Blockchains have ignited interest in Internet-scale consensus as a fundamental building block for decentralized applications and services, which in turn promise more egalitarian access and improved robustness to faults and abuse. While consensus has been studied in distributed systems for decades, Internet-scale consensus requires a fundamental rethinking of models, desiderata, and protocols. Participants are no longer a handful of computers in one company’s data centers, but numerous mistrustful entities distributed across the Internet. I will discuss two examples of challenges and solutions for this new setting: (1) Ethereum, the second largest blockchain, aims to strengthen consensus liveness under open participation where parties come and go at will, and to strengthen safety to enable accountability in case of any safety violation. However, we show that no traditional single-ledger protocol can satisfy both strengthened properties simultaneously. To resolve this dilemma, we develop the multi-ledger consensus paradigm that is now the security design-specification for Ethereum. A by-product of this work are attacks on Ethereum’s consensus protocol that prompted design changes. (2) Traditional network models do not capture rate constraints on communication and processing, leaving popular “provably secure” protocols vulnerable to attack. We show via a new queuing-based model how to schedule message handling securely. Our policies are simple enough to be forward-deployed at Internet service providers via a system that can also protect traffic of applications beyond blockchains.
Joachim Neu is a PhD candidate at Stanford University with David Tse. His research focuses on the science and engineering of Internet-scale consensus as a fundamental building block for decentralized systems, using tools from distributed systems, applied probability, networking and communications, and applied cryptography. While a Masters student at Technical University of Munich, he worked in information and coding theory. Joachim has received the Protocol Labs PhD Fellowship and the Stanford Graduate Fellowship.