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Leader Rotation Is Not Enough: Scrutinizing Leadership Democracy of Chained BFT Consensus

Yining Tang, Runchao Han, Jianyu Niu, Chen Feng, Yinqian Zhang

TL;DR

This work exposes a fundamental gap in chained BFT literature: leadership democracy is not guaranteed by leader rotation alone. By building a unified MDP-based evaluation framework with two metrics—chain quality and censorship resilience—the authors quantify how adversaries can undermine fairness and block-censorship resistance across four emblematic protocols (2CHS, CHS, Streamlet, FHS). The analysis reveals riskless forking opportunities and varying vulnerability patterns, motivating modular countermeasures such as broadcasting QCs in FHS and introducing randomness in proposing rules for 2CHS/CHS, which substantially improve leadership democracy with limited overhead. The study emphasizes design-component sensitivity, shows no scheme achieves perfect trade-offs among linear message complexity, responsiveness, and optimal leadership democracy, and points toward modular design and accountable forking as promising directions for future chained BFT protocols.

Abstract

With the growing popularity of blockchains, modern chained BFT protocols combining chaining and leader rotation to obtain better efficiency and leadership democracy have received increasing interest. Although the efficiency provisions of chained BFT protocols have been thoroughly analyzed, the leadership democracy has received little attention in prior work. In this paper, we scrutinize the leadership democracy of four representative chained BFT protocols, especially under attack. To this end, we propose a unified framework with two evaluation metrics, i.e., chain quality and censorship resilience, and quantitatively analyze chosen protocols through the Markov Decision Process (MDP). With this framework, we further examine the impact of two key components, i.e., voting pattern and leader rotation on leadership democracy. Our results indicate that leader rotation is not enough to provide the leadership democracy guarantee; an adversary could utilize the design, e.g., voting pattern, to deteriorate the leadership democracy significantly. Based on the analysis results, we propose customized countermeasures for three evaluated protocols to improve their leadership democracy with only slight protocol overhead and no change of consensus rules. We also discuss future directions toward building more democratic chained BFT protocols.

Leader Rotation Is Not Enough: Scrutinizing Leadership Democracy of Chained BFT Consensus

TL;DR

This work exposes a fundamental gap in chained BFT literature: leadership democracy is not guaranteed by leader rotation alone. By building a unified MDP-based evaluation framework with two metrics—chain quality and censorship resilience—the authors quantify how adversaries can undermine fairness and block-censorship resistance across four emblematic protocols (2CHS, CHS, Streamlet, FHS). The analysis reveals riskless forking opportunities and varying vulnerability patterns, motivating modular countermeasures such as broadcasting QCs in FHS and introducing randomness in proposing rules for 2CHS/CHS, which substantially improve leadership democracy with limited overhead. The study emphasizes design-component sensitivity, shows no scheme achieves perfect trade-offs among linear message complexity, responsiveness, and optimal leadership democracy, and points toward modular design and accountable forking as promising directions for future chained BFT protocols.

Abstract

With the growing popularity of blockchains, modern chained BFT protocols combining chaining and leader rotation to obtain better efficiency and leadership democracy have received increasing interest. Although the efficiency provisions of chained BFT protocols have been thoroughly analyzed, the leadership democracy has received little attention in prior work. In this paper, we scrutinize the leadership democracy of four representative chained BFT protocols, especially under attack. To this end, we propose a unified framework with two evaluation metrics, i.e., chain quality and censorship resilience, and quantitatively analyze chosen protocols through the Markov Decision Process (MDP). With this framework, we further examine the impact of two key components, i.e., voting pattern and leader rotation on leadership democracy. Our results indicate that leader rotation is not enough to provide the leadership democracy guarantee; an adversary could utilize the design, e.g., voting pattern, to deteriorate the leadership democracy significantly. Based on the analysis results, we propose customized countermeasures for three evaluated protocols to improve their leadership democracy with only slight protocol overhead and no change of consensus rules. We also discuss future directions toward building more democratic chained BFT protocols.
Paper Structure (48 sections, 2 theorems, 5 equations, 12 figures, 5 tables)

This paper contains 48 sections, 2 theorems, 5 equations, 12 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Preliminaries of Chained BFT Consensus
  3. Chained BFT Protocols
  4. Overview of chained BFT
  5. Varied design components
  6. Forking Attack of Chained BFT
  7. Leadership Democracy
  8. System Model and Metrics
  9. System Model
  10. Evaluation Metrics
  11. Chain quality
  12. Censorship resilience
  13. MDP Modeling
  14. Applying MDP to Chained BFT
  15. MDP Design
  16. ...and 33 more sections

Key Result

Theorem 1

FHS-C has optimal censorship resilience regardless of the adversary's strategy.

Figures (12)

  • Figure 1: The propose-vote paradigm of chained BFT protocols. The yellow crown denotes the leader in a view.
  • Figure 2: Four different voting patterns. Here, DV, BV, LRV, and LBV are short for "direct votes", "broadcasting votes", "leader relay votes" and "leader broadcasting votes", respectively. The yellow crown denotes the leader in a view.
  • Figure 3: The chain quality of 2CHS, CHS, FHS, and Streamlet. The $Q(\alpha)$ values of 2CHS, FHS, and Streamlet overlap with each other.
  • Figure 4: The censorship resilience of 2CHS, CHS, FHS, Streamlet. The $C(\alpha)$ values of 2CHS, FHS, and Streamlet overlap with each other.
  • Figure 5: Possible forks in 2CHS-C and CHS-C. Given forks, an honest leader first shortlists the longest forks in case (b) and follows a uniform tie-breaking rule when they are equal as shown in case (a).
  • ...and 7 more figures

Theorems & Definitions (4)

  • Definition 1: Attack threshold
  • Definition 2: Optimal censorship resilience
  • Theorem 1
  • Corollary 1