But please.. keep in mind, don't place the actual code in the map.
Use a serverside mutator to change the code in the keypad, as people can open the map using either SDK (if not protected) or Unreal Tournament Package Tool, which allows you to view all classes and their properties.
If you use a serverside mutator, noone can get the code

If anyone is actually interested in how it works and how it's unhackable, just request for it, and I will post.