Answered Essay: first number is: 01010000 01001100 01000001 01001001 01001110 01010100 01000101 01011000

Hi, i am having trouble with the question below, regarding DES block cipher. Could you help me?

1. [40 pts] For the given plaintext, what would be the state (intermediate cipher) after the first round of DES? A plaintext block right after the initial permutatiorn (01010000 01001100 01000001 01001001 01001110 01010100 01000101 01011000) 48 bit sub-key for the first round (11010111 00001010 01011101 00100010 10000011 11010101)

first number is: 01010000 01001100 01000001 01001001 01001110 01010100 01000101 01011000

second number is: 11010111 00001010 01011101 00100010 10000011 11010101

any help is appreciated thank you

Expert Answer

After initial permutation the 64 bit output is devide into to goup of 32 bit each

Say E_l left half and E_r right sub part

E_l=(01010000 01001100 01000001 01001001)

Er=(01001110 01010100 01000101 01011000)

E_i go to expanstion permutation to expand it to 48 bits size as following

E=(32 1 2 3 4 5 4 5 6 7 8 9 8 9 10 11 12 13 12 13 14 15 16 17 16 17 18 19 20 21 20 21 22 23 24 25 24 25 26 27 28 29 28 29 30 31 32 1) these are the bit position in E_l

E_lx=(10101010 00000010 01011000 00100000 00110010 01010010)

Now this 48 bits E_lx is mixed with 48 bits sub-key of the roud.

sub-key say ‘key’ is

key=(11010111 00001010 01011101 00100010 10000011 11010101)

So the state(intemediate cipher ) is : E_lx XOR key

(10101010 00000010 01011000 00100000 00110010 01010010)

$oplus$

(11010111 00001010 01011101 00100010 10000011 11010101)

———————————————————————————————————

01111101 00001000 00000101 00000010 10110001 10000111

Now substitution is applied which is lookup substitution calle s-box. The out put of S-box is 32 bits agian.

then output of s-box goes to P-box to rearange the 32 bit data.

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