In this thesis, we examine the security, performance, and privacy of Proof of Work-based (PoW) blockchains and digital currencies such as Bitcoin. The decentralized characteristics of blockchains have the benefit of removing trusted third parties; however, they create new challenges for security, performance, scalability, and privacy, which we investigate. The blockchain's security, for example, affects the ability of participants to exchange monetary value or participate in the network communication and the consensus process.In our first contribution, we observe the decentralized nature of Bitcoin and show that few individuals typically control vital operations in the Bitcoin ecosystem. Moreover, we show that a third party can unilaterally affect the fungibility of individual Bitcoins. Our second contribution provides a quantitative framework to objectively compare the security and performance characteristics of Proof of Work-based blockchains under adversaries with optimal strategies. Our work allows us to increase Bitcoin's transaction throughput by a factor of ten, given only one parameter change and without deteriorating the security of the underlying blockchain. In our third contribution, we highlight previously unconsidered impacts of the PoW blockchain's scalability on its security and propose design modifications that are now implemented in the primary Bitcoin client. In our fourth contribution, we investigate the privacy of lightweight Bitcoin clients, those that are the most critical to Bitcoin's mainstream adoption. Similarly, we propose appropriate design modifications that are being implemented to protect the user's privacy. Orthogonally, in our fifth contribution, we analyze the location privacy implications of public transaction prices. Surprisingly, we show that, given only a few prices from a consumer, we can accurately position the purchase location.

Given the increasing adoption of Bitcoin, the number of transactions and the block sizes within the system are only expected to increase. To sustain its correct operation in spite of its ever-increasing use, Bitcoin implements a number of necessary optimizations and scalability measures. These measures limit the amount of information broadcast in the system to the minimum necessary.In this paper, we show that current scalability measures adopted by Bitcoin come at odds with the security of the system. More specifically, we show that an adversary can exploit these measures in order to effectively delay the propagation of transactions and blocks to specific nodes-without causing a network partitioning in the system. We show that this allows the adversary to easily mount Denial-of-Service attacks, considerably increase its mining advantage in the network, and double-spend transactions in spite of the current countermeasures adopted by Bitcoin. Based on our results, we propose a number of countermeasures in order to enhance the security of Bitcoin without deteriorating its scalability.

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Abstract. In this article we present a new version of the Ocean Atmosphere Sea Ice Soil coupling software (OASIS4). With this new fully parallel OASIS4 coupler we target the needs of Earth system modelling in its full complexity. The primary focus of this article is to describe the design of the OASIS4 software and how the coupling software drives the whole coupled model system ensuring the synchronization of the different component models. The application programmer interface (API) manages the coupling exchanges between arbitrary climate component models, as well as the input and output from and to files of each individual component. The OASIS4 Transformer instance performs the parallel interpolation and transfer of the coupling data between source and target model components. As a new core technology for the software, the fully parallel search algorithm of OASIS4 is described in detail. First benchmark results are discussed with simple test configurations to demonstrate the efficiency and scalability of the software when applied to Earth system model components. Typically the compute time needed to perform the search is in the order of a few seconds and is only weakly dependant on the grid size.

Abstract-An internet user wanting to share observed content is typically restricted to primitive techniques such as screenshots, web caches or share button-like solutions. These acclaimed proofs, however, are either trivial to falsify or require trust in centralized entities (e.g., search engine caches). This motivates the need for a seamless and standardized internet-wide non-repudiation mechanism, allowing users to share data from news sources, social websites or financial data feeds in a provably secure manner.Additionally, blockchain oracles that enable data-rich smart contracts typically rely on a trusted third party (e.g., TLSNotary or Intel SGX). A decentralized method to transfer webbased content into a permissionless blockchain without additional trusted third party would allow for smart contract applications to flourish.In this work, we present TLS-N, the first TLS extension that provides secure non-repudiation and solves both of the mentioned challenges. TLS-N generates non-interactive proofs about the content of a TLS session that can be efficiently verified by third parties and blockchain based smart contracts. As such, TLS-N increases the accountability for content provided on the web and enables a practical and decentralized blockchain oracle for web content. TLS-N is compatible with TLS 1.3 and adds a minor overhead to a typical TLS session. When a proof is generated, parts of the TLS session (e.g., passwords, cookies) can be hidden for privacy reasons, while the remaining content can be verified.