Khufu is a Feistel cipher with 16 rounds by default (other multiples of eight between 8 and 64 are allowed). Each set of eight rounds is termed an "octet"; a different S-box is used in each octet. In a round, the least significant byte of half of the block is passed into the 8×32-bit S-box. The S-box output is then combined (using XOR) with the other 32-bit half. The left half is rotated to bring a new byte into position, and the halves are swapped. At the start and end of the algorithm, extra key material is XORed with the block (key whitening). Other than this, all the key is contained in the S-boxes.