Casper the Subjective Finality Gadget
Published in 2020 Symposium on Cryptography and Information Security, 2020
Recommended citation: Ryuya Nakamura. (2020). "Casper the Subjective Finality Gadget." SCIS 2020. https://www.iwsec.org/scis/2020/index.html
Casper the Friendly Finality Gadget (FFG) is a finality layer to construct a blockchain consensus protocol. Casper FFG overlays a proposal mechanism that produces blockchains, and clients commit blocks based on the votes by consensus nodes called validators. We present a variant of Casper the FFG called Casper the Subjective Finality Gadget (SFG), which allows clients to commit blocks subjectively, i.e., with their local assumptions on faults and network, extending Flexible BFT (Malkhi et al., 2019). We present two client commit rules of Casper SFG and prove their safety. The first rule works in a network that can suffer from temporal network failure, such as large message delay, and clients can parameterize the fault threshold for safety. The second rule works in a more optimistic setting where messages are guaranteed to be delivered within a known bound and allows clients to commit in lower latency than the first commit rule. Our work is in-progress since the liveness property is not yet proved. Instead, we present the problems around liveness, including concrete attack strategies we found in designing our protocol, which can also be applied for Casper FFG. We introduce our potential solutions to those problems towards our future work to complete the protocol.