Question

DES has eight S-boxes, which map 6 to 4 bits using a table made up of 4 rows and 16 columns. Given the following 6 bits 101101, how can you tell which row and which column to use for the mapping?

Answer 1

ANSWER:

GIVEN THAT:

The DES has 8 S-boxes.

Which map 6 to 4 bits usinga tablemadeup of 4 rows and 16 columns.

Given the following 6 bit 1011101

  

The DES's S-boxes map the 6 bits to 4 bits using the 16x4 s-box.

Suppose the 6-bit input is (x0,x1,x2,x3,x4,x5} then:

To choose the row x0,x5 are used to arrange a binary number. It's decimal equivalent denotes the row.

To choose the column, x1,x2,x3,x4 are used to arrange a binary number. It's decimal equaivalent denotes the column.Using this rule, we can tell which bits of the inputs are used to derive the columna nd row number.

*In the above example {101101}:

x0=1, x5=1

so we choose row 11 i.e. row 3

x1=0, x2=1,

x3=1, x4=0,

so we choose column 0110 i.e. column 6

*If you refer to the s-box, you can see that the cell at column 6 and row 3 contains the value 1 ie 0001 in binary. So 101101 is finally mapped to 0001.