TaiJi: Longest Chain Availability with BFT Fast Confirmation

TL;DR

TaiJi is a novel hybrid blockchain protocol combining longest chain and BFT methods, achieving near-deterministic confirmation times and full dynamic availability, enhancing security and efficiency in permissionless settings.

Contribution

It introduces the first dynamically available PoW protocol with almost deterministic confirmation and latency independent of security parameters, combining multiple longest chains for secure voting.

Findings

Achieves near-deterministic confirmation times.

Supports full dynamic participation similar to Bitcoin.

Secure against adaptive adversaries with up to 50% hash power.

Abstract

Most state machine replication protocols are either based on the 40-years-old Byzantine Fault Tolerance (BFT) theory or the more recent Nakamoto's longest chain design. Longest chain protocols, designed originally in the Proof-of-Work (PoW) setting, are available under dynamic participation, but has probabilistic confirmation with long latency dependent on the security parameter. BFT protocols, designed for the permissioned setting, has fast deterministic confirmation, but assume a fixed number of nodes always online. We present a new construction which combines a longest chain protocol and a BFT protocol to get the best of both worlds. Using this construction, we design TaiJi, the first dynamically available PoW protocol which has almost deterministic confirmation with latency independent of the security parameter. In contrast to previous hybrid approaches which use a single longest chain to sample participants to run a BFT protocol, our native PoW construction uses many independent longest chains to sample propose actions and vote actions for the BFT protocol. This design enables TaiJi to inherit the full dynamic availability of Bitcoin, as well as its full unpredictability, making it secure against fully-adaptive adversaries with up to 50% of online hash power.