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.

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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.

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