*3.1. Blockchain*

It can be strongly said that blockchain is the future of secure currency management. Security, integrity, worldwide availability, preservation of privacy, immutability, transparency, etc., are the basic services provided by blockchains [39]. Distributed storage and decentralized storing system are considered as additional advantages. To understand the popularity, the amounts of money invested (over the years) by different industries for blockchain are presented in Figure 1 (adopted from [40]).

Protection from several types of attacks is another special feature of blockchains. Due to its decentralized and distributed storage technique, many typical attacks, including Sybil attacks, unknown source attacks, man in the middle (MITM) attacks, unauthorized entry, and DDoS (distributed denial-of-service), are not possible to perform on a blockchain. Additionally, consensus protocols put on another level of security on this, which can ensure the integrity and stability of the information.

**Figure 1.** Worldwide blockchain spending by industry.
