Cryptography Reference
In-Depth Information
13
47
./
=
2
47
% %3
%7
7
'
3
13
3
7
4
$
./
Figure 10.2
The DES round function
f
.
Consequently,
S
5
(011001) refers to the decimal number 3 that can be written as a
sequence of bits (i.e., 0011). This sequence is the output of the S-box.
Last but not least, we must explain how the 16 round keys
k
1
,...,k
16
∈
48
are derived from the DES key
k
64
. We therefore define
v
i
for
{
0
,
1
}
∈{
0
,
1
}
i
=1
,...,
16:
v
i
=
1
if
i
∈{
1
,
2
,
9
,
16
}
2
otherwise (i.e., if
i
∈{
3
,
4
,
5
,
6
,
7
,
8
,
10
,
11
,
12
,
13
,
14
,
15
}
)
We also use two functions called
PC
1 and
PC
2.
11
PC
1 maps a 64-bit
string (i.e., a DES key
k
) to two 28-bit strings
C
and
D
(i.e.,
PC
1:
64
{
0
,
1
}
→
11
The acronym PC stands for
permuted choice
.
