This research explores ways to simultaneously improve both the scalability and privacy of blockchain technologies. A blockchain is a massively distributed, append-only log of transactions that is cryptographically protected from tampering. Thanks to their capacity towards facilitating fast and inexpensive transactions across the globe and their powerful scripting-language support for complex financial instruments, blockchains have already proven to be a highly disruptive force in the finance and e-commerce sectors. Nevertheless, at least two major hurdles still stand in the way of mainstream blockchain acceptance: 1) 'traditional' blockchain architectures are not sufficiently scalable to handle the ever-growing base of blockchain users and the resulting proliferation of transactions, and 2) despite myriad efforts to improve blockchain privacy, users of public blockchains remain susceptible to devastating attacks on the privacy of their accounts and transactions, which often lead to security breaches causing financial losses for the victims. The approaches explored in this project provide privacy for blockchain users where previous efforts have failed by rethinking the one-size-fits-all approach of using Tor to solve all problems requiring anonymity and/or unlinkability. Indeed, running complex protocols over Tor is fraught with risks and can open users to subtle-yet-devastating deanonymization attacks; the tailor-made solutions developed in this project leverage domain-specific knowledge to mitigate these risks.
The project addresses privacy concerns as they relate both to traditional blockchain transactions and to newer 'payment channel network' transactions. Payment channel networks promise greatly improved scalability by allowing secure (off-chain) payment requiring no interaction with the blockchain ledger. In the context of traditional blockchain transactions, this research develops innovative ways to 1) privately publish transactions to a blockchain by integrating tailor-made anonymous communication protocols directly into the blockchain communication infrastructure, and 2) privately retrieve transactions from a blockchain using carefully optimized private information retrieval (PIR) protocols that support expressive blockchain queries. In the context of payment channel networks, the project 1) explores the (im)possibility of performing off-chain transactions privately, and 2) develops a new theoretical framework and toolkit of algorithms for ensuring availability and quality of service for payment channel transactions in extreme adversarial conditions. Additionally, as part of this project, the multi-institution and transnational team of PIs are deploying a distributed instantiation of the new private blockchain transaction retrieval solutions, which will be open to use by the public. Along with training graduate students, the project puts a major emphasis on undergraduate involvement in this emerging area of blockchain research.
