Cryptography Reference
In-Depth Information
Therefore, the action of
IP
on
x
is given in the following.
IP
j
1 2 3 4 5 6 7 8
m
IP
(
j
)
m
2
m
6
m
3
m
1
m
4
m
8
m
5
m
7
Hence,
IP
(
m
)=
m
2
m
6
m
3
m
1
m
4
m
8
m
5
m
7
).
For instance, if
m
=
(10010111), then
IP
(
m
) = (01011101).
The next component is also a permutation used at various stages of S-DES.
Expansion Permutation
This permutation, denoted by
EP
, takes a
bitstring
(binary number) of
length 4 (its
bitlength
), and expands it into a byte according to the following.
EP
j
12345678
EP
(
j
) 41232341
For instance, if
x
=(
x
1
x
2
x
3
x
4
) is the input, then the following table gives
us the action of
EP
on it.
EP
j
12345678
x
E
P
(
j
)
x
4
x
1
x
2
x
3
x
2
x
3
x
4
x
1
Hence,
EP
(
x
)=(
x
4
x
1
x
2
x
3
x
2
x
3
x
4
x
1
). For example, if
x
= (1001), then
EP
(
x
) = (11000011).
A very important aspect of S-DES is the key schedule. In other words, we
need to understand how the keys are used and generated in the cipher.
S-DES Key Generation
S-DES uses a 10-bit secret (shared) symmetric key
k
=(
e
1
e
2
e
3
e
4
e
5
e
6
e
7
e
8
e
9
e
10
)
,
say, and employs
k
to generate two 8-bit (sub)keys for deployment at various
stages of the encryption and decryption process. Here is how that is accom-
plished.
First, a permutation
P
10
is applied to
k
according to the following.
P
10
j
12345 6 789 0
P
10
(
j
) 35274 0198 6
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