technology.

**Figure 1.** *Block in blockchain technology.*
