NNUS: Nothing New Under the Sun.
Actually, yes there is, the introduction of computing power to radar has revived the concept. From Lockheed-Martin's early-version Silent Sentry onto today's classified R&D projects, we are ourselves pushing to make stealth aircraft visible for detection and tracking. Although fire-control targetting may still be proving elusive.
Anyways, our letting Chinese espionage successfully glean the science behind this was simply criminal. As for counter-measures, perhaps decoys could work, and perhaps an ECM package can be devised. But the viability thereto probably depends on knowing where the Chinese detection systems are. All of them. And to revert back to dependence on ECM, when our stealth approach was based on low observability and close-to-zero EM emissions...seems rather contradictory.
The Czechs had passive radars out there some 20 years ago.
Always there is a conflict between smarter radars, and smarter countermeasures. My bet is on us. We design our aircraft software to be easily updated now, so what works on one day, doesn't the nest. The enemy never knows if what they pick up on one day is a real signal you could find in time of war, or a spoof, intentionally served up to preserve the effectiveness of "War Reserve" modes.
Certainly there are a lot more codes out there that are possible, than there are codes known to the enemy.