 operations such as bit shifts, rotations, substitutions, and permutations. This is the first of several rounds. A round refers to the basic processing logic of the algorithm. The out- put of the first round is fed into the second round along with the second subkey and SO on until the specified number of rounds and the required number of subkeys is exhausted. The number of rounds depends on the algorithm. For example, DES is specified with 16 rounds. Rijndael (AES) is specified with 9 rounds if both the key and block size are 128 bits long, 11 rounds if either the key or the block is 192 bits long and neither of them is longer than that, and 13 rounds if either the block or the key is 256 bits long. During the standardization process of the AES algorithm, it was settled that the official standard supports one block size only of 128 bits, whereas the key can be of 128, 192, or 256 bits. Rijndael supports all 9 possible combinations of key size versus block size, where each one them can be 128, 192, or 256 bits long. This issue of whether multiple key- and block-size combinations are possible is one of the many intercompatibility factors between communicating systems. Figure 17.2 shows the evolution of a run of DES over a block of plaintext. Ki is the subkey used in each round. We use DES processing as an example because it is still the bulwark of the Inter- net Protocol Security (IPsec) arena, which applies strongly and predominantly on the network- processing field. All concepts we discuss in this chapter are directly related to the cryptographic reality; therefore, they can almost always be applied to other algorithms as well, such as Rijndael. Figure 17.2, L and R denote the left half and the right half of the block-size reference, respectively. The 64-bit blocks would respectively correspond to the 32 MSB bits and the 32 LSB bits. The first block, called IP, represents an initial permutation where bits effectively are reshuffled according to a specific order. The last block is the inverse of that permutation a