The problem of scalability of blockchain platforms has raised a considerable amount of attention in the recent months. With the rise of the use of blockchain platforms and the prices of various cryptocurrencies hitting an all-time high in December of 2017, blockchain has now entered the eyes of the general public. Often seen as the future of digital transactions and applications, the decentralized platform faces many struggles in building networks capable of supporting the vast number of financial transactions modern systems currently handle, let alone serving as a replacement for the client-server model of the Internet. This paper introduces the basic structure of blockchain platforms that exist today, introduces the scalability problem, and examines the solutions that developers have proposed or implemented.

Cieľom tejto práce je navrhnúť platformu na spracovanie a preskúmavanie blockchainu vybraných kryptomien pri použití IPFS. Na riešenie tohoto problému bolo potrobné navrhnúť vlastný decentralizovaný a distribuovaný databázový systém, ktorý podporuje pokročilé dotazy. Vytvorené riešenie poskytuje prehľadné grafické užívateľské rozhranie, ktoré slúži na vizualizáciu dát a taktiež API, vďaka ktorému sa dá systém jednoducho napojiť na iné aplikácie. Prínosom tejto práce je nový pohľad na zpracovávanie blockchainu čo otvára nové možnosti v~jeho prehľadávaní.

Legal aspects of cryptocurrencies Abstract The aim of this work is to analyse the legal aspects of cryptocurrencies, especially focusing on their position within the legal system, legal binding of so-called smart contracts and their proper taxation in the field of personal income, legal entities and value added tax. The main content of the work is divided into six chapters. These six chapters are preceded by an introduction, which contains the reasons for choosing the topic, the chosen goal and the individual sub-goals and methods by which these goals are achieved. The last part is followed by a conclusion, which contains a summary of individual findings and recommendations for further research. The first chapter briefly deals with the history of cryptocurrencies, respectively describes the origin and life of two important precursors of the cryptocurrency bitcoin, E-Cash and E-Gold. The second chapter contains an analysis of the concept of cryptocurrency in relation to concepts such as currency, money, electronic money, digital and virtual currency. The content also includes the position of cryptocurrencies in the Czech legal system, an analysis of the fulfilment of the defining features of a security and the effects of their inclusion in the issue of combating the legalization of proceeds from crime. The...

Diese Dissertation untersucht die Auswirkungen der Blockchain-Technologie auf Geschäftsmodelle und Geschäftsprozesse anhand einer fallstudienbasierten Untersuchung von realen Phänomenen. Hierbei wird eine zentrale Forschungsfrage auf der Grundlage von vier grundlegenden Dimensionen eines Geschäftsmodells in Leitfragen zerlegt, die wiederum durch acht Forschungsbeiträgen mit Prozessablaufdiagrammen, Erklärungsmodellen, Taxonomien und einem 3-Schichtenmodell beantwortet werden. Die Grundlagen der Fallstudien sind Literatur Reviews zur Blockchain-Technologie, um auf dem aktuellen Stand der Forschung sowohl die Konzeptionierung von Fallstudien als auch die Analyse von realen Fallstudien durchführen zu können. Die Blockchain-Technologie wird als innovative Technologie bezeichnet und impliziert damit sozioökonomische Auswirkungen. Diese können in vielfältiger Weise auf die Unternehmen einwirken, ohne das bisher umfassende Untersuchungen aufgrund der unterschiedlichen Dimensionen eines Geschäftsmodells durchgeführt wurden. Die Blockchain-Technologie hat das Potenzial zur Disruption von bestehenden Geschäftsmodellen in vielerlei Hinsicht, es kann auf der einen Seite die Zusammenarbeit von Geschäftspartnern in einer Wertschöpfungskette wie beispielsweise der Logistik deutlich vereinfachen und auf der anderen Seite durch ein Initial Coin Offerings (ICO) als Crowdfunding über das Internet die Geschäftsbeziehungen von Start-ups mit Banken massiv verändern. Für bestehende Unternehmen am Markt als auch neuen Marktteilnehmern wie Start-ups besteht somit eine Unklarheit über die Auswirkungen der Disruption von bestehenden Branchenstrukturen und Geschäftsmodellen durch die Blockchain-Technologie. Dies ist zum einen der Komplexität der Blockchain-Technologie und deren vielfältigen Ausprägungen als auch zum anderen der Neuartigkeit des Phänomens zu verdanken, denn viele blockchain-basierte Geschäftsmodelle befinden sich in der Konzeptphase oder in einer Phase der Erprobung. Neben den zahlreichen vielversprechenden Whitepapers der blockchain-basierten Geschäftsmodellen gilt es, anhand eines konzeptionellen Modells die Ausgestaltung dieser Modelle zu analysieren und in einem Prozessablauf, Konzept- oder Erklärungsmodell die wesentlichen Auswirkungen darzustellen. Darüber hinaus ist die Gestaltung von Fallstudien und deren Konzeptionierung anhand des bestehenden Wissens über die Blockchain-Technologie verbunden mit aktuellen Herausforderungen eine weitere Möglichkeit, die Auswirkungen dieser Technologie darzustellen.

Indiana University-Purdue University Indianapolis (IUPUI) / Affordable and reliable supply chain visibility is becoming increasingly important as the complexity of the network underlying supply chains is becoming orders of magnitudes higher compared to a decade ago. Moreover, this increase in complexity is starting to reflect on the cost of goods and their availability to the consumers. Optimizing the physical distribution phase in supply chain by providing increased visibility to trading partners can directly reduce product cost. Current supply chain information systems often lack the ability to cost-effectively relay ground truth in- formation in near real time to all stakeholders and most importantly to the supplier and the customer during the transport of the shipment. This thesis presents a solu- tion that addresses this gap through a distributed architecture. The solution enables small, medium and large businesses to interact in a dynamic and shipment-centric manner through a private blockchain sub-ledger that digitizes the transfer of custody for each shipment. Information in this private ledger is augmented by a public event ledger that reflects the movement of the shipment in real time. Third party monitors are engaged in the validation of the geolocation of the shipments by posting their physical proximity in the form of events to the public ledger.

Blockchain techniques have received an outburst of interest both in academia and industry, with numerous benefits such as decentralization, persistency, anonymity and auditability. There has been widespread adoption of blockchain in various fields ranging from cryptocurrency, financial services, risk management, internet of things (IoT) to public and social services. As a distributed ledger, blockchain records data in the form of linked blocks secured by cryptography. Instead of being controlled by a central third party like a bank, this ledger is maintained by a P2P network. To work properly, it is essential that the nodes in the network have the same local view of the ledger. A consensus mechanism is the core of a blockchain system since it regulates how to achieve the same states among the nodes in a distributed fashion. Specifically, it grants a qualified miner the right of appending the ledger in a period of time. Despite its potential as a disruptive technology, blockchain is currently facing multiple challenges and problems, which incurs some issues on the performance, security, and fairness of blockchain-based systems. There are many underlying reasons for those challenges and problems, such as an inefficient networking protocol, a time-consuming consensus algorithm, a strategic mining node and so on. In this dissertation, we summarize our theoretical studies on the above-mentioned topic from three different aspects. Details are as follows: Design: A major weakness of the current blockchain systems is the low transaction throughput, which is resulted from the small block size, the long block generation time, and the delayed block propagation. To shorten the block propagation delay, we design a neighbor selection algorithm, improving the communication speed in the P2P network of the blockchain. This design is an attempt on topology optimization and reduces the frequency of blockchain forking. Forking is a situation where the blockchain temporarily diverges into two or more branches, which wastes mining power and causes security issues. Extension: Although a consensus mechanism aims to regulate mining nodes’ behavior, it also can be manipulate by strategic miners within legal limits due to their profit-driven nature. In the dissertation, we analyze strategies that miners apply for utility maximization under different consensus mechanisms from the perspective of game theory. And then we propose distributed algorithms to optimize resource allocation among miners. Application: We also propose a blockchain-based market where we believe the application of blockchain will enhance the corresponding applications. It is a blockchain-powered data market that allows multi-user cooperative search. We also integrate smart contract into these applications. All discussions in this dissertation are theoretical explorations. The theoretical results may not be realistically practical since we hold strong assumptions. We hope that, we could fill the gap between theory and practice in the future. / Computer and Information Science

En los últimos años, ha existido un creciente interés en la utilización de tecnologías inteligentes -propias de la denominada “Cuarta Revolución Industrial”- que ayuden a que las organizaciones se orienten hacia objetivos de sostenibilidad a largo plazo. Una de ellas es Blockchain, “disruptiva” desde su nacimiento, que implementada sola, o combinada con otras, podría mejorar la transparencia en las cadenas de suministros y, de esta forma, contribuir a una producción y consumo más sostenibles y responsables. En la presente Tesis nos preguntamos: ¿en qué medida puede llegar a impactar esta tecnología en los sistemas administrativos de las organizaciones? Asimismo, y con respecto a la información no tradicional que surja de nuevos sistemas integrados: ¿contribuirá para la toma de decisiones de sus directivos? ¿Podrá satisfacer las necesidades de los múltiples grupos de interés que interactúan con la organización? ¿Será creadora de valor? ¿Aportará a la sostenibilidad? ¿Cómo? Se abordaron todas estas cuestiones, partiendo del desarrollo de un marco teórico para definir el concepto de sostenibilidad social -soporte fundamental del presente trabajo-. Por medio de la metodología de múltiples estudios de casos se sistematizó la utilización de Blockchain a nivel mundial, principalmente en el sector primario -donde el uso de mano de obra intensiva es mayor y, por ende, la sostenibilidad social se encuentra más vulnerada en todos sus aspectos-. Sobre la base de la consulta a distintas fuentes, se estudió en profundidad la situación en Argentina en materia tecnológica, el marco legal existente y la utilización de Blockchain en el ámbito público y privado. Posteriormente, se procedió a relevar información sobre su puesta en marcha y utilización en las empresas argentinas a través de cuestionarios semiestructurados elaborados al efecto. Finalmente, se propusieron una serie de pautas para la integración de Blockchain en los sistemas administrativos en la idea de contribuir a la “sostenibilidad social”, considerando el rol que podría tener el Contador Público en la implementación y funcionamiento cotidiano de dicha tecnología en las áreas administrativas de empresas argentinas. / In recent years, there has been a growing interest in the use of smart technologies -typical of the so-called “Fourth Industrial Revolution”-, which help organizations to focus on long-term sustainability goals. One of those “disruptive” technologies since its emergence is Blockchain. Implemented by itself or combined with others, it can improve transparency in supply chains and, thereby, contribute to more sustainable and responsible production and consumption. In this thesis we ask ourselves: to what extent can this technology affect management systems? Likewise, and related to the non-traditional information produced by new integrated systems: will it contribute to their managers ́ decision-making? Will it be able to meet the needs of multiple stakeholders that interact with the organization? Will it be a value creator? Will it contribute to sustainability? How? All these issues were addressed, considering the development of a conceptual framework about social sustainability -which is the backbone of our approach. Through the methodology of multiple case studies, the use of Blockchain worldwide was systematized, mainly in the primary sector -characterized by intensive use of labour and infringement of social sustainainability in all its dimensions. Drawing information from different sources, the situation in Argentina in terms of technology, the existing legal framework, and the use of Blockchain in the public and private sectors were studied in depth. Then, information on its implementation and use in Argentine companies was collected through semi-structured questionnaires specially designed for this purpose. Finally, a series of guidelines were proposed for the integration of Blockchain in the management systems aiming to contribute to “social sustainability”, considering the role that accountants could have in the implementation and everyday operation with this technology in management areas of Argentine companies.

Despite the increasing interest in blockchain as a possible solution to replace centralized IoT architectures, previous work failed to provide a direct role for the sensing devices, i.e., direct interaction with the blockchain without additional components. Moreover, few studies focus on permissionless blockchains, even if it is the most secure platform for developing blockchain-based applications. This thesis presents an architecture that considers constrained sensing devices as direct actors on a public blockchain network. A public blockchain network allows the seamless inclusion of several unknown actors, and smart contracts provide a platform to develop complex IoT applications. The research followed an iterative DSR approach; designing, building, and evaluating new IT artifacts using two case studies in the agricultural IoT domain. These cases fostered two exploratory studies that diverged from the main IoT domain; however, they also provide novel contributions to blockchain-based applications. Thus, the novel architecture tackles three problems of current blockchain-based IoT systems i) constrained sensing devices as direct actors on a blockchain system, ii) permissionless blockchain networks and iii) smart contracts as an IoT application platform. Furthermore, the exploratory analyses examine two challenges of blockchain-based applications i) user experience and monetary costs and ii) data sharing and decentralized storage.

With current state-of-the-art privacy-preserving blockchain solutions, users can submit transactions to a blockchain while maintaining full anonymity and not leaking the contents of the transaction through cryptographic techniques like zero-knowledge proofs and homomorphic encryption. However, the architecture of a blockchain consists of a decentralized network where every network participant maintains their own local copy of the blockchain and updates it upon every added transaction. As a result, the volume of blockchain transactions and the timestamp of each blockchain transaction for an application is publicly available. This is problematic for applications with time-sensitive or volume-sensitive outcomes because users may want this information to be privatized, such as not leaking the lateness of student examinations. However, this is not possible with existing blockchain research. In this thesis, we propose a blockchain system for multi-party applications that does not leak any useful information from the volume and timing metadata of the application's transactions, including maintaining the privacy of a time-sensitive or volume-sensitive outcome. We achieve this by adding sufficient noise using indistinguishable decoy transactions such that an adversary cannot deduce which transactions actually impacted the outcome of the application. This is facilitated in a manner where anyone can publicly verify the application's execution to be correct, fair, and honest. We demonstrate and evaluate our approach by implementing a Dutch auction that supports decoy bid transactions on a private Ethereum blockchain network. / Master of Science / Blockchains are distributed, append-only, digital ledgers whose current state is continuously agreed upon through the consensus of network participants and not by any centralized party. These characteristics make them unique for many applications because they enable the application to be facilitated and executed in a public, verifiable, decentralized, and tamper-proof manner. For example, Bitcoin, the most popular cryptocurrency, uses blockchains to continuously maintain a permanent, verifiable ledger of payment transactions. However, one downside of this public architecture is that the volume of transactions and the timestamp of each transaction can always be publicly observed (e.g. the timestamp of every Bitcoin payment is public). This is problematic for applications with time-sensitive or volume-sensitive outcomes because users may want this volume and timing information to be privatized, such as not leaking the lateness of student examinations which could have severe consequences like violating student privacy laws. But currently with state-of-the-art blockchain research, privatizing this information is not possible. In this thesis, we demonstrate our approach that enables these time-sensitive and volume-sensitive applications to be implemented on blockchains in a manner that can maintain the privacy of these time-sensitive or volume-sensitive outcomes without sacrificing the application's integrity or verifiability. We then demonstrate and evaluate our approach through implementing a Dutch auction that supports decoy bid transactions on a private blockchain network.

Blockchain, the technology behind cryptocurrency, enables decentralized and distrustful parties to maintain a unique and consistent transaction history through consensus, without involving a central authority. The decentralization, transparency, and consensus-driven security promised by blockchain are unprecedented and can potentially enable a wide range of new applications that prevail in the decentralized zero-trust model. While blockchain represents a secure-by-design approach to building zero-trust applications, there still exist outstanding security bottlenecks that hinder the technology's wider adoption, represented by the following two challenges: (1) blockchain as a distributed networked system is multi-layered in nature which has complex security implications that are not yet fully understood or addressed; (2) when we use blockchain to construct new applications, especially those previously implemented in the centralized manner, there often lack effective paradigms to customize and augment blockchain's security offerings to realize domain-specific security goals. In this work, we provide answers to the above two challenges in two coordinated efforts. In the first effort, we target the fundamental security issues caused by blockchain's multi-layered nature and the consumption of external data. Existing analyses on blockchain consensus security overlooked an important cross-layer factor---the heterogeneity of the P2P network's connectivity. We first provide a comprehensive review on notable blockchain consensus protocols and their security properties. Then we focus one class of consensus protocol---the popular Nakamoto consensus---for which we propose a new analytical model from the networking perspective that quantifies the impact of heterogeneous network connectivity on key consensus security metrics, providing insights on the actual "51% attack" threshold (safety) and mining revenue distribution (fairness). The external data truthfulness challenge is another fundamental challenge concerning the decentralized applications running on top of blockchain. The validity of external data is key to the system's operational security but is out of the jurisdiction of blockchain consensus. We propose DecenTruth, a system that combines a data mining technique called truth discovery and Byzantine fault-tolerant consensus to enable decentralized nodes to collectively extract truthful information from data submitted by untrusted external sources. In the second effort, we harness the security offerings of blockchain's smart contract functionality along with external security tools to enable two domain-specific applications---data usage control and decentralized spectrum access system. First, we use blockchain to tackle a long-standing privacy challenge of data misuse. Individual data owners often lose control on how their data can be used once sharing the data with another party, epitomized by the Facebook-Cambridge Analytica data scandal. We propose PrivacyGuard, a security platform that combines blockchain smart contract and hardware trusted execution environment (TEE) to enable individual data owner's fine-grained control over the usage (e.g., which operation, who can use on what condition/price) of their private data. A core technical innovation of PrivacyGuard is the TEE-based execution and result commitment protocol, which extends blockchain's zero-trust security to the off-chain physical domain. Second, we employ blockchain to address the potential security and performance issues facing dynamic spectrum sharing in the 5G or next-G wireless networks. The current spectrum access system (SAS) designated by the FCC follows a centralized server-client service model which is vulnerable to single-point failures of SAS service providers and also lacks an efficient, automated inter-SAS synchronization mechanism. In response, we propose a blockchain-based decentralized SAS architecture dubbed BD-SAS to provide SAS service efficiently to spectrum users and enable automated inter-SAS synchronization, without assuming trust on individual SAS service providers. We hope this work can provide new insights into blockchain's fundamental security and applicability to new security domains. / Doctor of Philosophy / Blockchain, the technology behind cryptocurrency, enables decentralized and distrustful parties to maintain a unique and consistent transaction history through consensus, without involving a central authority. The decentralization, transparency, and consensus-driven security promised by blockchain are unprecedented and can potentially enable zero-trust applications in a wide range of domains. While blockchain's secure-by-design vision is truly inspiring, there still remain outstanding security challenges that hinder the technology's wider adoption. They originate from the blockchain system's complex multi-layer nature and the lack of effective paradigms to customize blockchain for domain-specific applications. In this work, we provide answers to the above two challenges in two coordinated efforts. In the first effort, we target the fundamental security issues caused by blockchain's multi-layered nature and the consumption of external data. We first provide a comprehensive review on existing notable consensus protocols and their security issues. Then we propose a new analytical model from a novel networking perspective that quantifies the impact of heterogeneous network connectivity on key consensus security metrics. Then we address the external data truthfulness challenge concerning the decentralized applications running on top of blockchain which consume the real-world data, by proposing DecenTruth, a system that combines data mining and consensus to allow decentralized blockchain nodes to collectively extract truthful information from untrusted external sources. In the second effort, we harness the security offerings of blockchain's smart contract functionality along with external security tools to enable two domain-specific applications. First, eyeing on our society's data misuse challenge where data owners often lose control on how their data can be used once sharing the data with another party, we propose PrivacyGuard, a security platform that combines blockchain smart contract and hardware security tools to give individual data owner's fine-grained control over the usage over their private data. Second, targeting the lack of a fault-tolerant spectrum access system in the domain of wireless networking, we propose a blockchain-based decentralized spectrum access system dubbed BD-SAS to provide spectrum management service efficiently to users without assuming trust on individual SAS service providers. We hope this work can provide new insights into blockchain's fundamental security and applicability to new security domains.