One-Way Function

http://en.wikipedia.org/wiki/One-way_functions

The existence of such one-way functions is still an open conjecture. In fact, their existence would prove that the complexity classes P and NP are not equal, thus resolving the foremost unsolved question of theoretical computer science.

Wait! How come hash fucking tables work and the shits like md5?

>>4
Try reading all the words, holy fuck.

You don't even need a good one-way for this. The standard solution is to use a random seed in the hash function. You can't predict collisions when you don't know where they will be. That technique has been in use successfully since around Y2K.