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

> maybe we could think about the XOR function. i recall it has a basic
> problem regarding unknownness of the precise inputs.

XOR can totally be reversed. 100%.

A B XOR
0 0 0
0 1 1
1 0 1
1 1 0

It's defined by a lookup table, so if you know the XOR output you know
an immediate function of one input given the other, and you know the
complete set of possible combinations of inputs and outputs.

It gets more complex when you have millions of them rather than just
one! I wouldn't put something that hard to rewind in the time travel
thing.

But maybe the xor function could be an example they would use?