Bitcoin Security: Enhancing Electronic Cash System Using Proof-of-Work (PoW)

Electronic payments normally rely on a trusted institution to stop the same money from being spent twice. Bitcoin, proposed by Nakamoto in 2008, removes that institution by letting network participants agree on one transaction history through proof-of-work (PoW). This paper studies how PoW strengthens the security of an electronic cash system. It gives a layered view of a PoW-secured system covering digital signatures, hash-linked blocks, the mining puzzle, difficulty adjustment, and the rule that selects the chain with the most accumulated work. It then analyses double spending using the catch-up probability published by Nakamoto, maps the main attacks (majority hash power, selfish mining, eclipse attacks, and zero-confirmation fraud) to the countermeasures that resist them, and discusses the limits set by energy use, throughput, and privacy. The work is a design and literature study: it reports no new experimental results, every statement about earlier work is attributed to its authors, and the numerical values are computed from a published formula. Four figures and seven tables summarise the architecture, the block structure, the mining flow, the attack probabilities, the threats, and the limitations.