The Checkpointed Longest Chain: User-dependent Adaptivity and Finality

Longest-chain protocols such as the one invented by Nakamoto for Bitcoin achieve liveness when the number of participating users is unknown, i.e., they are adaptive. However, they are not safe under network partitions, i.e., they do not achieve finality. On the other hand, there are many byzantine fault tolerant protocols that achieve finality, but not adaptivity. Indeed, the CAP theorem in the context of blockchains asserts that no protocol can simultaneously offer both adaptivity and finality. We propose a new and simple blockchain protocol, the checkpointed longest chain, that offers a single protocol along with two distinct confirmation rules: one that guarantees finality, and another that is adaptive, thus letting clients make a local choice between adaptivity and finality. Our solution builds on and sheds insight on the design of finality gadgets, which have become popular methods for achieving deterministic finality in proof-of-work protocols. Furthermore, like most deployed blockchains, our proposed solution allows for coupled validity, i.e., the finalized blocks lie on a single chain, and therefore, nodes can attest to the validity of transactions while proposing blocks.