Appendix A. Overview of the AES Block Cipher

The AES block cipher is the same as the Rijndael algorithm but with a fixed block size of 128 bits (see Chapter 12). In IEEE 802.11i RSN, a further simplification is made by restricting the key size as well as the block size to 128 bits. The following description relates only to the RSN version.

You can think of the encryption of the block of data as a sort of production process in which various operations are applied repeatedly until the finished product, the ciphertext, is produced. A medieval blacksmith made a sword by starting with a strip of iron and repeatedly heating it, hammering it, adding impurities, folding it, and quenching it in cold water. By folding the metal ten times, the sword ended up with a thousand fine layers. In AES the data is the raw material loaded into a state array. The state array is processed through ten rounds of manipulation, after which it is unloaded to form the resulting encrypted block of data. At each stage of the process, the state is combined with a different round key, each of which is created and derived from the cipher key.

Although this sounds like a lot of work, one of the key advantages of the Rijndael algorithm is that it uses only simple operations such as shift, exclusive OR, and table substitution. Many encryption approaches require multiplication operations that are very expensive to implement. Rijndael uses finite field byte multiplication, a special operation that can be simplified down to a few logical operations or lookups in a 256-byte table. This appendix begins with an overview of finite field arithmetic. If you are just interested in the encryption steps for AES, skip to the next section.