Abstract
Most existing blockchains either rely on a Nakamoto-style of consensus, where the chain can fork and produce rollbacks, or on a committee-based Byzantine fault tolerant (CBFT) consensus, where no rollbacks are possible. While the latter ones offer better consistency, the former tolerate more corruptions. To achieve the best of both worlds, we initiate the formal study of finality layers. Such a finality layer can be combined with a Nakamoto-style blockchain (NSB) and periodically declare blocks as final, preventing rollbacks beyond final blocks.
As conceptual contributions, we formalize the concept of a finality layer and identify the following properties to be crucial for finality layers: finalized blocks form a chain (chain-forming), all parties agree on the finalized blocks (agreement), the last finalized block does not fall too far behind the last block in the underlying blockchain (updated), and all finalized blocks at some point have been on the chain adopted by honest parties holding at least k units of the resource on which consensus is based, e.g., stake or computing power (k-support).
As our main technical contribution we propose the finality layer protocol Afgjort. We prove that it satisfies all of the aforementioned requirements in the setting with less than 1/3 corruption among the finalizers and a partially synchronous network.
We further show that tolerating less than 1/3 corruption is optimal for partially synchronous finality layers. Finally, we provide data from experiments ran with an implementation of our protocol; the data confirms that finality is reached much faster than without our finality layer.
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Notes
- 1.
We note that whenever we mention number of parties in this work it should be interpreted as the number of parties weighted by the underlying resource of the blockchain, e.g., in a PoS system k parties should be read as the fraction k/n of the total stake n in the system.
- 2.
We consider the partially synchronous network model of [11], where there is an upper bound \(\varDelta _{\mathtt {net}}\) on the network delay that is not known to the protocol designer or the honest parties. In particular, \(\varDelta _{\mathtt {net}}\) cannot be used by a partially synchronous protocol.
- 3.
Our finality layer repeatedly executes finalization in the \(\mathsf {FinalizationLoop} \). We require monotonicity only for each iteration separately, i.e., justified values can become unjustified in later iterations. We do not formalize this to simplify the presentation. This can in fact happen for the justification we use since they are with respect to \(\mathtt {FinalTree} \) and nodes get removed from \(\mathtt {FinalTree} \) after a successful finalization.
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Acknowledgements
We would like to thank Mateusz Tilewski for countless discussions during the design of the finality layer, his deep insights into practical distributed systems were valuable in designing a system which is at the same time efficient in practice and provably secure. We would like to thank Matias Frank Jensen and Emil Morre Christensen; their work on generalizing the Finality layer gave valuable insights which were adapted into the protocol presented in this paper. Finally, we thank the Concordium tech team that worked on the implementation and ran the experiments reported in this paper.
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Dinsdale-Young, T., Magri, B., Matt, C., Nielsen, J.B., Tschudi, D. (2020). Afgjort: A Partially Synchronous Finality Layer for Blockchains. In: Galdi, C., Kolesnikov, V. (eds) Security and Cryptography for Networks. SCN 2020. Lecture Notes in Computer Science(), vol 12238. Springer, Cham. https://doi.org/10.1007/978-3-030-57990-6_2
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