比特幣混合服務 TumbleBit 之阻斷服務預防措施

Abstract

比特幣是一個新興的加密貨幣與支付系統，於 2009 年由化名為 Satoshi Nakamoto 的人提出。其易於架設的特性，有潛力將安全且快速的支付系統遍及全球。比特幣系統是透過名為區塊鏈的公用帳本運作。在創始之初，區塊鏈因為使用化名，而被視為一個匿名的系統。然而，後續的研究和破密公司證實這種說法是錯誤的，許多維護使用者隱私的新方法也接著被提出。其中比特幣混合服務，是合於現行比特幣系統標準下，能有效對抗區塊鏈被監測和洩密的一系列方法。

然而，比特幣混合服務需要仰賴服務提供者以正常運作。這有違比特幣之核心精神。多數相關方法中，服務提供者有能力捲款而逃，或得知使用者的金錢流向。TumbleBit 是此系列方法中較新的著作，它克服了前項限制，但是容易受到大規模阻斷服務攻擊而癱瘓。在本研究中，我們提高發動此類攻擊的門檻：攻擊者必須在攻擊期間，凍結比 TumbleBit 服務提供者更多的資金。

Bitcoin is a cryptocurrency and payment method, proposed in 2009 by the mysterious Satoshi Nakamoto. It offers secure and fast payments across the globe while requiring little to no setup. The system is maintained by volunteers, through a public ledger called the blockchain. The Bitcoin system was first perceived to be an anonymous system, thanks to the use of pseudonyms in the blockchain. However, over the years research and surveillance services suggest otherwise. To protect user privacy, several solutions have been proposed, among which Bitcoin mixing counters blockchain surveillance and remains compatible with the current Bitcoin system.

Mixing methods, however, suffer from a problem of trust: the service provider is trusted not to steal the users’ funds or breach anonymity. TumbleBit, a recent work that incorporates other existing methods, does not suffer from the problem of trust. However, TumbleBit is vulnerable to large scale Denial-of-Service attacks. In this work, we make such attacks more difficult to accomplish. Attackers are required to escrow more funds than the service in order to paralyze it.