近年來，隨著網路科技的不斷發展，雲端儲存機制越來越流行。傳統的物理存儲不但設備昂貴，能夠使用的空間也有限。然而只要使用雲端儲存，就能以較低的價格租到足夠的空間，而且出門在外要使用資料時也方便很多。不過，有著中心控管的雲端，卻有諸多弊端。一是中心的權利過大，可以對用戶的資料動手腳，而用戶卻沒有辦法阻止。二是將所有資料集中儲存，也有很大的安全隱患。一旦中心的服務器受到影響，所有的資料都將處在危險之中。考慮到區塊鏈的去中心化和安全性，現在越來越多的雲端使用區塊鏈來做資料的儲存。然而這中間還有一個重要的問題。一般來說，一個完整的資料庫至少需要新增、刪除和修改功能。但是由於不可篡改性，區塊鏈的資料庫無法做到刪除和修改功能。因此我們提出使用側鏈的方式，來彌補這個缺陷。在我們的機制中，用戶可以將自己用不到的空間拿來出租獲得報酬。而資料擁有者也可以以較低的佣金租到足夠的空間。同時，為了減少維護成本，我們還用到變色龍雜湊函數來回收過期的側鏈。此外，我們也將一個公開稽核機制修改成了去中心化的版本，以檢查存放在雲端上資料的完整性。

Due to the development of network technique, the cloud storages become more and more popular. Traditional physical memories are expansive, and with limited usable storage space. If the cloud storages are adopted, one can rent large storage space in a lower price. However, centralized cloud has a number of risks. First, the right of the cloud center is over-large, and it means the cloud center can modify or delete data as it wants without being detected. Secondly, once the central services are compromised, all the data are also disclosed. Due to propertied decentralized immutability, more and more cloud storages adopt blockchain technology to store data. Traditionally, a complete database has at least three functions: Creation, Modification, and Deletion. However, since a blockchain is immutable, Deletion and Modification functions are difficult to be realized. Hence, we propose a storage architecture based on blockchain with sidechains to overcome the above limitations. In our scheme, users are able to provide spare storages to gain rewards. Meanwhile, we adopt a Chameleon-Hash function to reclaim the expired sidechains to reduce maintenance cost. Besides, we also proposed a public decentralized auditing scheme to verify the integrity in blockchain.

論文審定書 i
Acknowledgments iv
摘要 v
Abstract vi
List of Figures viii
List of Tables ix
Chapter 1 Introduction 1
1.1 Related work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Chapter 2 Preliminaries 5
2.1 Bitcoin blockchain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Sidechain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3 Chameleon-Hash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4 Public Auditing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Chapter 3 System Model 9
3.1 Parentchain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2 Sidechain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3 Downloading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Chapter 4 Our Scheme 13
4.1 Parentchain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2 Sidechain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.3 Downloading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Chapter 5 Discussion 19
5.1 Auditing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.2 Block-Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.3 Assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Chapter 6 Security Analysis 24
6.1 Security Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
6.1.1 Malicious O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
6.1.2 Malicious Miner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.1.3 Malicious SP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.1.4 Malicious U . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.2 Security Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.2.1 Against Malicious O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.2.2 Against Malicious Miner . . . . . . . . . . . . . . . . . . . . . . . . . . 26
6.2.3 Against Malicious SP . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
6.2.4 Against Malicious U . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Chapter 7 Comparison 28
Chapter 8 Conclusion 30
Bibliography 31

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