# Journal:A scoping review of integrated blockchain-cloud architecture for healthcare: Applications, challenges, and solutions

Full article title A scoping review of integrated blockchain-cloud architecture for healthcare: Applications, challenges, and solutions Sensors Ismail, Leila; Meterwala, Huned; Hennebelle, Alain United Arab Emirates University Email: leila at uaeu dot ac dot ae Yu, Keping 2021 21(11) 3753 10.3390/s21113753 1424-8220 Creative Commons Attribution 4.0 International https://www.mdpi.com/1424-8220/21/11/3753/htm https://www.mdpi.com/1424-8220/21/11/3753/pdf (PDF)

## Abstract

Blockchain is a disruptive technology for shaping the next era of healthcare systems striving for efficient and effective patient care. This is thanks to its peer-to-peer, secure, and transparent characteristics. On the other hand, cloud computing made its way into the healthcare system thanks to its elasticity and cost-effective nature. However, cloud-based systems fail to provide a secured and private patient-centric cohesive view to multiple healthcare stakeholders. In this situation, blockchain provides solutions to address security and privacy concerns of the cloud because of its decentralization feature combined with data security and privacy, while cloud provides solutions to the blockchain scalability and efficiency challenges. Therefore a novel paradigm of blockchain-cloud integration (BcC) emerges for the domain of healthcare.

In this paper, we provide an in-depth analysis of the BcC integration for the healthcare system to give the readers the motivations behind the emergence of this new paradigm, while also introducing a classification of existing architectures and their applications for better healthcare. We then review the development platforms and services and highlight the research challenges for the integrated BcC architecture, possible solutions, and future research directions. The results of this paper will be useful for the healthcare industry to design and develop a data management system for better patient care.

Keywords: blockchain, cloud computing, electronic health records, health data analytics, healthcare system, security, privacy

## Introduction

The healthcare domain has been revolutionized over the last century by technological advancement.[1] This revolution aims to improve the diagnosis of diseases and their causes, the quality of medical supplies, and the quality of medical treatment, as well as establish prevention plans on a global scale. The traditional client-server-based healthcare systems[2][3][4][5][6] suffer from security and privacy issues and lead to scattered patient’s medical history, delaying patient treatment.[7][8] Moreover, a patient needs to repeat medical tests when moving to another hospital. This increases the cost and time to the patient and affects the patient’s health due to repeated exposure to tests, such as X-rays and MRIs, each with their own potential side effects.[9] In addition, healthcare organizations are required to install and maintain infrastructure with up-to-date functionalities while complying with healthcare standards and regulations for the management of electronic health records (EHRs). This leads to a high total cost of ownership. To address these limitations of the client-server-based approach, the on-premises database migrated to the cloud, where health records are maintained by a cloud service provider.

Cloud computing[10] allows convenient and on-demand network access to a shared pool of configurable computing resources. Motivated by the pay-as-use cloud model, medical organizations use cloud computing to manage EHRs, reducing the cost of ownership. The five-year cost of $11 million for an on-premises healthcare system can be reduced to$3.2 million using cloud. This also reduces the infrastructure set-up time from 16 weeks to one week (Healthcare system cost reduction using cloud-based approach.[11] In addition, cloud computing provides multiple healthcare providers with efficient access to health records from a shared storage, improving patient care. The number of health records is increasing at a rapid pace with the introduction of smart healthcare and internet of things (IoT) devices with biosensors for personalized patient-centric healthcare. The scalability and elasticity of cloud computing aid in health records management, which requires powerful computing and large storage, for near real-time patient care. However, a cloud-based system suffers from the issues of security, i.e., data integrity, where the health records are under a constant threat of being modified, and privacy, i.e., unobservability or data leakage, in which the patients’ health records are being used without any tracking.[12]

Recent years have witnessed a bit of a revolution, with blockchain being adopted for many applications in the healthcare domain, such as health records management[13][14][15][16][17], medical supply chain management[18][19], and medical insurance claim management.[20][21] The characteristics of blockchain give it great potential for providing a patient-centric healthcare system, involving health stakeholders such as patients, health professionals, insurance providers, pharmaceutical firms, and health governmental authorities.

From a technical aspect, blockchain is a peer-to-peer distributed system, which enables users to maintain a ledger of transactions that is replicated over multiple servers.[22] The architecture allows all the network participants, i.e., healthcare stakeholders, to verify and process health data transactions without the need for a trusted third party. In addition, the data stored in the blockchain is immutable, i.e., once the data is stored, it cannot be modified or deleted, leading to enhanced security. This immutability enables an audit trail, bringing in accountability, adding trust to the system, and alleviating privacy concerns.[23][24] These distinctive features of blockchain have triggered its wide adoption for health records management to address information security and privacy issues, while providing access to patient’s health history to multiple stakeholders for patient-centric health services. However, blockchain poses scalability issues as the network grows[25], and consequently more hardware and human resources have to be provisioned for the operation and maintenance of the blockchain platform, thus increasing the health organization’s on-site cost. Moreover, blockchain suffers from the issues of high energy consumption[26][27], adding to blockchain operational cost.

Several works in the literature study the strengths and weaknesses of stand-alone cloud- or blockchain-based healthcare systems. To tackle security and privacy issues prevailing in cloud healthcare systems, the scalability issues inherent from the blockchain algorithms, and to develop more robust solutions for efficient patient care, research efforts have proposed applications using an integrated blockchain-cloud (BcC) paradigm, as shown in Figure 1. However, to the best of our knowledge, there is no work that analyzes the underlying BcC architectures or gives insights and research directions on how they can be enhanced for a patient-centric healthcare system.

 Figure 1. Evolution of the healthcare system toward an integrated blockchain-cloud (BcC) paradigm

The main contributions of this paper are as follows:

• We present the limitations of a healthcare system that is based on either cloud or blockchain and highlight the importance of implementing an integrated BcC system for better patient care.
• We present a scoping review and devise a taxonomy of existing integrated BcC healthcare system architectures into two different types based on the nature of integration. We analyze the effectiveness and limitation of these architectures.
• We compare and analyze blockchain as a service (BaaS) platforms provided by different cloud service providers.
• We identify the research challenges prevailing in an integrated BcC healthcare system and possible solutions that are proposed for these issues.

The rest of the paper is organized as follows. The next section presents an overview of related work. Afterwards, the concept of cloud computing, blockchain, and the importance of an integrated BcC healthcare system is discussed. The fourth section present a taxonomy, strengths, and weaknesses of the integrated BcC architectures, followed by an examination of different healthcare applications using the integrated BcC architecture, and then a discussion of the research challenges in BcC system, and possible solutions. Discussion and conclusions are presented in the final two sections.

## Related work

Traditional healthcare systems are based on the client/server approach, where the patients’ health records are stored in a hospital’s centralized database. Later on, the healthcare system migrated to the cloud-based approach, where the patients’ health records are stored and managed in third-party cloud storage by a cloud service provider. This change was intended to solve the issues of scalability, high cost, fragmented patient records, and repeated medical testing problems associated with the client/server-based approach. Several works in the literature present a review on this type of cloud computing-based healthcare.[28][29][30][31] Hu and Bai[28] in their review classify the work on cloud-based healthcare into three categories depending on the area of focus: a framework for data sharing, healthcare applications, and security and privacy. The authors highlight the issues of data integrity and confidentiality and suggest that a hybrid cloud model with appropriate access control can be a reliable solution. Ali et al.[29] review applications of the cloud-based healthcare system and highlight the issues of security and privacy with the involvement of a centralized third party, i.e., the cloud service provider. Mehraeen et al.[30] in their review on security challenges in cloud-based healthcare systems advise having authentication, authorization, and access control to ensure data security. The authors found that identity management, internet-based access, cybercriminals, authorization, and authentication are the major concerns in the cloud-based healthcare system. Ermakova et al.[31] classify the literature on cloud-based healthcare into four contribution categories: framework development, application development, broker development, and security and privacy mechanisms development. However, they found the broad cloud-based approach suffers from the issues of centralization, data security, and privacy.

Blockchain, a peer-to-peer network, solves the issues existing in the cloud-based approach. This is because of the immutability, replication, and decentralization characteristics of the blockchain. Several authors have contributed to the literature concerning the blockchain-based healthcare system.[32][33][34][35][36] Hölbl et al.[32] analyze the literature based on the contributions of blockchain, i.e., framework/architecture, algorithms, consensus mechanisms, bench-marking metrics, and applicability. The application domains are classified into data sharing, access control, audit trail, and supply chain. The authors highlight the significant use of the technology for data sharing and access control compared to that for supply chain management and audit trail. Kuo et al.[33] present a systematic review of different blockchain platforms with healthcare examples, highlighting the platforms’ features such as network type, consensus protocol used, hardware requirements, smart contract support, transaction throughput, scripting languages, and open source support. Agbo et al.[34] categorize the work in the literature based on their areas of contributions such as electronic medical records (EMR) sharing, supply chain management, biomedical research and education, remote patient monitoring, health insurance claims, and health data analytics. The authors reveal that the application of blockchain technology in the area of healthcare is limited due to the issues of interoperability, scalability, execution time, and patient engagement. Vazirani et al.[35] assess the feasibility of blockchain for efficient EHR management and conclude that the use of blockchain for healthcare can solve the issues of interoperability, security, and privacy. Hussien et al.[36] categorize the work on blockchain-based healthcare according to their applicability such as clinical/medical data sharing, remote patient monitoring, clinical trials, and health insurance.

Table 1 shows the existing reviews on either cloud-based or blockchain-based healthcare systems. However, to the best of our knowledge, no work analyzes the concept of an integrated BcC architecture for healthcare. In this paper, we highlight the motivation for the integrated BcC architecture for better patient-centric healthcare. In addition, we discuss the evolution of these architectures and capture the assumptions that led to their development. We provide a classification of the integrated architectures and their applications for better healthcare. We then review the BcC developments platforms and services and highlight the research challenges, possible solutions, and future research directions.

Work Healthcare system approach Area of focus Contribution(s) Table 1. Related reviews Hu and Bai[28] Cloud computing Cloud computing in e-health Categorization of cloud-based works, organized into four studied areas: (1) framework, (2) application, and (3) security and privacy. Ali et al.[29] Cloud computing Opportunities, challenges and applications of cloud-based healthcare Analysis of cloud computing-based healthcare in terms of opportunities (management and technical), issues (technical, legal, security, and privacy), and applications (information processing and monitoring). Discussion of research and implementation implication of the system. Mehraeen et al.[30] Cloud computing Security challenges in cloud-based healthcare Investigation of security challenges and recommendations for secure communication and interoperability in cloud-based healthcare. Ermakova et al.[31] Cloud computing Cloud computing in healthcare Categorization of cloud-based works based on four contribution areas: (1) framework development, (2) application development, (3) broker development, and (4) security and privacy mechanisms development. Hölbl et al.[32] Blockchain Blockchain in healthcare Categorization of blockchain-based works based on contributions (framework/architecture, algorithm, consensus protocol, and bench-marking metric) and applicability (data sharing, access control, audit trail, and supply chain management). Kuo et al.[33] Blockchain Blockchain platforms, with healthcare as an example. Comparison of blockchain platforms based on the following features: (1) network type, (2) consensus protocol used, (3) hardware requirement, (4) smart contract support, (5) transaction throughput, (6) scripting language, and (7) open source support. Agbo et al.[34] Blockchain Blockchain in healthcare Overview of blockchain and categorization of blockchain-based work based on six contribution categories: (1) EMR sharing, (2) supply chain management, (3) biomedical research and education, (4) remote patient monitoring, (5) health insurance claim management, and (6) health data analytics. Challenges and limitations of a blockchain-based healthcare system Vazirani et al.[35] Blockchain Blockchain implementation in healthcare Assessment of blockchain feasibility for efficient EHR management. Hussien et al.[36] Blockchain Taxonomy, challenges, and recommendations for a blockchain-based healthcare system Overview of blockchain and categorization of blockchain-based work based on four types of applicability: (1) clinical/medical data sharing, (2) remote patient monitoring, (3) clinical trials, and (4) health insurance. Motivation, challenges, and recommendation for a blockchain-based healthcare system. This work Integrate blockchain-cloud (BcC) Taxonomy of integrated BcC healthcare system architectures, challenges, and solutions Importance of an integrated BcC healthcare system and taxonomy of existing BcC architectures. Comparison of integrated BcC platforms. Survey of different healthcare application domains benefited by integrated BcC. Discussion of issues existing in an integrated BcC healthcare system, along with possible solutions for future research directions.

## Background and motivation

In this section, we first explain the fundamental concepts of cloud and blockchain technologies to aid in a better understanding of the rest of the paper. Then, we highlight the motivation of an integrated BcC healthcare system.

### Background

#### Cloud computing

Cloud computing technology offers a shared pool of configurable hardware resources and software services over the internet.[10] These resources can be speedily allocated and released without the system administrator’s intervention. Cloud computing is mainly characterized by on-demand service, rapid elasticity, pay-per-use model, and multi-tenancy. Figure 2 shows the general overview of the cloud system architecture.

 Figure 2. Overview of a cloud system architecture

Broadly speaking, a cloud architecture consists of

1. cloud consumers who are individual users (patients and allied healthcare professionals) and/or organizations (hospitals) that use the cloud services;
2. a cloud broker that enables the communication between the cloud consumers and the cloud; and
3. a cloud entity that makes the cloud services available to the consumers.

This architecture can then be organized into three layers: a physical resource layer, a resource abstraction and control layer, and a service layer. The physical layer consists of the hardware resources for processing, storage, and networking, and the facility resources for cooling, ventilation, power, and supply. The resource abstraction and control layer consists of the system components that enable access to the physical resources through a software abstraction. Abstraction components include virtual computing and virtual storage elements. This layer is also responsible for the efficient allocation and usage monitoring of the physical resources. The service layer consists of the interfaces required to access the cloud services. These cloud services can then be classified into software as a service (SaaS), platform as a service (PaaS), and infrastructure as a service (IaaS). SaaS makes software available remotely to multi-tenant users as a web-based service, e.g., Google Mail. PaaS provides the environment and tools required to develop web-based applications, e.g., Amazon Web Services (AWS). IaaS offers virtualized hardware hosted in cloud data centers to the end-users for operations. The hardware involves storage, computing servers, and network components. NTT Communications is an example of IaaS.

The cloud network can be divided into multiple categories. The three main categories are:

• Public cloud: Allows public access to systems and services without any restrictions and is less secure.
• Private cloud: Allows members of the organization that manages the cloud to access the systems and services and is more secure than a public cloud. A private cloud when shared among multiple organizations is known as a community cloud.
• Hybrid cloud: Combination of a public and private cloud that enables greater flexibility. The critical and confidential activities can be managed using the private cloud, while the general activities can be managed using the public cloud.

With the emergence of cloud computing, the healthcare system migrated from client-server-based infrastructure to cloud-based infrastructure. Cloud solves the issues of fragmented health records and the high total cost of ownership existing in the client-server-based healthcare system. This is thanks to the on-demand access, replication, and pay-as-you-go characteristics of the cloud. A cloud-based healthcare system is implemented using a private cloud to allow only authorized data access based on access control rights. Several cloud-based healthcare systems are proposed in the literature, where a patient or allied health professional can obtain a cohesive view of the patient’s medical history stored in third-party cloud storage.[37][38][39] Although the cloud-based approach improves system scalability and reduces the total cost of ownership, the health records managed by the cloud service provider are under constant security and privacy threats.[40][41] The patients’ records can be tampered with or can be accessed without their knowledge.[11] Consequently, a more robust healthcare management system is required to address the shortcomings of the cloud-based approach.

#### Blockchain

Blockchain is a peer-to-peer distributed system that maintains a synchronized ledger of transactions that is replicated over network participants. It was introduced for the exchange of e-currency in a network without the intervention of a third party.[42] Since then, blockchain has spread in several application domains such as healthcare, education, industry and marketplace, digital media, government, and entertainment. Blockchain has the following properties:

• Decentralization: Blockchain eliminates the intervention of a third-party entity for the processing of transactions and maintaining the ledger data. The transactions are validated and executed by the agreement of the majority of the participants that maintain the network.
• Immutability: The blockchain is a continuous chain of blocks where a block is connected to its preceding block by including the hash of the latter while hashing the former. A block is composed of a block header consisting of metadata and a block body consisting of valid transactions.[22] If a malicious entity attempts to tamper with the data of a block in the past, the hash of the block will change leading to a different hash value than the one used to calculate the hash of the succeeding block. Consequently, the malicious entity needs to re-hash all the subsequent blocks in the chain up till the last block. This re-hashing is compute-intensive, especially when there are several replicated copies of the ledger in the network. Thus, any data modification attempt is discouraged, leading to immutability.
• Transparency: Each operation performed in the network to access the data stored in the ledger is considered as a transaction in the blockchain. Each node in the network that holds the copy of the ledger can track any unauthorized or malicious data access, making the blockchain secure and transparent.
• Traceability: The replicated ledger in the blockchain enables efficient tracing of any transaction by the nodes maintaining the ledger. This discourages any malevolent activity, making the network more secure, efficient, and transparent.
• Consensus: Each transaction in the blockchain is verified and processed by the agreement of most of the participants holding the ledger copy. This enables transactions between participants who do not know and trust each other.

Figure 3 shows how a transaction is processed in the blockchain network. To initiate a new transaction, the transaction data is hashed by the transaction initiator, such as allied health professionals and patients. The digital signature of the transaction is generated by encrypting the hashed data. The encryption is performed using the private key of the transaction initiator. The transaction data and the corresponding digital signature are broadcasted to the network for processing. Each validating node in the network validates the transaction when received. This is by ensuring the authenticity of the transaction initiator and the integrity of the transaction data. The authenticity is verified if the digital signature is successfully decrypted using the transaction initiator’s public key. The integrity is verified if the hashed data obtained from the decryption operation matches the hash of the transaction data. The transaction, if valid, is broadcasted in the network to include it in the block. A miner (node that generates a block) creates a block of the received valid transactions after verifying each transaction for its validity. The selection of a miner that generates a block and the procedure of verifying and appending the generated block to the chain depends on the consensus protocol used by the blockchain network. The consensus protocols in blockchain are classified into compute-intensive-based, capability-based, and voting-based.[22] The selected miner generates the hash of the block, also known as the digital signature, and broadcasts the block in the network. The block’s hash is generated by first hashing the block header and then hashing the obtained hashed value. The version in the block header represents the version of the protocol used and the timestamp represents the block generation time. The Merkle root is a single hash value obtained from iterative pair-wise hashing of the transactions in the block data. Each validating node will update their ledger copy by adding the block if valid.[22]

 Figure 3. Processing of a transaction in blockchain

The blockchain network can be a public, private, consortium, or hybrid. The public network is the one where any entity can join the network with no prior permission and view the transaction data. On the other hand, a private network, organized by a single organization, is one where participation is subjected to prior permission and the data can be accessed based on access control rights. A private blockchain is suitable for healthcare as only authorized members can join the network and the ledger is queried and updated using access control rights. A consortium blockchain is such that a group of predetermined organizations governs the network. A hybrid blockchain lies somewhere between the public and the private versions, where the ledger can be viewed by any network participant, but the modifications to the ledger are subject to access control. The distinctive features of the blockchain described above are promising for the healthcare domain. A blockchain-based healthcare system has the following benefits:

• Provenance: The immutable blockchain ledger enables an audit trail, increasing the trust in the network. Any fraud in the network along with its source can be easily traced. This discourages malicious activities.
• Protection against natural disasters: In case of natural disasters such as forest fires, hurricanes, and floods, a database and its regional replicas might be unavailable. In such a scenario, the globally replicated blockchain ledger can aid in fault tolerance.
• Real-time data access: Patient’s health records can be accessed in real-time from the local or the nearest copy of the ledger to avoid life-threatening situations.
• Accurate patient care: The cohesive view of a patient’s health records provided by the blockchain enables allied health professionals in making a better prognosis/diagnosis.

Several blockchain-based healthcare data management systems have been proposed in the literature.[13][14][15][16][17] However, with the increasing amount of health records, the scalability[25][43] and energy consumption[26][27] of blockchain is an issue. In addition, on-premises blockchain deployment increases the total cost of ownership for healthcare organizations.

### Motivation of integrated BcC for healthcare

Security and privacy are the main requirements for an effective, trustworthy, patient-centric, and accurate healthcare system. The cloud-based system provides scalability and cost-effectiveness for managing ever-growing health records. However, security and privacy threats become a critical issue due to the involvement of a third-party service provider. Consequently, the healthcare domain seeks a more robust solution for the management of health records. Blockchain, a peer-to-peer network, allows transactions between multiple network participants, eliminating the need for a third party. Every event in the network is recorded on an immutable ledger, which is replicated over multiple network nodes. Blockchain enables transparent auditing, authorized data access, and immutability, thus providing secure and private management of health records. However, the scalability and the total cost of ownership question the implementation of blockchain in the healthcare domain where the number of health records is continuously increasing. As such, integrating cloud with blockchain (BcC) in healthcare enhances scalability and reduces cost while maintaining the security and privacy of health records.

Recently, there has been growing interest in AI-based healthcare, where the health records are analyzed using AI and machine learning algorithms to support allied health professionals with better prognosis and diagnosis of diseases. The accuracy of the AI and machine learning can be improved, resulting in a more accurate diagnosis and prognosis of a disease when more instances of data are used for training the models. In this context, an integrated BcC healthcare system would certainly revolutionize the way health professionals provide patient care. Blockchain would facilitate private and secure integration of data from multiple hospitals leading to a rich, secure and accurate database for the AI models, and cloud computing would enhance the scalability of the system. The incorporation of AI within an integrated BcC healthcare system could lead towards a better patient-centric, secure, and private healthcare, where the high availability of data from multiple sources, thanks to blockchain, can aid in better diagnosis and prognosis of disease using AI and machine learning techniques in a scalable cloud environment.

## Taxonomy and the strengths and weaknesses of integrated BcC healthcare system architectures

The individual benefits of cloud and blockchain technologies have led to the emergence of integrated BcC architectures, where the limitations of the standalone approaches are addressed. In this section, we present an analysis and classification of those architectures, and we compare the BcC development platforms and services.

### Encapsulated architecture

In this architecture, the blockchain platform and its underlying implementation are encapsulated within a cloud environment, as shown in Figure 4.

 Figure 4. Encapsulated BcC architecture for healthcare.

We formulate encapsulated architecture as:

${\displaystyle Encapsulated\ architecture=\left\{Cloud\mid Blockchain\in Cloud\right\}}$

This architecture has been proposed by several works in the literature.[44][45][46][47][48][49][50] The network participants (users) are the different health stakeholders, including allied health professionals, patients, health insurance companies, pharmaceutical firms, and health governmental authorities. The allied health professionals include doctors, nurses, dietitians, medical technologists, therapists, and pathologists. The users can connect to the platform via remote procedure call (RPC), representational state transfer (REST) application programming interface (API), web API, or SOAP (Simple Object Access Protocol). Health records can be generated by the allied health professional upon a patient’s visit or by the patient using sensors. A gateway device is used to process the sensor data. The cloud platform consists of a certificate authority, security management module, and operation management module, in addition to the blockchain (as a service). The security management module involves identity and access management, cloud firewall, and web application firewall, and the operation management module includes bill management, data replication and recovery, resource monitoring (CPU, memory, and storage usage), and a log service. The blockchain encapsulated within the cloud consists of an application layer, distributed computing layer, and storage layer. The blockchain ledger in the cloud database is stored using the InterPlanetary File System (IPFS)[51] or Storj Decentralized Cloud Storage.

The health transaction execution flow in this architecture is as follows:

Step 1: A transaction initiator (network participant) hashes the health record (transaction payload).
Step 2: The digital signature of the payload is generated by encrypting the hashed transaction.
Step 3: The transaction payload along with the digital signature is broadcasted to the blockchain nodes running in the cloud instances.
Step 4: The transaction is validated, and the block is generated based on the consensus mechanism.
Step 5: The block is updated to the ledger.

## Conclusions

In this paper, we highlight the importance of an integrated BcC healthcare system and present a taxonomy of BcC architectures. We also compare the BaaS platform offered by different cloud service providers. Additionally, we highlight the issues existing in the integrated architecture and present possible solutions for future research directions. In summary, the integration of cloud and blockchain for healthcare is promising to cope with the shortcomings of these individual technologies. Further research is still required to enhance the existing architecture to make it more scalable and energy-efficient with inter-cloud communication support.

## Acknowledgements

### Author contributions

Conceptualization, L.I.; methodology, L.I.; investigation, L.I., H.M. and A.H.; writing—original draft preparation, L.I., H.M. and A.H.; writing—review and editing, L.I. and H.M.; supervision, L.I.; project administration, L.I.; funding acquisition, L.I. All authors have read and agreed to the published version of the manuscript.

### Funding

This research was funded by the National Water and Energy Center of the United Arab Emirates under grant number 31R215.

### Conflicts of interest

The authors declare no conflict of interest.

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## Notes

This presentation is faithful to the original, with only a few minor changes to presentation, spelling, and grammar. In some cases important information was missing from the references, and that information was added. The original article pastes multiple URLs into the text body for some reason; for this version, most of those URLs were turned into formal citations, raising the citation count notably above the original 92. At the time of loading this article (30 October 2021) the link to SAP Cloud Platform Blockchain Service in the original article is dead, and a new URL could not be found; it's possible SAP discontinued its blockchain offering. The original article's citation #73 is said to be included in Table 3, but it unfortunately is not; that citation is removed for this version.