Albatross: An optimistic consensus algorithm

TL;DR

Albatross introduces a Proof-of-Stake consensus that merges speculative BFT performance with periodic finality guarantees from Tendermint. It achieves fast optimistic progress via single-slot micro blocks and robust finality through Tendermint macro blocks, while enforcing safety with fork/double-vote/double-proposal proofs and skip-block penalties. The protocol delivers a rigorous security analysis under standard BFT assumptions, supports scalable validator elections with a VRF-based random beacon, and includes an open-source Rust implementation with real-world measurements on the Nimiq PoS testnet demonstrating near-maximum theoretical performance. This design enables efficient syncing, even on constrained devices, and offers a practical path toward high-throughput, secure open networks with probabilistic finality and periodic ultimate finality. The work advances PoS blockchain consensus by balancing speed, finality, and robustness to adversarial behavior, validated by both theory and real-world experimentation.

Abstract

The consensus protocol is a critical component of distributed ledgers and blockchains. Achieving consensus over a decentralized network poses challenges to transaction finality and performance. Currently, the highest-performing consensus algorithms are speculative BFT algorithms, which, however, compromise on the transaction finality guarantees offered by their non-speculative counterparts. In this paper, we introduce Albatross, a Proof-of-Stake (PoS) blockchain consensus algorithm that aims to combine the best of both worlds. At its heart, Albatross is a high-performing, speculative BFT algorithm that offers strong probabilistic finality. We complement this by periodically guaranteeing finality through the Tendermint protocol. We prove our protocol to be secure under standard BFT assumptions and analyze its performance both on a theoretical and practical level. For that, we provide an open-source Rust implementation of Albatross. Our real-world measurements support that our protocol has a performance close to the theoretical maximum for single-chain Proof-of-Stake consensus algorithms.

Figures (5)

• Figure 1: Blockchain structure
• Figure 2: Tendermint phases (sourced from buchman2016tendermint)
• Figure 3: Legitimate fork at block height #4
• Figure 4: Maximum fraction of adversarial stake for different $n$
• Figure 5: Distribution of skip blocks in real-world measurements

Theorems & Definitions (4)

• Theorem 1: Protocol Security
• proof : Proof of \ref{['prop:prob-finality']}
• Theorem 2: Upgrade Security
• proof