Answer to Question #63374 in Computer Networks for Gene Moore

Question

#1. What is the output of the first round of the DES algorithm when the plaintext and the key are both all zeros?

Answer 1

Let the input string is X = zeros(64).
Let the input key is K = zeros(56).
After the initial permutation of X we have L0 = zeros(32), R0 = zeros(32).
After the fixed permutation of K we hawe the round-1 key K1 = zeros(48).
The round-1 formula is
R1 = L0 xor f(R0, K1)
here f is Feistel function:
1) R0 expands to zeros(48) using a fixed permuted expansion rule.
2) The resulting string is xor with K1 and produces a zeros(48)
3) The result is splitted into 8 x zeros(6) chunks. Each chunk is mapped with correspondig S-boxes and becomes
1110
1111
1010
0111
0010
1100
0100
1101
4) At the permutattion stage the resulting 32-bit string is permutted according to the P-table to the following