Substitution box
The 8 S-Boxes of DES were the subject of intensive studies for many years cause of a concern that a method of bypassing the DES cipher to obtaining access to the plaintext–a vulnerability (susceptibility) known only to its designers–might have been planted (inserted) in the cipher. In 1994, the S-Box design criteria were finally published by its designers after the public rediscovery of differential cryptanalysis, showing that they had been carefully tuned the design to increase resistance against differential cryptanalysis attacks. Other research had already indicated that even a very small modification to one of the 8 S-Box used by the DES could weaken it very much.