Re: Why unhashing is not possible?

Unruh wrote:


And at each step in the hash, information is thrown away.


Unless you reach its maxixum output length (which is typically very short in comparison to the input), any good cryptographic hash does its best in preserving as much information as possible.

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.


Are you implying a cryptographic hash here? The OP didn't. Sometimes we just want hashes that are only good at not randomly leading to collision, and CRC is a perfect hash in this sense - yet is cryptographically insecure.

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.

Nonsense again. You just have to know the key and for a small set of inputs you can perfectly reverse it.
.