The transponder keys should be updated frequently. (I do not know how they are handled.) Military GPS receivers are supposed to be tamperproof, and the keys (used to be) changed annually. If you had access to a military GPS receiver there was a rigmarole you had to go through to update keys.
thats one of the oldest tricks in the book....fly the other guys flag
shessh...
U.S. and NATO IFF are considered cryptologically secure. That means if an adversary had detailed design information, they would have no advantage in trying to break the cipher, the security is in the keys which are changed frequently.