**1.2 Security aspects of blockchain information**

The basic security properties of the blockchain stem from both advances in cryptography and the design and implementation of bitcoin. Theoretically, the first secure blockchain was formulated using cryptography in 1991.

A proposal to improve the efficiency of the cryptographic blockchain was published in 1993, incorporating Merkle trees and placing multiple documents in a block. The blockchain is designed to ensure a number of inherent security attributes, such as consistency, proof of forgery, resistance to a distributed denial of service (DDoS) attack, pseudonymity resistance to a double attack, and OWASP resistance. However, to use the blockchain for secure distributed storage, additional security and privacy properties are required. In this section, we describe the fundamental security and privacy properties of blockchains before addressing the topic of monitoring blockchain systems with the aim of proposing a methodology for exploring, analyzing, and visualizing the behaviors of blockchain actors. In this chapter, we will only deal with the monitoring of applications.

### *1.2.1 Controlling data consistency*

The criteria that are required for a functional blockchain are the following:


In a traditional database, it is possible to guarantee these properties by controlling access to the registry, which implies having confidence in the entity that maintains it.

The blockchain solution is to decentralize and replicate the maintenance of the registry between several locations. Thus, the participating entities do not need to trust each other, and it works as long as enough entities are actually trustworthy and do not form coalitions (of more than 51%). This honesty is motivated by a reward for producing blocks that are cryptographically protected. All of these blocks are replicated in a P2P network (with no central node), avoiding a single point of failure [11].

Satoshi Nakamoto's initial blockchain was permissionless, meaning that anyone could participate in maintaining the registry, without the need to register first. This meant that it would work efficiently regardless of the number of participating entities.

Later, a variation more suited to certain applications emerged: consortium blockchains, where participating entities are pre-registered. The registry can be faster and more reliable, while still being controlled by the majority of participants.
