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.
| 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 decryption, right rotate is used). In encryption, rotate amount in rounds 1, 2, 9 and 16 is one and in other rounds it’s two. In decryption, onebit 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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