Improving Collaborative Intrusion Detection System Using Blockchain and Pluggable Authentication Modules for Sustainable Smart City

Round 1

Reviewer 1 Report

The study covers interesting research on improving collaborative intrusion detection system using blockchain and pluggable authentication modules for sustainable smart city. The main problem in collaborative intrusion detection system is the concept of trust. Hosts in the network need to trust the data sent by other peers in the network. To bring in the concept of trust and implement the proof-of-concept, blockchain was used. Although this manuscript is submitted to the journal Sustainability, it minimally analyzes economic, environmental and social sustainability issues. Global research shows that the macro-level sustainability indicators significantly influence trust. Is it possible theoretically additionally analyze several macro-level economic, social and environmental sustainability indicators (for example, https://www.nature.com/articles/s41598-022-19131-6) influencing the trust? Also, the manuscript is written in style more like a lecture rather than a research article. The global innovativeness in research development hasn't been presented. Some figures and tables which involve world-wide novel research should be described and discussed with more details to emphasize the state-of-the-art-review all over the world novelty. Please use this the newest (2018-2022) Web of Science journal papers.

Author Response

Response to Editor/Reviewers Comments

Title: Improving Collaborative Intrusion Detection System using Blockchain and Pluggable Authentication Modules

The authors greatly appreciate the valuable and insightful comments made by the editor and reviewers and would like to thank them for their efforts. Their comments have undoubtedly helped us to improve the quality of our manuscript. We have carefully considered all the comments and the manuscript have been revised thoroughly to meet their expectations. Our point-by-point responses to each comment are listed below:

Reviewer: 1

The study covers interesting research on improving collaborative intrusion detection system using blockchain and pluggable authentication modules for sustainable smart city. The main problem in collaborative intrusion detection system is the concept of trust. Hosts in the network need to trust the data sent by other peers in the network. To bring in the concept of trust and implement the proof-of-concept, blockchain was used. Although this manuscript is submitted to the journal Sustainability, it minimally analyzes economic, environmental and social sustainability issues. Global research shows that the macro-level sustainability indicators significantly influence trust.

Comment 1: Is it possible theoretically additionally analyze several macro-level economic, social and environmental sustainability indicators (for example, https://www.nature.com/articles/s41598-022-19131-6) influencing the trust?

Response: Thank you for your valuable suggestion. We have included several indicators such as social, environmental and social in the updated introduction of the research article.

The role of blockchain in smart, sustainable cities is vital because it helps to foster the kind of trust necessary for smart cities. Blockchain should serve as the cornerstone for the development of a smart city and is a crucial assurance for the proper design and execution of the management strategy and planning scheme. A smart city is naturally combining smart energy, smart transportation, smart government, and other services under the same umbrella Decentralization and the availability of clear data place strict constraints on the big data service platform. Finding the problematic node among the hundreds of millions of nodes in a network is a time-consuming operation if a network encounters a problem or is the target of an attack. Most current Internet of Things networks are centralized. A huge server or centralized cloud is connected to hundreds of millions of nodes, which causes bottlenecks in the price and compute storage capacity. Blockchain distributed technology can guarantee that even if one or more nodes are hacked, the total network data remains trustworthy and secure. And distributed computing makes use of point-to-point computing to handle the hundreds of billions of transactions that the Internet of Things generates. This significantly lowers the cost of computing and storage by utilizing the computing and storage capabilities of a large number of idle devices deployed in unused locations.

In order to increase the degree of security for secure transmission and safe storage, additional protection mechanisms need to be implemented due to the privacy of numerous people involved. Blockchain has shown to be secure, dependable, and suitable for this purpose. The disaster recovery system cannot be enhanced due to the high ex-pense of putting up a data center and data storage. Therefore, a key issue at hand is how to lower storage costs while enhancing disaster recovery capabilities. Blockchain, which connects distributed and centralized services, can successfully stop an attack on the vital network infrastructure

Comment 2: The manuscript is written in style more like a lecture rather than a research article.

Response: Thank you for your valuable suggestion. We have updated entire paper as per your suggestions.

Comment 3: The global innovativeness in research development hasn't been presented. Some figures and tables which involve world-wide novel research should be described and discussed with more details to emphasize the state-of-the-art-review all over the world novelty.

Response: Thank you for your valuable suggestion. We have updated the innovativeness in the updated manuscript.

Collaborative Intrusion Detection System (CIDS) was designed to remove the inefficiency of the current Intrusion Detection which failed to detect coordinated distributed attacks. The main problem in CIDS is the concept of trust. Hosts in the network need to trust the data sent by other peers in the network. To bring in the concept of trust and implement the proof-of-concept, blockchain was used. Pluggable Authentication Modules (PAM) were also used to track login activity securely before an intruder could modify the login activity. To implement blockchain, an Ethereum based private blockchain was used.

Comment 4: Please use this the newest (2018-2022) Web of Science journal papers

Response: Thank you for your valuable suggestion. We have added some latest articles in the updated manuscript.

In the most recent years, many other IDS have been proposed [17-21]. These IDS can be used in any domain to identify intrusions or abnormalities, which can then lead to the development of a secure solution for smart cities. In the smart cities everything is connected to the internet so a smart IDS can play a significant role to provide the security for this created network. Moayad Aloqaily et al. [22], proposed an IDS for the securing the transportation. This IDS will help the vehicular service management to secure the network from the attach and ensure the quality-of-service availability. Mohamed Faisal Elrawy et al. [23], discusses the role of IDS and IoT in the smart environment. This article first discusses various existing work that have done to create the smart environment using IoT sensors and then discuss the existing IDS used to provide the security in IoT context. Asmaa Elsaeidy et al. [24], introduced smart IDS to prevent the distributed Denial of Service (DDoS) attacks for the smart cities. This article used Restricted Boltzmann Machines (RBMs) technique to design the IDS. Tanzila Saba et al. [25], proposed an ensemble-based IDS for the smart cities' hospitals.

The current IDS is not sophisticated enough to detect distributed, parallel attacks that take place throughout the nodes in the network instead of a single node. In case of CIDS, the ability to correlate the events is crucial. The events occurring across all the nodes in the network must be aggregated for further processing and raising of alerts. The concept of trust is crucial among the nodes. While the discussed approaches have their advantages, there was clearly a lack of scalable architecture in the case of CIDS. The main aim of this paper is to demonstrate an approach through which a scalable architecture could be developed, and the trust could be established among the nodes in the CIDS architecture. Blockchain was proposed to solve the trust issue in the case of CIDS. 

[17] C. Liang et al. “Intrusion Detection System for the Internet of Things Based on Blockchain and Multi-Agent Systems”, Electronics 2020, 9, 1120. Doi: 10.3390/electronics9071120

[18] A. Ghaleb et al. “Misbehavior-Aware On-Demand Collaborative Intrusion Detection System Using Distributed Ensemble Learning for VANET”, Electronics 2020, 9. Doi: 10.3390/electronics9091411

[19] Radoglou Grammatikis et al, “A Novel Multivariate Intrusion Detection System for Smart Grid. Sensors” 2020, 20. Doi: 10.3390/s20185305

[20] Iwendi, C.; Anajemba, J.H.; Biamba, C.; Ngabo, D. Security of Things Intrusion Detection System for Smart Healthcare. Electronics 2021, 10, 1375. doi: 10.3390/electronics10121375

[21] Kotecha, K. et al., “Enhanced Network Intrusion Detection System”, Sensors, 2021, 21. Doi: 10.3390/s21237835

[22] Moayad Aloqaily et al., “An intrusion detection system for connected vehicles in smart cities”, Ad Hoc Networks, vol. 90, July 2019. Doi: https://doi.org/10.1016/j.adhoc.2019.02.001.

[23] Mohamed Faisal Elrawy et al., “Intrusion detection systems for IoT-based smart environments: a survey”, Journal of Cloud Computing volume, vol. 7, 2018. Doi: https://doi.org/10.1186/s13677-018-0123-6

[24] Asmaa Elsaeidy et al., “Intrusion detection in smart cities using Restricted Boltzmann Machines”, Journal of Network and Computer Applications, vol. 135, 2019, pp. 76-83. Doi: https://doi.org/10.1016/j.jnca.2019.02.026

Author Response File: Author Response.docx

Reviewer 2 Report

The article discusses the results of the implementation of a distributed IDS, in which the log storage is implemented using blockchain technology. To simulate a distributed computer attack, the Doorknob Rattle Attack scenario was used. A private Ethereum based Ledger was used to implement the proof-of-concept. Pluggable-Authentication Modules were used to log the successful login attempts. Anomalies were recorded when the number of transactions exceeded the threshold, and the CPU load was additionally taken into account.

Despite the fact that the results are of undoubted interest, the manuscript has significant shortcomings that do not allow us to recommend it for publication in a journal.

1. The manuscript is designed as a chapter from a thesis, and not as an article intended for a journal. Indirect evidence of this are such words in the Conclusion as “This thesis showed…”, “… made in this thesis…”, “… main aim of this thesis…”, ‘… to explore in this thesis for additional work…’’. In addition, there are major flaws in the design. These include the following:

a. In the Introduction, the abbreviations HIDS and NIDS are expanded twice. The sentence “HIDS being HIDS and NIDS being Network based Intrusion Detection System” should be removed.

b. The figures are of poor quality. On Fig. 1 and Fig. 4, text inside geometric shapes is not readable. On Fig. 2, there is an overlay of the figure caption on the figure itself. Fig. 4 is broken in two parts due to a page break. The caption under Fig. 9 is not visible. Fig. 10 is not readable. On Fig. 12, the line “Linear (Number of attempts)”, which defines the threshold, must be strictly horizontal.

c. There are no references to many figures and tables in the text: figures - 2,4,5,7,12,13, tables - 1, 3. At the same time, the caption of Table 3 is located incorrectly; Table 4 has no caption at all.

d. Many figures and tables are either poorly commented or have no comments at all. So, Table 1 has no comments. Moreover, apart from it, there is nothing else in Subsection 2.3. This may be acceptable for a thesis, but it is not acceptable for a journal article! Figure 3 requires additional comments, as it is not clear why the O3 group cannot be considered as normal behavior. Fig. 4 has no comments. Classification in Fig. 5 is incomplete. There is no division into classes “Centralized”, “Decentralized” and “Distributed” in it. For Fig. 6, authors do not say where the logs came from. Fig. 7 is not informative, all the lines on it are the same. Dates in Fig. 8 and in Tables 3 and 4 do not match: in Fig. 8 - Apr 27, in Tables 3 and 4 - May 27. For Fig. 10, authors need to show the connection with 8 attempts. Table 6 and Fig. 12 do not specify the threshold value. Fig. 13 has no comments.

e. The first paragraph in Section 5 is broken.

2. The Introduction is short. It lacks a description of the goal, contribution (novelty), and structure of the article.

3. There are no mathematical expressions in the article. Based on the title of the article, the authors should show how the sustainability of Smart City is increasing. To evaluate sustainability, mathematical expressions are needed. Instead, the authors replace sustainability with trust. These are completely different properties! As a result, the authors moved away from sustainability and came to security! This is a methodological error!

4. We need a comparative assessment with well-known IDS-systems, not only in terms of sustainability, but also in other properties (speed, resource costs, and others).

5. We need a Smart City scenario and an experiment on this scenario.

6. The Conclusion is taken from the thesis. It should be rewritten.

7. The review of related work was carried out only for works related to the blockchain. There are no references to works on sustainability of Smart City. Also, the number of references is very small (only 20). It needs to be increased to 45-50.

8. There are some inaccuracies. So, in the comments to Fig. 1 authors mixed up Distributed and Decentralized systems.

Author Response

Response to Editor/Reviewers Comments

 Title: Improving Collaborative Intrusion Detection System using Blockchain and Pluggable Authentication Modules

The authors greatly appreciate the valuable and insightful comments made by the editor and reviewers and would like to thank them for their efforts. Their comments have undoubtedly helped us to improve the quality of our manuscript. We have carefully considered all the comments and the manuscript have been revised thoroughly to meet their expectations. Our point-by-point responses to each comment are listed below:

Reviewer: 2

Comment 1: The manuscript is designed as a chapter from a thesis, and not as an article intended for a journal. Indirect evidence of this are such words in the Conclusion as “This thesis showed…”, “… made in this thesis…”, “… main aim of this thesis…”, ‘… to explore in this thesis for additional work…’’. In addition, there are major flaws in the design. These include the following:

Comment 1.1: In the Introduction, the abbreviations HIDS and NIDS are expanded twice. The sentence “HIDS being HIDS and NIDS being Network based Intrusion Detection System” should be removed.

Response: Thank you for your valuable suggestion. The relevant changes have been made in the revised manuscript.

Comment 1.2: The figures are of poor quality. On Fig. 1 and Fig. 4, text inside geometric shapes is not readable. On Fig. 2, there is an overlay of the figure caption on the figure itself. Fig. 4 is broken in two parts due to a page break. The caption under Fig. 9 is not visible. Fig. 10 is not readable. On Fig. 12, the line “Linear (Number of attempts)”, which defines the threshold, must be strictly horizontal.

Response: Thank you for your valuable suggestion. All the suggested corrections have been done in the updated manuscript. I have removed the figure 3 and figure 4 because these figures are shown the general information.

Comment 1.3: There are no references to many figures and tables in the text: figures - 2,4,5,7,12,13, tables - 1, 3. At the same time, the caption of Table 3 is located incorrectly; Table 4 has no caption at all.

Response: Thank you for your valuable suggestion. We have mentioned the references if we take figures from the exiting work. All figures where reference is not mentioned are screenshots taken from the linux machine or graphs which are generated by the authors themselves.

Comment 1.4:  Many figures and tables are either poorly commented or have no comments at all. So, Table 1 has no comments. Moreover, apart from it, there is nothing else in Subsection 2.3. This may be acceptable for a thesis, but it is not acceptable for a journal article! Figure 3 requires additional comments, as it is not clear why the O3 group cannot be considered as normal behavior. Fig. 4 has no comments. Classification in Fig. 5 is incomplete. There is no division into classes “Centralized”, “Decentralized” and “Distributed” in it. For Fig. 6, authors do not say where the logs came from. Fig. 7 is not informative, all the lines on it are the same. Dates in Fig. 8 and in Tables 3 and 4 do not match: in Fig. 8 - Apr 27, in Tables 3 and 4 - May 27. For Fig. 10, authors need to show the connection with 8 attempts. Table 6 and Fig. 12 do not specify the threshold value. Fig. 13 has no comments.

Response: Thank you for your valuable suggestion. The relevant changes have been made. cpu.log was a file made by the authors which captured the cpu percentages at an interval of 2 seconds using the cron jobs. This would be helpful in finding whether an attack took place based on whether there was an increase in the CPU percentages. The second screenshot of log files is a linux generated log file coming from /var/log/auth.log which gives us information about the sessions opened or closed in the linux system.

Comment 1.5: The first paragraph in Section 5 is broken.

Response: Thank you for your valuable suggestion. The relevant changes have been made.

Comment 2: The Introduction is short. It lacks a description of the goal, contribution (novelty), and structure of the article.

Response: Thank you for your valuable suggestion. We have added contribution, novelty and organization of the paper in the revised manuscript.

The main contribution of this paper is:

• Proposed system will be able to detect coordinated distributed attacks.
• Hosts in the network need to trust the data sent by other peers in the network. To bring in the concept of trust and implement the proof-of-concept, blockchain was used.
• Pluggable Authentication Modules (PAM) were also used to track login activity securely before an intruder could modify the login activity.
• To implement blockchain, an Ethereum based private blockchain was used

This paper is organized as follow: section 2 discusses the basics of Blockchain along with the different components of blockchain, section 3 discusses different exiting Intrusion Detection System, section 4 explains the proposed an improving collaborative IDS which uses Blockchain and pluggable authentication modules. Section 5 discusses the result analysis and section 6 summarize the entire work and give the direction for the future work.   

Comment 3: There are no mathematical expressions in the article. Based on the title of the article, the authors should show how the sustainability of Smart City is increasing. To evaluate sustainability, mathematical expressions are needed. Instead, the authors replace sustainability with trust. These are completely different properties! As a result, the authors moved away from sustainability and came to security! This is a methodological error!

Response: Thank you for your valuable suggestion. In this work, we have proposed an intrusion detection system that have implemented in real time scenario.

Comment 4: We need a Smart City scenario and an experiment on this scenario.

Response: Thank you for your valuable suggestion. Smart city scenario has been added in the updated manuscript.

The role of blockchain in smart, sustainable cities is vital because it helps to foster the kind of trust necessary for smart cities. Blockchain should serve as the cornerstone for the development of a smart city and is a crucial assurance for the proper design and execution of the management strategy and planning scheme. A smart city is naturally combining smart energy, smart transportation, smart government, and other services under the same umbrella Decentralization and the availability of clear data place strict constraints on the big data service platform. Finding the problematic node among the hundreds of millions of nodes in a network is a time-consuming operation if a network encounters a problem or is the target of an attack. Most current Internet of Things networks are centralized. A huge server or centralized cloud is connected to hundreds of millions of nodes, which causes bottlenecks in the price and compute storage capac-ity. Blockchain distributed technology can guarantee that even if one or more nodes are hacked, the total network data remains trustworthy and secure. And distributed computing makes use of point-to-point computing to handle the hundreds of billions of transactions that the Internet of Things generates. This significantly lowers the cost of computing and storage by utilizing the computing and storage capabilities of a large number of idle devices deployed in unused locations.

In order to increase the degree of security for secure transmission and safe storage, additional protection mechanisms need to be implemented due to the privacy of nu-merous people involved. Blockchain has shown to be secure, dependable, and suitable for this purpose. The disaster recovery system cannot be enhanced due to the high ex-pense of putting up a data center and data storage. Therefore, a key issue at hand is how to lower storage costs while enhancing disaster recovery capabilities. Blockchain, which connects distributed and centralized services, can successfully stop an attack on the vital network infrastructure

 

Comment 5: The Conclusion is taken from the thesis. It should be rewritten.

Response: Thank you for your valuable suggestion. The conclusion section has been updated in the revised manuscript.

The sharing of information is extremely crucial between the nodes in a CIDS sys-tem in order to prevent the system from attacks as a whole. Information sharing is ex-tremely important in a scenario where distributed attacks are taking place increasing-ly. CIDS along with blockchain appears to be highly suitable for the ingesting of data specially in the case of building a smart sustainable city. This paper showed that com-mercial and open-source blockchain technologies may be used to create an information sharing system that records both the doorknob rattling attack using Pluggable Authentication Modules and CPU utilization data as blockchain transactions. This also proves that a blockchain system can also be used as a logging mechanism for multiple machines and hence can be used to aggregate data which could be later processed for intrusion detection. This research provides positive indications that blockchain technology could be used on a large scale for solving the intrusion detection problem and building a CIDS at a very large scale. The most significant contribution made in this paper is that it provides an end-to-end proof-of-concept for CIDS. It also showed at an initial level that the attacks or intrusions can be detected using blockchain as a backbone of the CIDS framework. However, there is a need to consider the cost of setting up such a system and how sound it is. The proof-of-concept which was discussed in the literature was not implemented at an end-to-end level.

The main aim of this paper was to build an IDS which could be potentially used to detect system abnormalities and intrusions. There are several avenues which are left to explore in this paper for additional work. The main aim, going further would be to create a large-scale system which could detect anomalies, block them and trigger alerts to the system administrator. Further research is also required to see how the overhead cost of running the blockchain client would be taken care of. Currently, Ganache (a private blockchain running at a particular node) is used for testing and carrying out transactions in the blockchain. Later, public or other test nets could be used to carry out the system tests.

Comment 6: The review of related work was carried out only for works related to the blockchain. There are no references to works on sustainability of Smart City. Also, the number of references is very small (only 20). It needs to be increased to 45-50.

Response: Thank you for your valuable suggestion. We have added few references in the updated manuscript.

In the most recent years, many other IDS have been proposed [17-21]. These IDS can be used in any domain to identify intrusions or abnormalities, which can then lead to the development of a secure solution for smart cities. In the smart cities everything is connected to the internet so a smart IDS can play a significant role to provide the security for this created network. Moayad Aloqaily et al. [22], proposed an IDS for the securing the transportation. This IDS will help the vehicular service management to secure the network from the attach and ensure the quality-of-service availability. Mo-hamed Faisal Elrawy et al. [23], discusses the role of IDS and IoT in the smart envi-ronment. This article first discusses various existing work that have done to create the smart environment using IoT sensors and then discuss the existing IDS used to provide the security in IoT context. Asmaa Elsaeidy et al. [24], introduced smart IDS to prevent the distributed Denial of Service (DDoS) attacks for the smart cities. This article used Restricted Boltzmann Machines (RBMs) technique to design the IDS. Tanzila Saba et al. [25], proposed an ensemble-based IDS for the smart cities hospitals.

The current IDS is not sophisticated enough to detect distributed, parallel attacks that take place throughout the nodes in the network instead of a single node. In case of CIDS, the ability to correlate the events is crucial. The events occurring across all the nodes in the network must be aggregated for further processing and raising of alerts. The concept of trust is crucial among the nodes. While the discussed approaches have their advantages, there was clearly a lack of scalable architecture in the case of CIDS. The main aim of this paper is to demonstrate an approach through which a scalable architecture could be developed and the trust could be established among the nodes in the CIDS architecture. Blockchain was proposed to solve the trust issue in the case of CIDS. 

[17] C. Liang et al. “Intrusion Detection System for the Internet of Things Based on Blockchain and Multi-Agent Systems”, Electronics 2020, 9, 1120. Doi: 10.3390/electronics9071120

[18] A. Ghaleb et al. “Misbehavior-Aware On-Demand Collaborative Intrusion Detection System Using Distributed Ensemble Learning for VANET”, Electronics 2020, 9. Doi: 10.3390/electronics9091411

[19] Radoglou Grammatikis et al, “A Novel Multivariate Intrusion Detection System for Smart Grid. Sensors” 2020, 20. Doi: 10.3390/s20185305

[20] Iwendi, C.; Anajemba, J.H.; Biamba, C.; Ngabo, D. Security of Things Intrusion Detection System for Smart Healthcare. Electronics 2021, 10, 1375. doi: 10.3390/electronics10121375

[21] Kotecha, K. et al., “Enhanced Network Intrusion Detection System”, Sensors, 2021, 21. Doi: 10.3390/s21237835

[22] Moayad Aloqaily et al., “An intrusion detection system for connected vehicles in smart cities”, Ad Hoc Networks, vol. 90, July 2019. Doi: https://doi.org/10.1016/j.adhoc.2019.02.001.

[23] Mohamed Faisal Elrawy et al., “Intrusion detection systems for IoT-based smart environments: a survey”, Journal of Cloud Computing volume, vol. 7, 2018. Doi: https://doi.org/10.1186/s13677-018-0123-6

[24] Asmaa Elsaeidy et al., “Intrusion detection in smart cities using Restricted Boltzmann Machines”, Journal of Network and Computer Applications, vol. 135, 2019, pp. 76-83. Doi: https://doi.org/10.1016/j.jnca.2019.02.026

Comment 7: There are some inaccuracies. So, in the comments to Fig. 1 authors mixed up Distributed and Decentralized systems.

Response: Thank you for your valuable suggestion. We have made the relevant changes in the updated manuscript.

Author Response File: Author Response.docx

Reviewer 3 Report

I recommend authors to specify why they have chosen Etherium framework and provide a clear comparison with other private Blockchain platform, for example Exonum

Author Response

Response to Editor/Reviewers Comments

 Title: Improving Collaborative Intrusion Detection System using Blockchain and Pluggable Authentication Modules

The authors greatly appreciate the valuable and insightful comments made by the editor and reviewers and would like to thank them for their efforts. Their comments have undoubtedly helped us to improve the quality of our manuscript. We have carefully considered all the comments and the manuscript have been revised thoroughly to meet their expectations. Our point-by-point responses to each comment are listed below:

Reviewer: 3

Comment 1:  Recommend authors to specify why they have chosen Etherium framework and provide a clear comparison with other private Blockchain platform, for example Exonum

Response: Thank you for your valuable suggestion.

It is easily scalable and allow programmer to create the blockchain network from the scratch by using an all-in-one SaaS platform like Hyperledger Besu.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

In the future, when writing articles, authors should pay much more attention to revealing the worldwide novelty of the research.

Reviewer 2 Report

Some comments were omitted by the authors. Still, they need to be eliminated.

Here they are:

1. Page 2, 3^rd line  from the bottom: “In Distributed CIDS, the SPoF disadvantage is …” replace with “In Decentralized CIDS, the SPoF disadvantage is …”

2. Page 2, 1^st line from bottom: “Hence, we come to Decentralized CIDS.” replace with “Hence, we come to Distributed CIDS.”

3. Should be disclosed in the comments to Fig. 1, which is represented by the letters “A”, “M” and “MA” in this figure.

4. In Tab. 3 and Tab. 4: it should be "Wed Apr 27", not "Tue May 27" or "Wed May 27".

5. Page 12, line 9 from top: “The time instances were 10:40, 11:20 and 11:45.” replace with “The time instances were 10:40 and 11:35.”