NIST didn't do that. The NSA did. NIST submitted the AES candidates to the NSA for feedback and the NSA proposed S-box changes without explaining why. Turns o

NIST didn't do that. The NSA did. NIST submitted the AES candidates to the NSA for feedback and the NSA proposed S-box changes without explaining why. Turns out hose changes made AES more resilient to differential cryptography.