The 56 bits key becomes the source for extracting 16 48bit subkeys as follows: The 56 bits are permutated according to a preset order: T = 57, 49, 51, ......20, 12, 4 The permutation works on the 64 bits key (including the 8 parity bits, which are ignored in the permutation). The permutated 56 bits are divided to left and right halves (28 bits each half). The halves are processed 16 times consecutively, and each such round produces a 48 bits sub-key, which in turn is used in each of the 16 rounds of DES (exercising the f-function).