Conventional Cryptography - A Classical Introduction to Cryptography: Applications for Communications Security - page 23

K 1

⊕ F

K 2

⊕

F

K 3

⊕

F

Figure 2.2. Function ( F K 1 , F K 2 , F K 3 ).

is XORed to the other part. We obtain two parts which are then exchanged (except in
the final round). The round function uses subkeys derived from a secret key.

This elementary process is iterated, and the number of round function applications
is called the number of rounds. We usually denote F r ) the permutation ob-

tained from an r -round Feistel scheme in which the round functions are F 1 ,...,

( F 1 ,...,

F r . All
F i may come from a single function F with a parameter K i defined by a subkey. We de-
note F i = F K i . Fig. 2.2 illustrates a 3-round Feistel scheme. DES consists of 16 rounds.

More precisely, DES starts by a bit permutation IP, performs the Feistel cipher
using subkeys generated by a key schedule, and finally performs the inverse of the IP
permutation. This is illustrated in Fig. 2.3.

X K

IP

K 1
K 2

Feistel Schedule

K 16

IP − 1

Y

Figure 2.3. DES architecture.