Protecting Against Eavesdroppers with Symmetric Cryptography - Implementing SSL/TLS Using Cryptography and PKI - page 62

Initial Key Input Key Schedule

bytes 1-4

bytes 5-8

bytes 9-12

bytes 13-16

bytes 17-20

bytes 21-24

bytes 25-28 =
bytes 1-4 xor
bytes 21-24

bytes 29-32 =
bytes 5-8 xor
bytes 25-28

bytes 33-36 =
bytes 9-12 xor
bytes 29-32

Figure 2-8: AES 192-bit key schedule computation

Rotation is straightforward and easy to understand. The fi rst byte is overwrit-
ten with the second, the second with the third, the third with the fourth, and
the fourth with the fi rst, as shown in Figure 2-9 and Listing 2-29.

Figure 2-9: AES rotation