A Secured Privacy-Preserving Multi-Level Blockchain Framework for Cluster Based VANET

Round 1

Reviewer 1 Report

In this paper, the authors propose a blockchain-based secured, decentralized, and distributed authentication protocol for Cluster-based MAC (ACB-MAC) for VANETs. The formation of the authentication centers, vehicles registration, and key generation processes are explained with the secure and faster authenticating methods. Further, a multi-level blockchain is proposed. To avoid the hidden node problem, packet overloading, packet dropping, etc. of the traditional MAC protocols, a modified control packet format of IEEE 802.11 standards is proposed. Moreover, to preserve the privacy of the vehicles the original identity of them are securely stored in the LAC and only the public keys are shared between the CHs. The paper is technically sound and organized well. However, the following concerns need to be addressed before publication.

1. The abstract is too lengthy. There is no need to provide details in the abstract.
2. Please write down the shortcoming of the literature provided in [14] and [15].
3. Please provide motivation of the research work and a section i.e., problem formulation in which the authors should describe what problem the authors are going to solve and how they will solve it.
4. The authors provided 11 keywords. This will make the searching of the paper a little bit less optimized. Can you please provide a smaller number of keywords? i.e., 6-8.
5. Please provide the implementation parameters in a table.
6. Please describe the results by stating how the proposed scheme achieved better results i.e. by mentioning the cons of the proposed scheme.

Author Response

Thank you very much for reviewing the paper, helping us to rethink and to make the paper more efficient. Attached is the point-to-point reply of the reviewer's comments.

Best Regards, 

A F M Suaib Akhter

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors propose an authentication scheme with the blockchain technology and integrate the scheme with the WiFi MAC protocol.The authors claim that the proposed scheme is more efficient than previous works. My comments are as follows.

1. The authors claim that blockchain is a distributed storage system so that the designed scheme does not has single point of failure. Undoubtedly, blockchain is a distributed storage system. However, in the proposed scheme, there is still a central point, which is called GAC, in this paper. So I do not agree that the scheme does not avoid the single point of failure problem. The authors may think that even the GAC has some problem, all LACs have the blockchain storage and can be used to check the vehicle information from local blockchain. For me, the benefit comes from the information duplication and the blockchain technology is not mandatory.
2. In line 275, when receiving a MPK, CH will generate a transaction in the LABC. The time cost of this step should also be considered.
3. Actually, PKI is also a hierarchical architecture. In most scenarios, the Root CA does not need to participate in the authentication process since most entities have the certificate of the Root CA and the certificate can be used to authenticate other entities.
4. Since VANET is dynamic, it is undoubtedly that the transaction and the block are changed frequently. How about the mining issue in the proposed scheme?
5. Actually, I think this paper is an integration work from a blockchain authentication and CB-MAC. However, the authors do not provide enough reasons to convince me the necessity of the combination. In section 6, we can see that the analysis looks like two independent works.
6. English must be improved.

Author Response

Thank you very much for reviewing the paper, helping us to rethink and to make the paper more efficient. Attached is the point-to-point reply of the reviewer's comments.

Best Regards,

A F M Suaib Akhter

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

1. In line 297, the authors claim that one-bit data signature and verification is lightweight. I think there are some problems here. First, even the message space is one bit, how about the signature space? If the signature space is large, the computation cost cannot be ignored. If the signature space is small, this is not a secure signature scheme. Moreover, in most signature scenarios, there should be a random number used in the signature and therefore, the signed data is not one bit.
2. In the authors' response, the authors claim that certification-based solutions require higher computational time in the signature and verification process. The authors also claim that they prefer to use RSA-1024. However, RSA-1024 can also be applied to the certification-based solutions. The authors should explain this in detail instead of listing some references. Moreover, in the reference 16 and 17, where I download these two papers and read them, I think they can also belong to the certificate-based solution. So I am not sure if they can support the authors' claim.
3. The authors claim that they use instant mining rather than using any sort of consensus protocol to get faster transactions. Will this bring another problem that the blockchains stored in all members are not synchronized? Will this bring another problem that some member in this system is compromised?
4. The authors add section 6.3 to describe their achievements. However, I still cannot get the reason of this combination. For me, it looks like two independent works and the lightweight  authentication part is not convincing enough. The most important design in the authentication part is that the authors create a new class for the emergency vehicle.

Author Response

The reviewer#2 has our deepest thanks as the comments are useful to enhance the quality of our manuscript. In this third round of review, we sincerely hope that our manuscript is worthy of publication. We tried to address the points raised by the honourable reviewer#2 and also specified that, some of the issues are going to be part of our future works. Therefore, this manuscript will not only serve as a reference for blockchain integration in VANET, but also will generate more futuristic research questions to address by the research community.

Round 3

Reviewer 2 Report

All my concerns are addressed. The authors do lots of experiments to evaluate their work. I think if it will be better that the authors can evaluate their work with some VANET simulation tools.