Question

Consider two DES keys:

K1 = 11 22 33 44 55 66 77 88 AA BB CC DD EE FF FF FF

K2 = 11 22 33 44 55 66 77 88 11 22 33 44 55 66 77 88

One of the two keys enables “fast” decryption with the 1-DES cipher (a single application of 1-DES). Which one? Explain your answer.

Answer 1

The answer is the key K1. This is because during decryption in DES, we have to split the key in half multiple time and do operations on the partitions. Here, since the partitions will be same, we can reuse the results of one partition during the second one.