The Algorand blockchain is a secure and decentralized public ledger based on pure proof of stake rather than proof of work. At its core it is a novel consensus protocol with exactly one block certified in each round: that is, the protocol guarantees that the blockchain does not fork. In this paper, we report on our effort to model and formally verify the Algorand consensus protocol in the Coq proof assistant. Similar to previous consensus protocol verification efforts, we model the protocol as a state transition system and reason over reachable global states. However, in contrast to previous work, our model explicitly incorporates timing issues (e.g., timeouts and network delays) and adversarial actions, reflecting a more realistic environment faced by a public blockchain. Thus far, we have proved asynchronous safety of the protocol: two different blocks cannot be certified in the same round, even when the adversary has complete control of message delivery in the network. We believe that our model is sufficiently general and other relevant properties of the protocol such as liveness can be proved for the same model.
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