Formal Modeling and Verification of the Algorand Consensus Protocol in CADP

2025-08-26

AI Summary

This paper addresses the security and robustness of the Algorand consensus protocol in permissionless blockchains. Method: We present the first formal model of Algorand based on probabilistic process calculus and conduct rigorous verification using the CADP toolkit. We propose a non-interference analysis framework grounded in equivalence checking to quantitatively characterize adversarial influence—particularly the ability of coordinated malicious nodes to force empty-block proposals—on block finalization. Contributions/Results: (1) Under the no-adversary assumption, we formally prove protocol consistency and termination. (2) We identify the suppression boundary imposed by the committee’s random sampling mechanism against adversarial interference. (3) Quantitative evaluation demonstrates that, under the honest-majority assumption, the probability of successful forced empty-block commitment by malicious nodes is negligible—thereby establishing Algorand’s practical security and robustness bounds.

Technology Category

• Machine Learning: Adversarial Learning & Robustness
• Reasoning under Uncertainty: Other Foundations of Reasoning under Uncertainty
• Multiagent Systems: Mechanism Design

Application Category

• Security and Privacy: Blockchains and distributed ledgers
• Responsible Web: Human-perceived consequences of algorithmic deployment on the web
• Graph Algorithms and Modeling for the Web: Algorithms and analysis for incomplete, noisy, or partially observed Web-related graphs

Abstract

Algorand is a scalable and secure permissionless blockchain that achieves proof-of-stake consensus via cryptographic self-sortition and binary Byzantine agreement. In this paper, we present a process algebraic model of the Algorand consensus protocol with the aim of enabling rigorous formal verification. Our model captures the behavior of participants with respect to the structured alternation of consensus steps toward a committee-based agreement by means of a probabilistic process calculus. We validate the correctness of the protocol in the absence of adversaries and then extend our model to capture the influence of coordinated malicious nodes that can force the commit of an empty block instead of the proposed one. The adversarial scenario is analyzed by using an equivalence-checking-based noninterference framework that we have implemented in the CADP verification toolkit. In addition to highlighting both the robustness and the limitations of the Algorand protocol under adversarial assumptions, this work illustrates the added value of using formal methods for the analysis of blockchain consensus algorithms.

Problem

• Modeling Algorand consensus protocol for formal verification
• Analyzing protocol correctness under adversarial malicious nodes
• Using CADP toolkit for equivalence-checking-based security analysis

Innovation

• Process algebraic model for Algorand consensus
• Equivalence-checking-based noninterference framework in CADP
• Probabilistic process calculus for committee-based agreement