php中的加密/验证功能

Question

There is a form with 2 fields: X and Y. X is known, Y is generated later via ajax based on X. After the submit I would like to check if Y was generated by me or not.

I'm looking for a function (and its inverse) that is more complicated to figure out than the example below:

x = 7 (random number)
y = f(x)
x = g(y)

example:
f = +2
g = -2

x = 7
y = 9
x = 7

Answer 1

Why not use f(x) again and check its output against the posted value of y? That way you are even able to make irreversible valies for y (like md5 or sha)?