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The concept of Proof-of-Stake (PoS) first emerged in Peercoin [1] as an approach to reduce the demanding energy consumption of the Proof-of-Work (PoW) mechanism, where the mining difficulty of the PoW can be reduced by burning stakes, ie, consuming digital tokens. Since then, PoS has gradually evolved to become a standalone consensus mechanism with significant advantages over the PoW mechanism, including negligible energy consumption and higher transaction processing capability. These advantages have shifted the research focus from PoW to PoS in academia, and, in turn, given rise to numerous PoS-based real-world applications. For example, Ethereum, the cryptocurrency with the second highest market capitalization (more than $200 billion [2]), has recently switched from PoW to pure PoS [3]. Cardano, another pure PoS cryptocurrency, has a market capitalization of more than $10 billion [2]. Besides their monetary values, these cryptocurrencies also support numerous decentralized applications in diverse areas such as finance, web browsing, gaming, advertising, identity management, and supply chain management [4]. Although the PoS mechanisms offer significant advantages, they still face various security threats. Particularly, there are many attacks targeting the PoS mechanisms, including double-spending, Sybil, race, bribery, transaction denial, grinding, desynchronization, eclipse, long-range, nothing-at-stakes, past majority, and 51% attacks. Some of these attacks can be completely prevented by most PoS-based blockchain networks. However, for some attacks (eg, 51% attacks), there is no effective solution to mitigate …