Scalable solutions for next-generation distributed applications

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Powered by the blockchain technology, next-generation distributed applications enable anyone to participate in activities such as finance without any permission, accounts, or identifiers from a government or trusted organizations. Importantly, these applications do not need to rely on any intermediary/trusted third-party as they can rely on the trust offered by the blockchain technology. This revolutionary technology, however, must scale before it can be widely adopted. Bitcoin's blockchain, for instance, can handle up to about ten transactions per second, whereas mainstream payment systems, such as Visa, can handle tens of thousands of transactions per second. The goal of this research is to pave the way to mainstream adoption of next-generation distributed applications by designing scalable solutions for blockchain. We approach this goal in two major themes. In Theme 1, we focus on the network layer of blockchain systems. This is the layer that is responsible for propagating blocks and transactions, hence is critical for scalability. Furthermore, the layer is critical for the security and privacy of the blockchain, and imposes unique security requirements such as denial of service resistance and topology hiding. This makes designing fast information propagation methods for blockchain particularly challenging and different from traditional networks, which I have been working on over the past 15 years. In Theme 2 of this research, we will explore the so-called Layer-two solutions. These solutions allow parties to conduct transactions outside the slow blockchain, while relying on a smart contract on the blockchain to resolve any potential dispute. For instance, a payment channel allows Alice and Bob to transfer payments between themselves without involving the blockchain. The smart contract that created the payment channel guarantees that these off-chain transfers are enforceable; if Bob, for example, closes the channel with an outdated state, Alice will be able to dispute by presenting the latest state of the channel to the smart contract. Over the past four years, we have designed several new methods to improve the efficiency and security of Layer-two solutions. There are, however, many remaining challenges in this new field. For instance, parties involved in Layer-two solutions are often required to remain online and observe the blockchain for possible frauds. A party who decides to go offline should employ a third-party, referred to as watchtower, to watch the blockchain and protect her interest. Designing a watchtower protocol, however, is challenging, because in a digital world, there is often no trust between a party and the employed watchtower. A solution to such challenges quickly finds its way to practice, and in turn pushes the mainstream adoption of next-generation distributed applications one step forward.