Cryptography Reference
In-Depth Information
Table 10.6
The S-Boxes
S
1
to
S
8
of the DES
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
S
1
0
14
4
13
1
2
15
11
8
3
10
6
12
5
9
0
7
1
0
15
7
4
14
2
13
1
10
6
12
11
9
5
3
8
2
4
1
14
8
13
6
2
11
15
12
9
7
3
10
5
0
3
15
12
8
2
4
9
1
7
5
11
3
14
10
0
6
13
S
2
0
15
1
8
14
6
11
3
4
9
7
2
13
12
0
5
10
1
3
13
4
7
15
2
8
14
12
0
1
10
6
9
11
5
2
0
14
7
11
10
4
13
1
5
8
12
6
9
3
2
15
3
13
8
10
1
3
15
4
2
11
6
7
12
0
5
14
9
S
3
0
10
0
9
14
6
3
15
5
1
13
12
7
11
4
2
8
1
13
7
0
9
3
4
6
10
2
8
5
14
12
11
15
1
2
13
6
4
9
8
15
3
0
11
1
2
12
5
10
14
7
3
1
10
13
0
6
9
8
7
4
15
14
3
11
5
2
12
S
4
0
7
13
14
3
0
6
9
10
1
2
8
5
11
12
4
15
1
13
8
11
5
6
15
0
3
4
7
2
12
1
10
14
9
2
10
6
9
0
12
11
7
13
15
1
3
14
5
2
8
4
3
3
15
0
6
10
1
13
8
9
4
5
11
12
7
2
14
S
5
0
2
12
4
1
7
10
11
6
8
5
3
15
13
0
14
9
1
14
11
2
12
4
7
13
1
5
0
15
10
3
9
8
6
2
4
2
1
11
10
13
7
8
15
9
12
5
6
3
0
14
3
11
8
12
7
1
14
2
13
6
15
0
9
10
4
5
3
S
6
0
12
1
10
15
9
2
6
8
0
13
3
4
14
7
5
11
1
10
15
4
2
7
12
9
5
6
1
13
14
0
11
3
8
2
9
14
15
5
2
8
12
3
7
0
4
10
1
13
11
6
3
4
3
2
12
9
5
15
10
11
14
1
7
6
0
8
13
S
7
0
4
11
2
14
15
0
8
13
3
12
9
7
5
10
6
1
1
13
0
11
7
4
9
1
10
14
3
5
12
2
15
8
6
2
1
4
11
13
12
3
7
14
10
15
6
8
0
5
9
2
3
6
11
13
8
1
4
10
7
9
5
0
15
14
2
3
12
S
8
0
13
2
8
4
6
15
11
1
10
9
3
14
5
0
12
7
1
1
15
13
8
10
3
7
4
12
5
6
11
0
14
9
2
2
7
11
4
1
9
12
14
2
0
6
10
13
15
3
5
8
3
2
1
14
7
4
10
8
13
15
12
9
0
3
5
6
11
that only 48 bits are taken into account and that
b
9
,
b
18
,
b
22
,
b
25
,
b
35
,
b
38
,
b
43
,
and
b
54
are discarded.
To derive the 16 round keys
k
1
,...,k
16
from the DES key
k
, (
C
0
,D
0
)
are first initialized with
PC
1(
k
) according to the construction given earlier. For
i
=1
,...
16,
C
i
is then set to the string that results from a cyclic shift left of
C
i−
1
for
v
i
positions, and
D
i
is set to the string that results from a cyclic shift left of
D
i−
1
for
v
i
positions. Finally, the round key
k
i
is the result of concatenating
C
i
and
D
i
, and applying the function
PC
2 to the result (i.e.,
k
i
=
PC
2(
C
i
D
i
).The
resulting key schedule calculation is illustrated in Figure 10.3.
In the literature, many numeric examples can be found to illustrate the working
principles of the DES encryption algorithm or to verify the correct input-output
behavior of a specific DES implementation, respectively.
10.2.1.3
Decryption Algorithm
As mentioned earlier, the DES is a Feistel cipher and as such the decryption
algorithm is the same as the encryption algorithm. This means that Algorithm 3.1
can also be used for decryption. The only difference is that the key schedule must
