International Data Encryption Algorithm
Each of the eight round uses six sub-keys, while the half-round uses four; for a total of 52 sub-keys. Each sub-key is a 16-bit in length. The first eight sub-keys are extracted directly from the 128-bit key, with K1 being the lowest sixteen bits and K8 is the highest sixteen bits; further groups of eight keys are created by rotating the main key left 25 bits after the creation of the previous group; six rotations generate all sub-keys.