"I assign it a unique code. Is there any unique code for each microcontroller provided by the chip manufacturer? "

Little bit of confusion there. When you are assigning the unique code why worry about the manufacturer ? In any case I dont think the manufacturer's assign any such unique code for every MCU - they might run out of codes very fast ;-)

Best would be write your ID code in a location in code memory and lock the lockbits. Could be "foolproof" to start with.

Raghu