Answered Essay: Exercise 1. Compute the Round 1 DES key given the original key input is

Exercise 1. Compute the Round 1 DES key given the original key input is

K[0] = 0101010101010101HEX

Note: Here we’re assuming odd parity bits.

Expert Answer

 

Numbers 14 17 11 24 1 5 3 … come from permuted choice PC-2. To explain 16 round DES key scheduling:

DES input key size is 64 which contains 56 bit key and 8 bit parity. Parity bits are 8th bit of every 8 bit (one byte). So they are all multiple of eight: 8, 16, 24, 32, 40, 48, 56 and 64. Permuted choice PC-1 is used to remove these bits from the 64 bit input key. So PC-1 gives 56bits as output.

In round i (1 <= i <= 16), there is a 56 bit input, C_(i-1) as left half and D_(i-1) as right half (each 28 bit). These two halves are rotated left (for decryptionright rotate is used). In encryption, rotate amount in rounds 1, 2, 9 and 16 is one and in other rounds it’s two. In decryptiononebit right rotation in rounds2,9,16 and two bit right rotation in all other rounds. As inputs of rotation are C_i and D_i, outputs of rotation are C_(i+1) and d_(i+1) which are passed to the next round,i+1, as input.

After all, in round i there is a 48 bit output, named K_i. This sub key is generated from permuted choicePC-2, which takes C_i and D_i as a 56 bit input and outputs the sub key.

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