In DES (Data Encryption Standard) every time a block is encrypted the user requi
ID: 3797837 • Letter: I
Question
In DES (Data Encryption Standard) every time a block is encrypted the user requires to supply 16 keys comprised of 56 bits each.
Mark the correct answer below:
a) The rounds in DES, each uses a sub-key derived from the single key provided by the user.
b) No, 16 plus 2, because of the transposition stage at the beginning and at the end.
c) No, every 16 blocks it is necessary to use a different key to keep the entropy in order.
d) No! 3 key bits are used for each round, and four for each transposition, so together 3*16+4+4=56, which is the size of the DES key.
Explanation / Answer
In the fi rst approach, round 16 is different from other rounds; there is no swapper in this round. This is needed to make the last mixer in the cipher and the fi rst mixer in the reverse cipher aligned. We can make all 16 rounds the same by including one swapper to the 16th round and add an extra swapper after that (two swappers cancel the effect of each other). We leave the design for this approach as an exercise. Key Generation The round-key generator creates sixteen 48-bit keys out of a 56-bit cipher key. However, the cipher key is normally given as a 64-bit key in which 8 extra bits are the parity bits, which are dropped before the actual key-generation process