Server side only anti-cheat is one of the problem domains that I'd really love to work on at some point in my career. This is the type of adversarial arms race that just seems really fun to think long and hard about.
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Now add in that I'm running a physics-heavy game with 120 tickrate, (considering higher after more tests), with fine motor control action combat, aimed to scale to mmorpg size, and it really becomes a challenge!