Towards the Uses of Blockchain in Mobile Health Services and Applications: A Survey <s> The Blockchain Dilemma <s> Data is nowadays an invaluable resource, indeed it guides all business decisions in most of the computer-aided human activities. Threats to data integrity are thus of paramount relevance, as tampering with data may maliciously affect crucial business decisions. This issue is especially true in cloud computing environments, where data owners cannot control fundamental data aspects, like the physical storage of data and the control of its accesses. Blockchain has recently emerged as a fascinating technology which, among others, provides compelling properties about data integrity. Using the blockchain to face data integrity threats seems to be a natural choice, but its current limitations of low throughput, high latency, and weak stability hinder the practical feasibility of any blockchain-based solutions. In this paper, by focusing on a case study from the European SUNFISH project, which concerns the design of a secure by-design cloud federation platform for the public sector, we precisely delineate the actual data integrity needs of cloud computing environments and the research questions to be tackled to adopt blockchain-based databases. First, we detail the open research questions and the difficulties inherent in addressing them. Then, we outline a preliminary design of an effective blockchain-based database for cloud computing environments. <s> BIB001 </s> Towards the Uses of Blockchain in Mobile Health Services and Applications: A Survey <s> The Blockchain Dilemma <s> Technological innovation and consequential decentralisation are driving forces in the ongoing evolution and increasing openness of digital infrastructures and services. One of the most discussed and allegedly disruptive innovations is the distributed database technology referred to as blockchain. Although it is still in its technological infancy, experimental adoption and customization seem to be in full progress in various potential fields of application ranging from decentralized grids for computation and storage to global financial services. However, the technology and its path of development still entail a lot of common unknowns for practitioners and researchers alike. Especially regarding the question how the technology could amend or be incorporated into the existing landscape of digital services, processes and infrastructures. Hence, in this article we develop an ontology that (1) clearly delineates common terminology, core concepts and components, their relationships as well as innovative features of blockchain technology. It further (2) connects these insights with implications for relevant types of digital market models. Our framework is of high theoretical and practical value as it provides researchers and practitioners a common basis for communication and means for guided analysis of blockchain applicability. <s> BIB002 </s> Towards the Uses of Blockchain in Mobile Health Services and Applications: A Survey <s> The Blockchain Dilemma <s> Bitcoin and Ethereum, whose miners arguably collectively comprise the most powerful computational resource in the history of mankind, offer no more power for processing and verifying transactions than a typical smart phone. The system described herein bypasses this bottleneck and brings scalable computation to Ethereum. Our new system consists of a financial incentive layer atop a dispute resolution layer where the latter takes form of a versatile"verification game."In addition to secure outsourced computation, immediate applications include decentralized mining pools whose operator is an Ethereum smart contract, a cryptocurrency with scalable transaction throughput, and a trustless means for transferring currency between disjoint cryptocurrency systems. <s> BIB003
Inherent to the nature of the blockchain protocol itself, key characteristics such as real-time transaction record, immutable history, anonymity and its open APIs may translate into several desirable advantages to healthcare systems. As such, the main appeal associated with the integration of blockchain into existing healthcare systems would come in the form of increased interoperability between health service providers, increased access control by the user who owns the data and immutable history of accesses and changes to data. Obviously, the introduction of blockchain technology into the field brings several concerns and challenges, such as the cost of transition and continued operation, the means of storage, the consensus mechanics that are to be used and scalability concerns. These challenges have to be faced and addressed in order for this technology to form a credible and reliable mean of improving the capabilities of existing health systems. The advantages that come from the successful integration of blockchain into existing health systems have to be weighed against the cost of the integration process. Transitioning these systems to be able to take advantage of blockchain technologies may have a cost of adaptation and continued operation that is just too large in comparison to continuing to utilize the system as is. When considering the adaptation of blockchain to new systems, the structure and architecture of the system has to be designed from the ground considering the blockchain architecture. For example, factors such as, how new transactions are verified and added to the network, how the transaction verification system works, how changes are propagated to other participants of the network and how can access control and monitoring be achieved are dependent on the system that interacts with the blockchain. In addition, specifics to the cryptographic structure of the blockchain also have to be designed with its performance and reliability in mind. Aspects, such as, the format of the chain, how and what data is encrypted in the chain and, most importantly, how the blockchain is to be used to improve the system, also have to be given special attention. Most of these factors are complex on their own and slight changes in any of them can originate entirely different systems with different advantages and disadvantages. In fact, the precise cost of operating these new systems incorporated with blockchain technology is still not know, as well as the scalability concerns that may arise from the growing number of transactions recorded in the blockchain. This is due to these implementations still being in a proof-of-concept stage, as there is not enough test data to precisely conclude on the feasibility and even performance gains that these systems propose when com-pared against the cost of development in a real environment. Because of this, it becomes difficult to accurately gauge the extent to which these changes affect the performance of new systems that make use of the blockchain protocol especially regarding how the verification, addition and broadcast of new transactions to the other network participants would work. While there is a significant amount of theoretical data that seems to indicate advantages, this may prove to be less indicative of the advantages of early systems using blockchain than expected. There is the need for expectations and researches to be adjusted to reflect the results from tests made in production environments  . When considering blockchain storage, while the size of a block can be negligible, the combined size of the blockchain, replicated through several devices and storage, can have a very large impact in the way that a healthcare service provider handles its storage limits. A Bitcoin block is comprised of five main fields, these being a "magic" identifier of the message origin network, a command, a length, a checksum and a payload. Considering a very simplistic scenario of implementation of the blockchain protocol in a m-Health application, it can be expected that, as the user base of the application grows, the size of the chain becomes a problem. By taking this into consideration, both the transmission and storage of the whole chain becomes extremely complicated. One of the biggest challenges that future health systems with blockchain integration have to overcome is how to handle the verification process of transactions that are to be added to the chain. In traditional blockchain systems, such as Bitcoin, after a network user manages to solve the challenge associated with a new transaction, he then receives a compensation in the form of some value of Bitcoin. However, when adapting blockchain to a health system, this process becomes slightly more complex. Questions such as who will handle the verification of transactions, what kind of reward is given and how is the reward given, pose challenges that are difficult to solve without some sort of compromise. The following proposals are primarily meant towards m-Health systems, but can be transposed to traditional health systems with some adjustments. The easiest way to approach the aforementioned issues is to use the device of the patient as a network participant, using the resources of the device to verify transactions. In terms of reward for the patient, it would be the ability to use the application allowing more control over his data. While this may seem like a suitable solution to the problem, as the number of users increases, the number of transactions would be expected to increase even more, and challenges would increase over time. This, coupled with the relative low processing power of mobile devices and with the fact that only a small part of this processing power can be asked of from the user, increases the solving time of the challenges to a point where the systems would become unusable. Inversely, instead of using the patient's devices, the computational resources could be gathered from a larger infrastructure, like a national health system or a service provider backed and monitored by a government entity. This Table 1. TABLE THAT INDICATES THE ADVANTAGES THAT A   BLOCKCHAIN BASED IMPLEMENTATION CAN HAVE ON PROBLEMS THAT  MANIFEST IN CURRENT TRADITIONAL HEALTH SYSTEMS. approach, however, renders the decentralized nature of the blockchain system quite mute. Another approach to this problem is to provide the adaptation of blockchain to a m-Health application as a paid service. For example, consider an application that allows patients to control access to their health data, where the data is encrypted and stored in a cloud server accessible by several different health service providers. By having a user fee, much like in a form of health insurance, their healthcare data would be aggregated, encrypted and stored in a cloud server. When an attempt to access this data, the user is prompted to allow the access. This access request and response is then stored in a blockchain comprised of past access requests and the users fee serve as incentive for successful validation of the block by a participant in the blockchain network. In addition to choosing what approach to take, the consensus mechanism also needs to be chosen, having a deep impact on the morphology of the whole system  ,  . Due to the above-mentioned issues, it becomes clear that blockchain presents several scalabilities concerns that must be addressed in order for these systems to be able to withstand large user bases. In addition to this, implementations of the blockchain protocol still present security issues, with sybil attacks being a looming threat, and events such as  are still a reality. Another problem that has to be addressed is the cost of operation in the blockchain. While reading data from the blockchain is a fairly simple process, writing information into new blocks is costly in terms of time, computer resources and power. As such, while in a proof-of-concept phase this cost can be ignored as to test the technical viability of the system, it has to be addressed when considering an implementation in a real environment. Since every time a user medical data is changed it has to be recorded into the blockchain, the cost of writing this data becomes a considerable bottleneck. This can be amended by implementing second-layers to the blockchain, platforms and protocols that sit on top of the blockchain base and improve on the disadvantages of blockchain implementations BIB001 , BIB002 , with several rising ones such as [145] , BIB003 ,  . A resume of the above-mentioned challenges, advantages and constraints of applying blockchain to m-Health systems are presented in table 1 and 2. Table 2. TABLE THAT INDICATES BOTH THE BIGGEST CHALLENGES AND   THE MAIN ADVANTAGES OF IMPLEMENTING THE SPECIFIC SOLUTIONS   TO THE PROBLEMS MENTIONED IN TABLE 1 .