[martinced]

> How do you know it can't be reversed easily?

It is impossible to reverse because information is lost. There's an infinite number of "plain text" leading to a same hash.

If you could reverse, say, SHA-1, then you'd just have invented the best compression scheme ever and the world as we know it would be no more: the implication would be huger than anything we can imagine. You could compress a full movie in a SHA-1 hash. Not. Gonna. Happen. It is impossible.

The problem is not reversing information: reversing is simply impossible.

The problem is finding collisions and hence being able to create plain texts that shall lead to the same hash.