Video - Bitcoin Q and A Security In Proof-Of-Work vs. Proof-Of-Stake

Security of proof-of-work versus proof-of-stake. Proof-of-work, i.e. thermodynamic cost, is not a waste. Proof-of-stake secures transactions going forward but does not provide robust immutability. Proof-of-work relies on resources outside the network (electricity). Both systems are vulnerable to DoS attacks if you get the majority of the consensus mechanism, but getting a majority of the intrinsically-native blockchain capital (ex. through a smart contract bug) is a lot easier than getting a majority of the mining infrastructure and widely distributed sources of electricity. The security differential is worth it.

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