Blockchain for IoT Applications: Taxonomy, Platforms, Recent Advances, Challenges and Future Research Directions

Round 1

Reviewer 1 Report

This paper presents a survey (tutorial style) focused on blockchain for IoT applications. It is interesting but could be more illustrative for beginners.

Suggestions and questions (answers can/should be used to improve the paper):
1. The way the abstract is presented is strange: "first, second, third... etc.". My suggestion is to rephrase this 'list' to better present the flow of the paper content.
2. The paper organization should be presented at the end of the introduction section.
3. Avoid short paragraphs - 3 or fewer lines. Revise the whole manuscript, looking for short paragraphs such as lines 55-57; 210-211; 364-365; etc.
4. Table 1:
a) when a cell is not filled, enter N/A;
b) improve the content of the column 'Blockchain Technology Covered' of your study;
5. Figure 3 is strange and not explained. What is the rationale behind it? It requires an explanation.
6. Related works: this is my main concern in this paper.
a) Consider the sentences "In summary, most of the current review studies highlight blockchain technologies of smart contracts and consensus protocols to secure IoT applications. In addition, a few of the studies presented distributed ledger technology to provide security and privacy for IoT applications. In contrast, this review provides an exhaustive analysis of all the blockchain technologies, protocols (e.g., smart contract, consensus protocol and ledger technology) and properties (e.g., decentralization, immutability, transparency, security, and trust) that offer a critical role in supporting security and trust for IoT applications.", why is 'In contrast' used? This review presents the same thing that others have already presented, right? Recheck what you are arguing in such sentences. What are the differences?
b) Paragraph in lines 181-199 is too verbose, what does not allow me to identify the novelty of this review.
c) Paragraph in lines 200-208 is repetitive. Paper organization should be provided in the introduction.
7. Paragraph in line 958-962 requires an explanation. Why do few recent studies have made advances and novel contributions to the literature? Is there a justification?

Specific comments:
- et al ⇾ et al. (add comma)
- Figure 1: properties ⇾ Properties (initial capitalized) - check all figures
- Most numbers of figures and tables are wrong in the captions and texts. Check all of them.

Author Response

Thank you so much for your comments

Author Response File: Author Response.pdf

Reviewer 2 Report

Apprezzo l'enorme lavoro degli autori per l'attività di ricerca a tavolino che hanno condotto, tuttavia l'articolo è una panoramica ampia (e non completa) del paradigma Blockchain. L'articolo non ha lo scopo di introdurre e utilizzare Blockchain nelle applicazioni IoT, ma è un'ampia revisione della letteratura sui fattori più importanti relativi alla tecnologia Blockchain. Al riguardo, infatti, non sono stati affatto presi in considerazione aspetti come la “latenza” o la “tokenizzazione” o proprietà come la “privacy”. La discussione sui protocolli non è affatto chiara e sufficiente. La rassegna sulle minacce legate ai servizi web legate all'utilizzo della Blockchain nelle applicazioni IoT è inesistente. Gli autori non si limitano alle caratteristiche, ai vantaggi, alle minacce e alle sfide che l'introduzione della tecnologia Blockchain nell'IoT ha comportato, ma prova ad approfondire e fare un'analisi esaustiva di tutte le tecnologie, i protocolli e le proprietà Blockchain senza finalizzare l'analisi alla sicurezza e alla fiducia delle applicazioni IoT (come altrimenti indicato nella riga 187 o nelle righe 193-194). Si raccomanda pertanto di modificare la riga 15 dell'abstract "Questo articolo fornisce un'indagine completa" in "Questo articolo fornisce un'indagine" e che l'intero articolo sia rielaborato sulla base degli elenchi puntati forniti all'autore, facendo riferimento a ciascun paragrafo di la carta.

1. Introduction

In this paragraph the authors have only mentioned, and not all, the technologies currently connected to the Blockchain paradigm such as: "smart contract" and "distributed ledger", but for a reader, even a less experienced one, to understand, from the beginning, the technical comparisons reported by the authors later in the paragraphs "2. Related Works" and "3. Taxonomy" it is recommended to expand this paragraph and make the reader better understand the concepts of "smart contract" and "distributed ledger" (and Ledger Type in general) and also insert the concepts of "Consensus process" for the evaluation of consensus protocols or for the understanding of the authors' statements, such as the one reported in line 280 "which are generally differentiated by the exchange of a distributed ledger". the authors are also recommended to include "encryption options" (only mentioned in line 131, or line 567, but not described in the entire paper) and "protection techniques" in this paragraph, which are critical to the security, trust, and privacy of IoT applications.

 

2. Related Works

This paragraph needs improvement. Some papers, such as the work of Hassan et al [12] (line 101-108) are extreme summaries that have no value. At line 103 the authors merely mention "five privacy protection techniques" without going into them or pointing them out. Similarly, the work of Lo et al. [17] (lines 135-139) is summarily described and refers to the entire paper for discussion, stating "The processes and methods used for the assessment of these works are frequently documented and evaluated in this review" (lines 137-138). For the work of Azbeg et al. [23] the authors talk about difficulties and possible solutions without going into the merits with a critical sense (lines 169-171). It is recommended a complete rewriting of this paragraph by critically highlighting the characteristics of the papers analyzed, not just reporting the abstracts of each paper.

 

3. Taxonomy

For this paragraph, the authors claim to have presented a classification and thematic taxonomy of Blockchain approaches for IoT applications. In this regard, however, it is believed that the authors do not describe the type of analysis conducted and that determined the taxonomy, which are important elements for a publication in an electronic journal. The paragraph thus articulated seems to list some concepts related to Blockchain technology such as: the basic components of an abstract model of architecture, the modes, the main related technologies and protocols, the fundamental properties and application fields.

Having said that, it is requested to the authors the entire rewriting of this paragraph suggesting the reading of a paper on Blockchain taxonomy: Tasca, Paolo & Tessone, Claudio. (2019). A Taxonomy of Blockchain Technologies: Principles of Identification and Classification. DOI: 10.5195/ledger.2019.140.

It is also noted that the different modes should initially be described based on the analysis conducted on the taxonomy, also reporting a summary of the comparison given in Table 2 "Camparison of blockchain modes" so that the reader can understand the difference between an authorized/unauthorized, centralized/decentralized network, etc. It should be noted that the "Chain efficiency" and "Immutability" features are not described within the various modes, and the "Consortium blockchain mode" is not fully described.

Line 282 "Table 1. Comparison of blockchain modes" is not Table 1, but Table 2, to be corrected, please, also re-enumerating the other tables correctly (Table 2 in Table 3, etc.).

Line 309 - Attention the "Private Blockchain" is a centralized-based network, not "decentralized-based network", to be corrected, please.

 

3.3. Blockchain technologies and protocols

This section of the paragraph 3, describing Blockchain technologies and protocols, provides a table (Table 3 "Blockchain services and benefits for IoT applications") that lists the benefits for each type of "Blockchain technologies and protocols," however these benefits are not made explicity within each sub-section and the section does not provide the reader with a discussion of the consequential advantages and disadvantages of using these protocols and technologies in the IoT. 

 

3.4. Properties of blockchain protocols (Pay attention error in the paragraph header - line 389)

The properties are well written, except for the "Decentralization" property, which contains errors in the form:

Line 407 - "In fact, blockchain services are decentralized, with no single authority" is not always the case, maybe the authors wanted to say that services in the IoT are decentralized ? Write better.

Line 416 - "One of the most exciting services" - exciting ? Inappropriate term. Write better.

For the "Transparency" property, the statement in lines 467-468 "All public sector transactions using personal data can also be monitored via an audit log accessible through the government portal to protect privacy." is incorrect, as these choices provide transparency, but do not solve the privacy protection problem.

It is suggested that the authors also include in this section an analysis on "privacy" and "latency", which are important properties for IoT applications.

3.5.1. Smart healthcare

Lines 523-524 - “This approach will remove the patient from the hospital's centralized structure” “Write it better.

4. Blockchain-based platforms for IoT

Paragraph 4 describes some important platforms, but for a more complete analysis, the authors are advised to enter the description of other features related to the platforms and useful for a technical comparison, such as: Ledger Type and Consensus Algorithm.

 

5. Role of blockchain in IoT

Line 747 - "Figure 2. The role of blockchain in IoT applications." To be corrected is not figure 2, but figure 3. Rename correctly the other figures as well (line 972). Furthermore, it is advisable to move this figure before the beginning of paragraph "5.1. Providing high scalability ".

5.1. Providing high scalability

Lines 738-745 contain concepts repeated and described previously. Review this part.

Line 770 - "5.3. Orchestration of connected iot devices " rewrite the “iot” acronym in" IoT "

5.4. Ensuring interoperability

Lines 787-788 - "Cross-chain communication archives data through five main approaches,…." Five main approaches? Which ? Specify better.

6. Recent advances

In the reported analysis for these works, only for the work Singh et al. [88] the platform on which the framework was created is indicated. For a more comprehensive analysis, the authors are asked to enter the blockchain platform also used for the other works that describe a "framework" or "System" type solution, such as for the works of: Rathee et al. [87], Latif et al. [89], Lin et al. [90], etc. For works that describe “Architecture” type solutions, eg. Singh et al. [95] or Memon et al. [96], you are asked to include the main and important features of the architecture.

Lines 852-853 - “Gong et al. [92] explored a simple four-layer IoT blockchain modeling approach comprising various types of IoT computers. " . “… A simple four-layer IoT blockchain modeling approach…”, what does it represent? Specify better.

Line 894 - “based on HashMaps” specify better or add a bibliographic reference.

Line 898 - “a SmartME project” specify better or add a bibliographic reference.

8. Conclusion

This paragraph contains concepts repeated and reported previously in the paper. Authors are recommended to rewrite the paragraph. Furthermore, on lines 1125-1126 the authors cite “the main platform of blockchain,” which platform are they referring to? Specify better.

 

English: A complete and extensive revision is required. Particular revision is asked in the use of vocabulary.

Author Response

Thank you so much for your comment

Author Response File: Author Response.pdf

Reviewer 3 Report

The Internet of Things (IoT) has become a prevalent paradigm of computing technology. It is increasingly used to facilitate the processes of human life via a range of different applications, such as smart healthcare, smart grids, smart finance and smart cities. However, IoT applications face a variety of challenges, including scalability, interoperability, security and privacy, and trustworthiness. Recently, blockchain technologies have been introduced to assist in overcoming these challenges. This manuscript has the following merits over the existing works of literature:

• This manuscript provides a comprehensive survey and tutorial on the application of blockchain for IoT systems.
• This manuscript discusses the importance of blockchain technology in terms of features and benefits for constituents of IoT applications.
• This manuscript introduces a thematic taxonomy of blockchain for IoT applications considering the most important factors.
• This manuscript describes the common blockchain platforms adopted for IoT applications.
• This manuscript highlights the roles of blockchain technology in expanding the scope of IoT applications.
• This manuscript discusses the challenges and future research directions of the use of blockchain for the IoT.

Thus, I can recommend this manuscript for publication in Electronics.

Author Response

Thank you so much for your appreciation

Author Response File: Author Response.pdf

Reviewer 4 Report

The paper presents the discussion of the importance of blockchain technology in terms of features and benefits for constituents of IoT applications, introduces a thematic taxonomy of blockchain for IoT applications considering the most important factors, describes and discusses the common blockchain platforms adopted for IoT applications, highlights the roles of blockchain technology in expanding the scope of IoT applications, discusses the recent advances and solutions offered for IoT environments, and discusses the challenges and future research directions of the use of blockchain for the IoT. The paper presents and interesting flow, and the abstract is interesting. However, the introduction presents a background research, but it is not presenting the scope and purpose of the study. However, it is incorrectly presented at the end of section 2. The section 3 needs to start with the contextualization of the taxonomy. However, the sections that described each part of the taxonomy are not relevant in this context. The remaining sections are interesting, but it needs to be more connected.

Author Response

Thank you so much for your comments

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have improved the paper. Some minor concerns remain:

1. The text "[On blockchain and its integration with IoT. Challenges and opportunities]" looks wrong in the text (2 times).
2. My comment: "5. Figure 3 is strange and not explained. What is the rationale behind it? It requires an explanation."
Authors' answer: "Figure 3 “Blockchain challenges and future research directions.” Related to challenges and issues And the figure 2 title “Abstract architecture of blockchain” removed and text related."
Reviewer's reply: Figure 3 is still in the manuscript. I could not understand this answer.
3. I strongly recommend an English review in the manuscript, looking for typos, grammar and punctuation errors. For example:
- paragraph in lines 900-906 should be revised
- programming language -> languages
- Java Scripts -> Java Script
- Latif et al., [86] - there is no need for comma (check all occurrences in citations) - only a dot should be used (Latif et al. [86])
- in the IoT -> in IoT

Author Response

Thank you so much for your comment

Author Response File: Author Response.pdf

Reviewer 2 Report

I appreciated the way the authors have modified the paper, making it more organic. Also the inclusion of the paragraphs on "latency" and "privacy" are in line with the paper,  bringing  to the reader's attention also the threats related to web services and the use of the Blockchain. While appreciating the in-depth study of the paragraph "3.2.4. Consensus protocol", I think  the authors have to  describe in this paragraph  the specific characteristics too and relative to important algorithms such as "PBTF", "Raft", "IBFT", etc., (not only PoS, PoW and DPoS), algorithms not described nor mentioned in the paper, but algorithms which are then cited in the table "Table 4. Blockchain platforms" of the paper and describing, in a complete way, the Blockchain platforms. The authors are invited to include the main features of these algorithms in the paragraph, making the reader understand how the consensus algorithm can be an important means of ensuring data consistency (Consistency problem and Byzantine failures) in these platforms.

Author Response

Thank you so much for your appreciation

Author Response File: Author Response.pdf

Reviewer 4 Report

The authors considered the previous comments, and the manuscript can be accepted.

Author Response

Thank you so much for your appreciation