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Thought Email
List: unsolvable puzzle fun
Subject: reversing simple hashes
From: excited naive puzzler

i was thinking a little about the time travel idea and how you could
generalize that to reversing of math!
this seems like a reasonable gentle puzzle that's known to be hard
enough that thinking about it wouldn't impact much
maybe we could think about the XOR function. i recall it has a basic
problem regarding unknownness of the precise inputs.