Cryptography Reference
In-Depth Information
Description of RC4
RC4 is a stream cipher, i.e., a secret key is used to create a secret byte sequence
(
K
i
)
, and simple XOR is used for encryption and decryption:
Ciphertext byte
i
=
plaintext byte
i
⊕
K
i
Plaintext byte
i
=
ciphertext byte
i
⊕
K
i
Internal Key
Two bytes,
i
and
j
, and a permutation,
(P
i
)
i
=
0
,...,
255
, of the numbers 0
,...,
255 are
used as internal key, i.e., each of these numbers occurs exactly once in sequence
(
P
i
)
. The size of the key space is approximately 2
1700
(
=
256
2
∗
256!).
Ciphering
A step to create a key byte,
K
, looks like this:
i = i + 1 mod 256
j=j+P
i
mod 256
swap P
i
and P
j
t=P
i
+P
j
mod 256
K=P
t
.
(A modulo-256 addition is simply a bytewise addition — in C adding two data of
the type
unsigned char
.)
Creating the Internal Key
Let the key entered consist of
l
bytes,
S
0
,...,S
l
−
1
.Weset
i=0
j=0
P
k
= k (k=0,...,255)
and compute the following 256 times:
j=j+P
i
+S
i
mod 256
swap P
i
and P
j
i = i + 1 mod l.
With this,
i, j
, and the
P
k
are initialized.
Comment
: In [SchnCr, 17.1],
i
traverses all indices from 0 to 255, and the key
is iterated as often as needed until all 256 bytes,
S
0
,...,S
255
, are filled. I didn't
check whether or not the two versions are equivalent. My variant stems from tested
C programs.
Figure 5.21:
Description of the RC4 stream cipher.
