This post let me thinking about ...


A trapdoor one-way function is a special type of one-way function, one with a secret trapdoor. It is
easy to compute in one direction and hard to compute in the other direction. But, if you know the
secret, you can easily compute the function in the other direction. That is, it is easy to compute f(x)
given x, and hard to compute x given f(x). However, there is some secret information, y, such that
given f(x) and y it is easy to compute x.
Taking a watch apart is a good example of a trap-door one-way function. It is easy to disassemble a
watch into hundreds of minuscule pieces. It is very difficult to put those tiny pieces back together
into a working watch. However, with the secret information—the assembly instructions of the
watch—it is much easier to put the watch back together.

ISBN: 978-1-119-09672-6
Page,30




As far as I concern, SHA-2, does not have trapdoor ... BUT ....