Cryptography Reference
In-Depth Information
always the same (all the integers ranging from 0 to 255) and are permuted by the
algorithm based on the values of the two counters
i
and
j
. In order to initialize the
inner state of the cipher, RC4 performs a first permutation of the
S
array (described
in Algorithm 4.1) according to the key supplied, which may be from 5 to 256 bytes
long. The initialization operates 256 swaps among the variables, employing an eight-
bit slice of the key together with the value of
j
to determine the index of the array
cells to swap.
Algorithm 14.1
: The RC4 key schedule
Input
:
K
=
(
k
0
,
k
1
,...,
k
l
)
: cipher key
l
: key length in bytes
Output
:
i
,
j
,
S
=
(
s
0
,...,
s
255
)
: initialized inner state
begin
1
S
←
(
0
,
1
,
2
,...,
255
)
2
j
←
0
3
i
←
0
4
for
i
<
255
do
5
j
←
(
j
+
s
j
+
k
i
mod
l
)
mod 256
6
temp
←
s
i
7
s
i
←
s
j
8
s
j
←
temp
9
i
←
i
+
1
10
end
11
i
←
0
12
j
←
0
13
return
i
,
j
,
S
14
end
15
Algorithm 14.2
: The RC4 step function
Input
:
i
,
j
,
S
=
(
s
0
,...,
s
255
)
the inner state of the cipher
Output
:
o
the output byte
i
←
i
+
1 mod 256
1
j
←
(
j
+
s
i
)
mod 256
2
temp
←
s
i
3
s
i
←
s
j
4
s
j
←
temp
5
return
o
←
s
s
i
+
s
j
6
After the initialization step is completed, the cipher is able to output an eight-bit
value per step, obtaining it as described by Algorithm 4.2. At first, the value of
i
,
which acts as a counter, is incremented by 1, and employed to update the value of
j
based on the value of the
i
th cell of the state. After the updates, the values of
i
and
j
are used to perform a swap between two values of the inner state and output
a single byte based on their contents. Due to the nature of the updating algorithm, it
