CacheCash: A Cryptocurrency-based Decentralized Content Delivery Network

Online content delivery has witnessed dramatic growth recently with traffic consuming over half of today’s Internet bandwidth. This escalating demand has motivated content publishers to move outside the traditional solutions of infrastructure-based content delivery networks (CDNs). Instead, many are employing peer-to-peer data transfers to reduce the service cost and avoid bandwidth over-provision to handle peak demands. Unfortunately, the open access work model of this paradigm, which allows anyone to join, introduces several design challenges related to security, efficiency, and peer availability. In this dissertation, we introduce CacheCash, a cryptocurrency-based decentralized content distribution network designed to address these challenges. CacheCash bypasses the centralized approach of CDN companies for one in which end users organically set up new caches in exchange for cryptocurrency tokens. Thus, it enables publishers to hire caches on an as-needed basis, without constraining these parties with long-term business commitments. To address the challenges encountered as the system evolved, we propose a number of protocols and techniques that represent basic building blocks of CacheCash’s design. First, motivated by the observation that conventional security assessment tools do not suit cryptocurrency-based systems, we propose ABC, a threat modeling framework capable of identifying attacker collusion and the new threat vectors that cryptocurrencies introduce. Second, we propose CAPnet, a defense mechanism against cache accounting attacks (i.e., a client pretends to be served allowing a colluding cache to collect rewards without doing any work). CAPnet features a bandwidth expenditure puzzle that clients must solve over the content before caches are given credit, which bounds the effectiveness of this collusion case. Third, to make it feasible to reward caches per data chunk served, we introduce MicroCash, a decentralized probabilistic micropayment scheme that reduces the overhead of processing these small payments. MicroCash implements several novel ideas that make micropayments more suitable for delay-sensitive applications, such as online content delivery. CacheCash combines the previous techniques to produce a novel service-payment exchange protocol that secures the content distribution process. This protocol utilizes gradual content disclosure and partial payment collection to encourage the honest collaborative work between participants. We present a detailed game theoretic analysis showing how to exploit rational financial incentives to address several security threats. This is in addition to various performance optimization mechanisms that promote system efficiency and scalability. Lastly, we evaluate system performance and show that modest machines can serve/retrieve content at a high bitrate with minimal overhead

Probabilistic micropayments with transferability

Micropayments are one of the challenges in cryptocurrencies. The problems in realizing micropayments in the blockchain are the low throughput and the high blockchain transaction fee. As a solution, decentralized probabilistic micropayment has been proposed. The winning amount is registered in the blockchain, and the tickets are issued to be won with probability $p$, which allows us to aggregate approximately $\frac{1}{p}$ transactions into one. Unfortunately, existing solutions do not allow for ticket transferability, and the smaller $p$, the more difficult it is to use them in the real world. We propose a novel decentralized probabilistic micropayment Transferable Scheme. It allows tickets to be transferable among users. By allowing tickets to be transferable, we can make $p$ smaller. We also propose a novel Proportional Fee Scheme. This is a scheme where each time a ticket is transferred, a portion of the blockchain transaction fee will be charged. With the proportional fee scheme, users will have the advantage of sending money with a smaller fee than they would generally send through the blockchain. For example, sending one dollar requires only ten cents

Breaking the Binding: Attacks on the Merkle Approach to Prove Liabilities and its Applications

Proofs of liabilities are used for applications, function like banks or Bitcoin exchanges, to prove the sums of money in their dataset that they should owe. The Maxwell protocol, a cryptographic proof of liabilities scheme which relies on a data structure well known as the summation Merkle tree, utilizes a Merkle approach to prove liabilities in the decentralized setting, i.e., clients independently verify they are in the dataset with no trusted auditor. In this paper, we go into the Maxwell protocol and the summation Merkle tree. We formalize the Maxwell protocol and show it is not secure. We present an attack with which the proved liabilities using the Maxwell protocol are less than the actual value. This attack can have significant consequences: A Bitcoin exchange controlling a total of $n$ client accounts can present valid liabilities proofs including only one account balance in its dataset. We suggest two improvements to the Maxwell protocol and the summation Merkle tree, and present a formal proof for the improvement that is closest in spirit to the Maxwell protocol. Moreover, we show the DAM scheme, a micropayment scheme of Zerocash which adopts the Maxwell protocol as a tool to disincentivize double/multiple spending, is vulnerable to an multi-spending attack. We show the Provisions scheme, which adopts the Maxwell protocol to extend its privacy-preserving proof of liabilities scheme, is also infected by a similar attack

Micropayments for Decentralized Currencies

Electronic financial transactions in the US, even those enabled by Bitcoin, have relatively high transaction costs. As a result, it becomes infeasible to make \emph{micropayments}, i.e. payments that are pennies or fractions of a penny. To circumvent the cost of recording all transactions, Wheeler (1996) and Rivest (1997) suggested the notion of a \emph{probabilistic payment}, that is, one implements payments that have \emph{expected} value on the order of micro pennies by running an appropriately biased lottery for a larger payment. While there have been quite a few proposed solutions to such lottery-based micropayment schemes, all these solutions rely on a trusted third party to coordinate the transactions; furthermore, to implement these systems in today\u27s economy would require a a global change to how either banks or electronic payment companies (e.g., Visa and Mastercard) handle transactions. We put forth a new lottery-based micropayment scheme for any ledger-based transaction system, that can be used today without any change to the current infrastructure. We implement our scheme in a sample web application and show how a single server can handle thousands of micropayment requests per second. We analyze how the scheme can work at Internet scale

SoK: A Taxonomy for Layer-2 Scalability Related Protocols for Cryptocurrencies

Blockchain based systems, in particular cryptocurrencies, face a serious limitation: scalability. This holds, especially, in terms of number of transactions per second. Several alternatives are currently being pursued by both the research and practitioner communities. One venue for exploration is on protocols that do not constantly add transactions on the blockchain and therefore do not consume the blockchain\u27s resources. This is done using off-chain transactions, i.e., protocols that minimize the interaction with the blockchain, also commonly known as Layer-2 approaches. This work relates several existing off-chain channel methods, also known as payment and state channels, channel network constructions methods, and other components as channel and network management protocols, e.g., routing nodes. All these components are crucial to keep the usability of the channel, and are often overlooked. For the best of our knowledge, this work is the first to propose a taxonomy for all the components of the Layer-2. We provide an extensive coverage on the state-of-art protocols available. We also outline their respective approaches, and discuss their advantages and disadvantages