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PoW rate limiting has been around for way longer than mainstream cryptocurrencies:

https://en.wikipedia.org/wiki/Hashcash

Uh. Ignore the "Cash" in the name. That's mostly a coincidence.

Or if you need another antique example: opentnl, the network library use in a 2001 released game Tribes 2 had „client puzzles“ (aka PoW): https://github.com/kocubinski/opentnl/blob/master/tnl/client...

The problem of energy waste applies to proof-of-work in general. It could be argued that mitigating DDoS attacks might make it a net negative, but someone would have to do the calculations.

Eh. Nah, probably not. Proof-of-work energy waste with cryptocurrencies is a problem specifically because of its scale, and the relative worthlessness of the activity.

Human basal metabolic rate is around 100W. Actual energy use per capita averages up to 10,000W, depending on how wealthy and boreal you are. If you want to count "energy waste", a successful DDOS costs each blocked user multiple hours, at multiple kilowatts.

It's no more a waste of energy than using advanced video encodings, or running `bcrypt` on passwords. The energy cost is comparatively miniscule, and it solves or mitigates other problems.

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