Cryptography Reference
In-Depth Information
to transform 64 bits of plaintext blockwise into 64 bits of ciphertext, and
vice versa,
which is done in 16 key-dependent rounds, where
a fixed, bitwise transposition (i.e., permutation) is done before the first
round and after the last round. The final permutation is the reversion of
the first one.
You know from Section 4.2 that the blocks in a Feistel network are split into
equally large left and right halves, and that each round has the following form:
L i
1 =R i
+
R i
1 =L i
f S , i (R i )
+
So DES looks roughly as shown in Figure 4.7.
How is the round- and key-dependent function, f , structured in DES?
First, the 56-bit key is modified depending on the round, and 48 bits are
selected.
key
plaintext
EP
fixed input permutation
S 1
S 2
16 Feistel rounds
S 15
S 16
AP
fixed output permutation
ciphertext
Figure 4.7: Rough scheme of DES.
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