Title: From network-wide to localized: Some consensus but not too much!

Abstract:

Albeit initially devoted to cryptocurrency, the blockchain model has found applications in a number of settings, such as logistics, self-sovereign identity systems, or e-voting. Today, however, we know that the main blockchain application, cryptocurrency, does not need the full power provided by the blockchain abstraction. In particular, it does not require the total order of operations provided by consensus. However, the question remained open regarding other blockchain applications. In this talk, I will consider three of them: anonymous cryptocurrency, self-sovereign identity, and decentralized e-voting. I will show that, unlike plain cryptocurrency, their implementation does require consensus. However, contrary to general practice, this consensus does not require the involvement of the entire network. The talk introduces two novel distributed objects: the AllowList and the DenyList. It shows that the AllowList can be implemented without consensus, while the DenyList only needs a restrained form of consensus involving a small set of participants. The talk then shows that AllowList and DenyList can implement the three considered applications. Finally, it introduces CAC, a novel primitive that generalizes the concept of restrained consensus.