# Answer to Question #63374 in Computer Networks for Gene Moore

Question #63374

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

Expert's answer

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

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

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