Can’t the steps be reversed one by one to get back the original?
In: Mathematics
There are plenty of mathematical operations that are easy to do one way but very hard to do the other way.
For example, you could multiply two prime numbers like 4952019383323 and 16123689073 fairly easily. However, if I gave you the result and asked you “which two primes multiply to form this number” it would be much harder.
Hash functions make use of these kinds of operations.
That said, since hash functions are deterministic people have created mappings between inputs and outputs to common hash functions (called rainbow tables). There are ways of dealing with issues like this (i.e., salting).
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