Cryptography Reference
In-Depth Information
⊕
⊕
⊕
⊕
⊕
⊕
⊕
⊕
⊕
⊕
⊕
⊕
⊕
⊕
⊕
⊕
⊕
⊕
⊕
⊕
⊕
S
1
S
2
S
3
S
4
S
5
S
6
S
7
S
8
Figure 2.4.
DES round function.
The round function of DES has a main 32-bit input, a 48-bit subkey parameter
input, and a 32-bit output. For every round, the 48-bit subkey is generated from the
secret key by a key schedule. Basically, every 48-bit subkey consists of a permutation
and a selection of 48 out of the 56 bits of the secret key. As illustrated in Fig. 2.4, the
round function consists of
an expansion of the main input (one out of two input bits is duplicated) in order
to get 48 bits,
a XOR with the subkey,
eight substitution boxes which transform a 6-bit input into a 4-bit output,
a permutation of the final 32 bits (which can be seen as a kind of transposition).
The substitution boxes (called S-boxes) have a 6-bit input and a 4-bit output. We
have eight S-boxes called
S
1
,
S
8
. They are defined by tables in the standard.
The tables however need to be read in a special way. For instance,
S
3
is defined by
S
2
,...,
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
10
0
9
14
6
3
15
5
1
13
12
7
11
4
2
8
13
7
0
9
3
4
6
10
2
8
5
14
12
11
15
1
13
6
4
9
8
15
3
0
11
1
2
12
5
10
14
7
1
10
13
0
6
9
8
7
4
15
14
3
11
5
2
12
Search WWH ::
Custom Search