…, which is guarding access to a node’s limited resources, such as disk, bandwidth, memory, and CPU. In a permissionless cryptocurrency network, peers must not be trusted. So, to prevent against resource exhaustion attacks, Bitcoin nodes first check the PoW for any received blocks before committing more resources, such as storing the block in RAM or on disk. However, it turns out that checking a Proof-of-Stake is a lot more complicated and context-sensitive than validating a Proof-of-Work. As a result, many chain-based PoS implementations have skimped on the appropriate validation.