Re: Why unhashing is not possible?

Randell_D <fiprojects.com@xxxxxxxxx> writes:

I've always led the belief that if something can be created/built,
then it can be "uncreated/unbuilt". In technology this is sometimes
referred to as reverse-engineering. I understand too how hashing can
be used within programs and fast database lookups but I failed
miserably discussing the subject of hashes with someone who could not
understand when I told them that you cannot reverse the process... I
know you can crudely hack it (using John the Ripper for example) but
you cannot "uncompress it" so to speak.

So... my question here to you good people is... why can't a hash be...
well... unhashed?

Surely if the hash is unique, we know the process that got us the hash
we have only to reverse the process... we can't... so... but why not?

No, a hash is NOT unique. Many many many many inputs give the same hash.
That is the essense of a hash.
And at each step in the hash, information is thrown away. Ie, in running
the hash in reverse one would have to keep guessing at the thrown away
material.

Secondly to find even one of those may be very difficult. Yes, If I try
2^128 inputs there is good possiblity I will find the one giving me the
hash I have, but 2^128 is a very large number and I cannot try that many.

For an encryption, which is one to one, there are 2^60 or 2^128 ( depending
on the key length) by which that output could have been generated from the
input ( the process depends on the key). Thus I do NOT know how the output
was generated and thus cannot reverse it.

.

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