"Abe" wrote
In article 4xzvb.153$US3.85@okepread03, says...
Basically the system is a challenge/response system. You challenge
the target with an encrypted word. This word is decoded if you have
the right code of the day. The word is changed every challenge, so
in effect, you may get a stream of 100 words, and you have to
decode about 95% of them in your 100 replies, or you will not be
upgraded to friend.

What if a plane was captured, along with its IFF system? Wouldn't that
invalidate the IFFs in every other friendly aircraft?

Without the crypto code, the hardware is of no use. There's all sorts of
safeguards (procedural and electro/mechanical) to coding the unit.

But even if captured intact, the code could be changed before the
scheduled time. The whole air defense system isn't centered on IFF,
it's just a tool. If none of the IFF's were used, the defense system
could still function with procedural agreements. Stealth fighters almost
never use IFF, but are able to integrate into the air defense system.