الفهرس 
Chapter 1Only 14 pages are availabe for public view
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Abstract
Blockchain is a Distributed Ledger Technology (DLT) that allows users to exchange values and make transactions directly without a need for a trusted third party. Bitcoin (BTC) is one of the most popular digital cryptocurrencies that is based on blockchain technology because of its decentralization, transparency, trust-less, and immutability features. However, blockchain can be considered the cause of Bitcoin scalability issues including transaction throughput limitation, transaction latency, transaction starvation, and the huge storage requirement. Bitcoin transaction throughput is very little compared to traditional payment methods. Additionally, many Bitcoin transactions suffer from delays and starvation as miners prefer transactions with higher fees. On the other hand, nodes in the Bitcoin network need to store the full blockchain to validate transactions. Over time, the blockchain size will be extremely huge. So, the full nodes will prefer to leave the network. This leads to the blockchain being centralized and trusted, and therefore, security will be adversely affected. Much of the current research focuses on how to enhance the scalability of Bitcoin by improving the performance of consensus techniques, dividing the network into smaller ones with different parts of the blockchain, or completely changing the data structure of the blockchain. Unfortunately, engaging in this problem usually affects either decentralization or security; this is called the blockchain Trilemma. In this thesis, we propose and develop a Cooperative Mining System (CMS), which depends on enhanced proof of work consensus algorithm to increase the transaction throughput and eliminate the transaction latency and starvation without affecting decentralization and security. Also, we propose a stateful layered chain model which is based on storing accounts’ balances to reduce the size of the Bitcoin blockchain. In CMS, miners cooperated to create one super block containing more transactions than the traditional Bitcoin block for each epoch. Whereas miners create their traditional blocks simultaneously, broadcast them, wait to receive other miners’ blocks, and lastly create a super block that contains all the transactions of the gathered blocks. The CMS and Bitcoin system simulations are developed using C# .NET programming language. The scalability is measured based on three parameters (number of peers, number of waiting transactions, and the difficulty level). The simulation results of the CMS and Bitcoin system using different case scenarios show a significant improvement of CMS compared to the current Bitcoin system, as the CMS has greatly increased the transaction throughput and eliminated the transaction latency. On the other hand, the proposed stateful layered chain model changes the structure of the traditional blockchain from blocks to layers. This model is implemented using 100 Bitcoin blocks from a height of 710327 to 710426. The experimental results demonstrated that this model reduces the size of the blockchain by about 50.6 %. Implicitly, the transaction throughput can also be nearly doubled using this model.