It is much more difficult to scan chips these days, because the dimensions of key components are less than a wavelength of light. So you can't use normal microscopes. It has to be done with electron microscopes.

There are also implant layers that cannot be seen. Implanted layers allow you to insert "gotcha"s. These are structures that look like a logic gate, but are not. Imagine something that looks like an inverter, but whose output is always a logic 1. The copier puts it into their design as an inverter, and that causes the design to fail.




I would keep the checksum internal to the chip, and only output a pass-fail signal.