I think you have a serious misunderstanding of the DES story: the NSA made changes to the S-box without explanation in the 1970's, and everyone was suspicious a

I think you have a serious misunderstanding of the DES story: the NSA made changes to the S-box without explanation in the 1970's, and everyone was suspicious at the time, but then 15 years later two researchers "discovered" differential cryptanalysis, and realized that the changes the NSA made actually protected it against this form of attack. So instead of weakening it, the changes the NSA made protected it against a then unknown (in UNCLASS) attacks. The NSA did push to reduce the key size from 64 to 48, which is why the eventual standard was to the always bizarre 56 bits.