Hi guys, i have a special problem to solve :P

this is the thing, i need a pin system, so if a user enters a pin (a 12 digits code might be) he will gain acces to certain area.

The problem is i just can't figure out a correct and secure way to generate those codes, should i have to generate them randomly, how many should i generate or anything...?

if anyone of you have info about a similar system please let me know.