Cryptography Reference
In-Depth Information
32
32
56
L
i
R
i
S
i
32
28
28
L
R
cyclic
rotation
Expansion
permutation
48
28
28
56
compression
permutation
48
8
∗
6
S-boxes
8
∗
4
32
P-box
32
32
56
L
i+1
R
i+1
S
i+1
Figure 4.10:
A DES round; the numbers to the right of the boxes denote the
width in bits.
denotes the table column. The number in the corresponding row and column
is the output value. You can see that 4 bits are sufficient for the output: the
largest table entry is 15.
Figure 4.10 shows a schematic representation of a DES round.
Design Features of the Algorithm
If you think that the DES algorithm is rather complicated you are right, but it is
extremely hardware-friendly: no step involved does additions or multiplications,
everything is limited to bitwise shifts, fixed permutations (that are easy to
implement in hardware), and XOR operations. There is a system behind the
complexity:
•
The expansion permutation and the P-box are responsible for diffusion
(more specifically, for the avalanche effect).
•
In addition, the P-boxes ensure that a plaintext bit traverses another S-box
in each round on 'its way through the algorithm'.
