This. DES was strengthened against differential cryptanalysis, a technique not discovered by the public for over a decade. https://en.wikipedia.org/wiki/Diffe

This. DES was strengthened against differential cryptanalysis, a technique not discovered by the public for over a decade. https://en.wikipedia.org/wiki/Differential_cryptanalysis https://en.wikipedia.org/wiki/Differential_cryptanalysis In 1975 brute-forcing of 56-bit keys was a NOBUS capability.