# Blockchain and IoT Convergence--A Systematic Survey on Technologies, Protocols and Security
## 10 Conclusions  
This paper has provided a systematic review by discussing the application prospects of Blockchain technology in the IoT industry, the foundations of both systems, and the strategic importance of the Blockchain-IoT convergence. 500 articles and whitepapers has been screened as documented in PRISMA 2009 flowchart (Supplementary File S2). Some were discarded as non-innovative, too generic or not sufficiently IoT-oriented. At the end of the study 118 sources were used for the drafting of the article, including 23 online available Internet resource. 36 are whitepapers with a high risk of bias. In order to mitigate the bias issue, we proceeded to analyze the projects that had source code available and we also tested the related Blockchains. In summary, Blockchain technology has huge potential in IoT systems like supply chain, medical transportation and smart city [77]. But like any system still in an embryonic state, there are many challenges to be faced and risks to be considered. Specifically, this paper first introduced the principal security risks connected to IoT systems in Section 4, then the core theory of Blockchain technology, going deep into the 'consensus' algorithms and cryptography concepts in Section 5, ending with an excursus on the main projects in the Blockchain-IoT area. PRISMA 2009 checklist (Supplementary File S1) has been compiled. The following conclusions were drawn:  
* Europe hosts the most important project, but Asia is the most powerful in promoting the link between Blockchain and IoT.
* The number of projects in Blockchain-IoT domain is growing fast and many are already in production stage.
* Big international players like Microsoft, Volkswagen, Fujitsu and countries like China have established important partnerships with existing projects.
* In financial terms, cryptocurrency traded on exchanges suffers extreme volatility, those related to the Blockchain and IoT world are also of a considerable scarcity of volumes. Here are the remaining open issues and research directions:
* Storage: one of the main advantages of the Blockchain is its decentralization, but the ledger must be stored on the nodes themselves and IoT devices have low computational resources and very low storage capacity.
* Processing Power: Encryption and consensus algorithms can be very CPU-intensive and IoT systems have different types of devices which have very different computing capabilities [118], and not all of them will be able to run the same encryption algorithms at the required speed.
* Legal and Compliance: Blockchain is the very first architecture able to connect the entire world without a central control. Connecting countries with different laws without a legal supervision is a serious issue for both manufacturers and service.
* Scalability: In the Blockchain world there is a famous trilemma that says that if you want security and decentralization, it will be necessary to sacrifice scalability. Overcoming the Blockchain trilemma will lead to a new level of adoption for distributed ledgers.  
Finally, the authors have briefly presented the future research trend that includes the introduction of AI mechanisms to enhance the capability of IoT devices in Blockchain systems.  
**Supplementary Materials:** The following are available online at [[http://www.mdpi.com/2076-3417/10/19/6749/s1](http://www.mdpi.com/2076-3417/10/19/6749/s1)]([http://www.mdpi.com/2076-3417/10/19/6749/s1](http://www.mdpi.com/2076-3417/10/19/6749/s1)), Supplementary File S1: PRISMA 2009 checklist, Supplementary File S2: PRISMA 2009 flow diagram.  
**Author Contributions:** Conceptualization, L.F. and A.P.; Methodology, A.P. and N.S.; Validation, A.P.; Investigation, L.F.; Resources, L.F.; Data curation, L.F. and A.P.; Writing--Original draft, L.F. and A.P.; Writing--Review and editing, L.F., A.P. and N.S.; Supervision, A.P. and N.S. All authors have read and agreed to the published version of the manuscript.  
**Funding:** The APC was funded by Universita degli Studi Guglielmo Marconi.