The Subbytes operation is a nonlinear substitution. This is a major reason for the security of the AES. There are different ways of interpreting the Subbytes operation. In this application report, it is sufficient to consider the Subbytes step as a lookup in a table. With the help of this lookup table, the 16 bytes of the state (the input data) are substituted by the corresponding values found in the table (see Figure 4-3).

Figure 4-3 Subbytes Operation