Blockchain security and applications

Cryptocurrencies, such as Bitcoin and Ethereum, have proven to be highly successful. In a cryptocurrency system, transactions and ownership data are stored digitally in a ledger that uses blockchain technology. This technology has the potential to revolutionize the future of financial transactions and decentralized applications. Blockchains have a layered architecture that enables their unique method of authenticating transactions. In this research, we examine three layers, each with its own distinct functionality: the network layer, consensus layer, and application layer. The network layer is responsible for exchanging data via a peer-to-peer (P2P) network. In this work, we present a practical yet secure network design. We also study the security and performance of the network and how it affects the overall security and performance of blockchain systems. The consensus layer is in charge of generating and ordering the blocks, as well as guaranteeing that everyone agrees. We study the existing Proof-of-stake (PoS) protocols, which follow a single-extension design framework. We present an impossibility result showing that those single-extension protocols cannot achieve standard security properties (e.g., common prefix) and the best possible unpredictability if the honest players control less than 73\% stake. To overcome this, we propose a new multi-extension design framework. The application layer consists of programs (e.g., smart contracts) that users can use to build decentralized applications. We construct a protocol on the application layer to enhance the security of federated learning