[Cryptography] Solving Matt's hash problem
Jerry Leichter
leichter at lrw.com
Mon Nov 14 21:01:07 EST 2022
> In Matt Levine's long article about cryptocurrencies, he describes in
> accurate detail how hashing works, and mentions in an aside that he
> cannot include the hash of the article in the article itself, you (the
> reader) figure out why.
>
> I know this is a preimage problem, but is there anything about making
> the hash itself part of the input that would make it harder or easier
> to solve?
Call the article with all 0's (say) as a placeholder for the hash A0. Now replace the 0's with H(A0) to create A1. Almost certainly, H(A0) != H(A1). You can replace replace H(A0) with H(A1) to get A2, but hash changes again. What you're looking for is essentially a fixed point - but even assuming it exists (there's no particular reason I can see why it has to) - how would you find it?
-- Jerry
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