Question

Compute the bits number 15, 26, 49, and 61 of the output L1 and R1 of the first round of DES encryption, assuming that the input plaintext block consists of the sequence F0F0F0F0F0F0F0F0, and the key consists of all zeros. Use the S-Box Below

	0000	0001	0010	0011	0100	0101	0110	0111	1000	1001	1010	1011	1100	1101	1110	1111
00	14	4	13	1	2	15	11	8	3	10	6	12	5	9	0	7
01	0	15	7	4	14	2	13	1	10	6	12	11	9	5	3	8
10	4	1	14	8	13	6	2	11	15	12	9	7	3	10	5	0
11	15	12	8	2	4	9	1	7	5	11	3	4	10	0	6	13

Answer 1

Solution:

The following is the given table of S-box.

	0000	0001	0010	0011	0100	0101	0110	0111	1000	1001	1010	1011	1100	1101	1110	1111
00	14	4	13	1	2	15	11	8	3	10	6	12	5	9	0	7
01	0	15	7	4	14	2	13	1	10	6	12	11	9	5	3	8
10	4	1	14	8	13	6	2	11	15	12	9	7	3	10	5	0
11	15	12	8	2	4	9	1	7	5	11	3	4	10	0	6	13

It is also given that input plain text consists of the sequence F0F0F0F0F0F0F0F0 and the key consists of all zeroes.

let us first know the figure of DES, the following is the DES encryption figure-

From the above figure it is clear that

let us substitute i=1 to get the

so, the bits number 15, 26, 49, and 61 of the output L1 will equal to the bits number 15, 26, 49, and 61 of the output R0

Therefore, L1= 14A7D678. ( pink colors in the below table)

and R1=18CA18AD. (Green colors in the below table)