區塊鏈所建立的虛擬貨幣交易系統 (Virtual Currency Trading System)中，雙重支付攻擊 (Double-Spending Attack)是最被廣為討論的一種攻擊；最近的研究提出一套針對工作量證明 (Proof of Work, PoW)機制設計的個人化難度調整 (Personalized Difficulty Adjustment, PDA)規則，以對抗惡意礦工 (Malicious Miners)發動如此攻擊；另一方面，則將雙重支付攻擊與女巫攻擊 (Sybil Attack)結合，以有效減低於比特幣網路中發動雙重支付攻擊時所需的算力成本。於是，本論文首先對考量之攻擊做明確定義，且在工作量證明網路環境下，提出改善之防禦機制以補強PDA，此一防禦機制稱為EPDA-T (Enhanced PDA with the Target Computing Power to Consume)；透過適當的分析，EPDA-T及PDA之防禦效果可以被成功攻擊之機率來呈現，數值結果說明EPDA-T能夠比PDA在防禦效果上大幅提升，其可獲95%以上的改善，顯現EPDA-T對所考量之攻擊具優越之防禦能力。

In the blockchain-based virtual currency trading system, the double-spending attack is one of the most wildly discussed attacks. Recently, designed a personalized difficulty adjustment (PDA) strategy to counter the malicious miners to launch such an attack. On the contrary, combined the double-spending attack and the sybil attack to lower the computing power required to launch the double-spending attack. Therefore, this thesis first define the behavior of the attacker considered precisely in the Proof of Work (PoW) network. Then, the defending mechanism called the enhanced PDA with the target computing power to consume (EPDA-T) is proposed to strengthen PDA. The defending effect of EPDA-T and PDA can be demonstrated through the proper analysis in terms of the probability to be compromised. Our numerical results to be presented can reveal the superiority of EPDA-T over PDA in defending, showing that more than 95% of improvement can be achieved by EPDA-T as compared to PDA. This explicitly demonstrates that the excellent capability of EPDA-T to defend against the attack considered in this thesis.

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