Formal Modeling and Verification of the Algorand Consensus Protocol in CADP

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.