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  3. 3 the feistel cipher does not use reversible substitutions in...

Question: 3 the feistel cipher does not use reversible substitutions in...

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3. The Feistel cipher does not use reversible substitutions in the Round function. The S-bo support the avalanche criterion. In particular, flipping the value of any one of the 32-bits Right plaintext bits that are input to an S-Box will flip the value of at least two bits of the output. This exercise wants you to show this for particular values. To help you we first illustrate this for one input; you then must show this for another input.In both cases, assume that all round keys are: Case 1 (illustration): Let the input be: a. 1000 0000 0000 0000 0000 0000 0000 0001 (32 bits) The table below is used to expand it 234 5682/27 28 29 30)31 32 12 345456 78924125 26)27 28 29)28)2) 30 31 32 110000 000000 000000 000000 000000 000000 000000 000011 (48 bits)

The result is XORed with the key round key to get Then we use the substitution below to get 10 /010 01000/0101 0130 011 1003 101 1010 101012114 9 01 0001 1101 1101 1101 1101 This is the output. Now flip the first bit of the input, to get 1101 1101 0101 0000 0000 0000 0000 0000 0000 0000 0001 expand it to get, 100000 000000 000000 000000 000000 000000 000000 000010 XOR it with the round key to get, (48 bits) then substitute it by 1000 1101 1101 1101 1101 1101 1101 0110 This is the output. Notice that one flipped bit in the input has caused 4 bits to flip in the output (distanced far apart!) b. Case 2, Now do this for input: 1111 0000 0000 0000 0000 0000 0000 0001 and then flip the first bit to see how many bits get flipped in the output.

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