What is S-Box Substitution?

S-Box Substitution is a procedure that accepts the 48-bit input from the XOR operation containing the compressed key and expanded RPT and creates a 32-bit output utilizing the substitution technique.

The substitution is implemented by the eight substitution boxes (also known as the S-boxes). Each 8-S-boxes has a 6-bit input and a 4-bit output. The 48-bit input block is divided into 8 sub-blocks (each including 6 bits), and each sub-blocks is provided to an S-box.

The substitution in each box follows a pre-decided rule depends on a 4-row by 16- column table. The sequence of bits one and six of the input represent four rows and the sequence of bits two through five represent sixteen columns.

Because each S-box has its own table, we require eight tables, as display in table 1 to table 8, to represent the output of these boxes. The values of the inputs (row number and column number) and the values of the outputs are given as decimal numbers to store space. These need to be changed to binary.

S-Box 1

S-Box 2

S-Box 3

S-Box 4

S-Box 5

S-Box 6

S-Box 7

S-Box 8