The main defense against AES or similar being secretly broken is that people (nominally) on the same side also use it. If NSA had discovered a flaw in AES and

The main defense against AES or similar being secretly broken is that people (nominally) on the same side also use it. If NSA had discovered a flaw in AES and not announced it - then when this was innevitably discovered, there may have been a few conversations with the CIA/Secret Service/The Whitehouse/SAC etc along the lines of: "so there is a flaw in this algorithm and you have been letting us use it all this time for our secrets?" >"Yes but it's OK only we know about it - nobody else could have discovered it independently" "Really? Prove it!" This did actually happen with Bletchley park. After the war various UK/US allies were given copies of the Enigma machine and told that it was unbreakable.