Des is both an example and a counterexample - the same changes that shortened its key to 56 bits also changed its S-boxes to be resistant to differential crypta

Des is both an example and a counterexample - the same changes that shortened its key to 56 bits also changed its S-boxes to be resistant to differential cryptanalysis. That was a previously unknown (although later separately discovered) attack vector to the public community. It's important to always think of the attacks you're describing as the best publicly known attacks.