Review of Offline Payment Function of CBDC Considering Security Requirements

Round 1

Reviewer 1 Report

Dear Authors
Thanks for submitting your manuscript to MDPI journal Applied Sciences. As you know, the journal is indexed in prestigious indexing sources and has an impact factor of 2.679. Therefore a rigorous peer-review process is required.

The paper you submitted is a simple review of academic papers that should shed light on the offline payment mechanism for CBDC. This is a fascinating subject to investigate due to the nascency of the topic. Unfortunately, the paper you submitted lacks many critical elements for it to be appropriately evaluated by peers. Please accept some suggestions to improve the article.

 

ABSTRACT

From lines 15 to 24, the abstract is quite confusing and hard to read. Plus, it does not really communicate the scope and content of the paper. I believe that just by reading the abstract, a reader has no idea of what is going to find in the paper text. Therefore I think it has to be rewritten. Also, sentences such as lines 15-16 have to be reworded and re-elaborated since they are not clear. Please focus on the paper's objectives and methodology. Also, mind that references should not be inserted in the abstract; therefore, find another way to describe the inspected literature.

INTRODUCTION

The first lines introduce the paper sufficiently well. From lines 49 to 56 instead, it is not clear how figure 1 is drawn. At the end of the sentence, there is citation (5) which I could not find online, therefore:

• Adjust reference (5), adding more information so that it can be found online. Maybe a link to the resource?
• Please elaborate on the figure explaining if it is taken from another resource or it is elaborated by the author, specifying the sources of the extracted information.

From lines 58 to 61, I start to understand what the focus of the paper is, which is actually interesting, but it should have been clarified before. The introduction then ends with a confused outline of the paper's progression, which does not help the reader to understand the paper's content. Please consider adding more information in the introduction to specify how the analysis is undertaken and what the objectives are. Thus no research questions can be found….what do you wish to achieve with the paper?

METHODOLOGY

The methodology is not clear and is not well outlined in the abstract as well as the introduction. It seems that it is a review, and there are paragraphs that are a summary of related papers. However, this is something that is rarely seen in research papers. Reviews can be simple or structured but have to be built toward a research question that is missing here. Please elaborate on a research methodology based on prior or similar research.

LITERATURE BACKGROUND

The literature background provides some characteristics of CBDC but is not well organized. It is interesting as a stand-alone source, but it does not provide any additional information for the reader that tries to understand the scope of the paper. It lacks a clear outline that explains the reason for providing that information.

REVIEWS

The reviews are interesting, but similarly to the literature, the background seems to be just packed together without an apparent reason. How are those resources selected, and why? This should be extremely clear. Above this, the number of selected resources is quite limited, and it raises some doubts on the robustness of the results in the first place.

DISCUSSION

Unfortunately, the discussion is hard to understand due to the lack of a clear interpretation key that is not provided in the introductory part of the paper. Lacking a lens of the analysis, it is hard to evaluate the discussion brought by the authors appropriately.

CLOSING THOUGHTS

The research paper contains some good hints for research, but at the moment, the article requires more work to be evaluated for publication. The author should rewrite the abstract and introduction clearly stating what the scope of the paper is and the methodology used. Furthermore, they should try to expand the dataset of investigated paper, maybe also relying on grey literature (preprints, online blogs, books, newspaper articles). Lately, due to the lack of academic papers in the blockchain field, it is very common to see reviews that utilize both academic and grey literature entries. With the newly retrieved data, the authors can elaborate some exciting research questions that can better guide readers to understand and appreciate their research.
Good luck with your study.

Author Response

We are grateful for your constructive and valuable comments, which were very helpful in improving our manuscript. We have read the comments carefully and tried to provide appropriate answers. Please check the response attached.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper in question deals with an up-to-date topic which is central bank digital currencies. More specifically it explores the aspect of an offline payment function in CBDC systems by checking whether the methods proposed in a series of research studies satisfy all six security requirements, namely: double-spending, unforgeability, non-repudiation, verifiability, anonymity and DDoS attack prevention. Three studies are analysed in detail, that is: Camenish et al., Dmitrienko et al. and Christodorescu et al. The last study corresponds with the payment organisation Visa offline payment solution for CBDC using trusted execution environments (TEE).

The authors well structure the entire paper. It starts with a clear explanation of all mentioned above security requirements followed by a proper review of literature. Then they move on to examine the three studies selected for a detailed investigation. It is done correctly as well as table 2 seems to summarise correctly to what extent approaches taken in the three studies satisfy all the six security requirements. The next section elaborates on limitations of the methods presented by all the three studies. In the discussion the authors aptly point at different techniques (such as blockchain technology, TTP involvement, etc.) that could be used to help meeting the security requirements. It is shown that these techniques are not perfect in each aspect for offline payments systems. The authors conclude by proposing two methods that supplement the TEE technology proposed by Christodorescu et al. and used by Visa in their solution. These, proposed by the authors methods, allow for preventing DDoS attacks and satisfy the unforgeability requirement. Still, however, the potential vulnerability exists in the TEE technology when it would be hacked. Therefore the final conclusion is that, to make the CBDC system for offline payments fully robust to tampering, a periodic synchronisation of offline wallets and online accounts would be needed.

Clearly the authors prove that they have a very good understanding of the subject. The merits of the paper is high. The paper is a valuable review of others studies on offline payment function of central bank-issued digital currency. It also contains the author’s sole contribution which enriches the literature and adds to the field of research on CBDC. Therefore it deserves to be published.

On the downsides of the paper, surprisingly, it does not mention any of the CBDCs that are already up and running: Sand Dollar in The Bahams, eNaira in Nigeria, SOV in the Republic of the Marschall Islands or the e-yuan (e-CNY, digital renminbi) in China. It would be interesting to find out how those systems meet the offline security requirements. The authors should at least write in the introduction section that such systems exist. And in the conclusion section the authors could say, that exploring these CBDCs by checking how they meet offline and online security requirements could be a worthwhile avenue for further research on this topic.

Moreover, the paper requires some language editing. It is debatable whether to use plural “researches”. Perhaps “research studies” would sound better and could be used instead.

There are also other language or technical glitches in the paper, some of them listed below:

Line 20: the methodologies dealing therewith the security requirements mentioned in other prior studies were also reviewed.

Line 35:  The project E-korena (should be: E-krona)

Line 37: Uruguay where facing the technical issues in financing management system due to the under-developed infrastructures for the payment

Line 82: Section 2.1. “No Double-spending” should begin in a new line.

Line 87:  Brans [16] reviewed one show blind signatures and wallets with observers methods

Lines 438-439: 3.3.4. Protocols Included in the Transaction Process of Offline Payment System

suggested by Christodorescu et al. [3] ???

Line 504: In the standard case that fraudulent actions did not occurr, offline payment is processed while maintaining anonymity between users.

Lines 538 -542: excessive spaces

Author Response

We are grateful for your constructive and valuable comments, which were very helpful in improving our manuscript. We have read the comments carefully and tried to provide appropriate answers. Please check the response attached.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Dear Authors.
Thanks for submitting the revised version of your manuscript.
I can observe that substantial rework has been done to the paper.

Unlike the previous submission, this paper provides more content that may be of interest to the reader or for further studies.
Although I can confirm that the paper has improved, I am still concerned with some aspects to which I hope you could provide improvements.

• Paper Organization. The organization of the paper is still unclear. There is a paper outline in section 1.3. But the reason why the paper investigates the discussed areas is still unclear. The introduction lacks a scheme that explains how content is organized and why. Although interesting at a higher level, the study is really difficult to follow without a clear organization of the content. Alternatively, a methodology section would greatly help understand the paper structure and data gathering process.
• Research Questions. I Still cannot find the research questions of the paper. I also tried with a word processor typing research questions, or questions or just the question mark (?), but I couldn’t find anything. Could you please outline clearly what the research questions of this paper are? Please put them in the query form so that it is clear for the reader what this paper is going to answer. Without clear research questions, the paper’s purpose is still too foggy.
• English and editing. The paper needs thorough and specialized English editing. In my opinion, it cannot be published in this state. Many sentences are still unclear, while others are pretty naïve in the form. There are also many problems with spacing, so it clearly requires a specialized rework.

Additionally, please clearly state the limitations of this paper. It’s good practice that the authors honestly declare the study's limitations and the reasons why those could not be addressed at the time of writing the paper.
Good luck with your research.

 

 

Author Response

We are grateful for your constructive and valuable comments, which were very helpful in improving our manuscript. We have read the comments carefully and tried to provide appropriate answers. Please check the response attached.

Author Response File: Author Response.pdf