One way functions

One-way-functions are functions where it is easy to compute f(x) but almost infeasible to compute f-1(y). f is collision free, so there are no two elements x and x’ with f(x)=f(x’). A good example for this is a telephone Book. It’s very easy to find a telephone number for a given name, but it’s really hard to find a name for a given number if you have to do it all by yourself.