Cryptography Reference
In-Depth Information
the second output position, and so forth. The final permutation
IP
−
1
performs the
inverse operation of
IP
as shown in Fig. 3.9.
IP
−
1
408481656246432
397471555236331
386461454226230
375451353216129
364441252206028
353431151195927
342421050185826
33141 9 49175725
IP
58 50 42 34 26 18 10 2
60 52 44 36 28 20 12 4
62 54 46 38 30 22 14 6
64 56 48 40 32 24 16 8
57 49 41 33 25 17 9 1
59 51 43 35 27 19 11 3
61 53 45 37 29 21 13 5
63 55 47 39 31 23 15 7
Fig. 3.9
Final permutation
IP
−
1
Fig. 3.8
Initial permutation
IP
3.3.2 The
f
-Function
As mentioned earlier, the
f
-function plays a crucial role for the security of DES.
In round
i
it takes the right half
R
i
−
1
of the output of the previous round and the
current round key
k
i
as input. The output of the
f
-function is used as an XOR-mask
for encrypting the left half input bits
L
i
−
1
.
R
i−1
32
Expansion
i−1
E(R )
48
48
k
i
48
6
6
6
6
6
6
6
6
S
1
S
2
S
3
S
4
S
5
S
6
S
7
S
8
4
4
4
4
4
4
4
4
32
Permutation
P
32
Fig. 3.10
Block diagram of the
f
-function
