Title:             A Semi-Synchronous Finality Layer for Blockchains
Authors:           Bernardo Magri; Jesper Buus Nielsen; Daniel Tschudi
Key words:         Blockchain, Finality, Consensus

Abstract:
Most existing blockchain protocols provide immutability up to the last ``few
blocks" in the chain. The immutability guarantees that a new block inserted in
the chain will not be rolled-back and completely erased if one waits until
that block is ``deep enough" in the chain. The concept of ``deep enough" comes
from the common prefix property (as defined by Garay et. al. EC'15), and as it
turns out it is very difficult to measure it in practice. As a consequence,
one would have to wait for a confirmation time of $80$ new blocks to have a
roll-back probability of $2^{-80}$ considering a $50\%$ adversary. 

In this paper we propose a finality layer that can be composed with any
semi-synchronous blockchain that has the property of common prefix. Our
finality layer allows to dynamically ``checkpoint" the blockchain by using
Byzantine agreement to identify and then mark common blocks in the honest
user's chain as final, essentially turning the new final blocks into genesis
blocks. The goal of our finality layer is to guarantee that a block becomes
final much faster than the $80$ blocks mentioned above.