RC4 (Rivest Cipher 4) Algorithm
RC4 is a stream cipher and
variable length key algorithm. This algorithm encrypt one byte at a time (or
larger units on a time).
A key input is pseudorandom bit generator
that produces a stream 8-bit number that is unpredictable without knowledge of
input key, The output of generator is called key-stream, is combined one byte
at a time with the plaintext stream cipher using X-OR operation.
Encryption
|
Decryption
|
10011000
Å
01010000
= 11001000
|
11001000
Å
01010000
= 10011000
|
Key-Generation
Algorithm:
A variable-ength key of from 1 to 256 byte is used to
initialize a 256-byte state vector S, with elements S[0] to S[255]. For
encryption and decryption, a byte k is generated from S by selecting one of te
255 entries in systematic fashion, then the entries in S are permuted again.
1- Key-Scheduling Algorithm
Initialization: The entries of S are
set equal to the values from 0 to 255 in ascending order, a temporary vector T,
is created.
If length of the key k is 256 bytes , then
k is assigned to T. Otherwise, for a key with length(k-len) bytes, the first
k-len elements of T as copied from K and then K is repeated as many times as
necessary to fill T. The idea is
illustrated as follow:
for i = 0 to 255 do
S[i] = i;
T[i] = K[i mod k-len];
S[i] = i;
T[i] = K[i mod k-len];
we use T to produce the initial
permutation of S. Starting with S[0] to S[255], and for each S[i] algorithm
swap it with another byte in S according to a scheme dictated by T[i],but S
will still contain values from 0 to 255 :
j = 0;
for i = 0 to 255 do
{j = (j + S[i] + T[i]) mod 256;
Swap (S[i], S[j]);}
for i = 0 to 255 do
{j = (j + S[i] + T[i]) mod 256;
Swap (S[i], S[j]);}
2- Pseudo random generation algorithm (Stream Generation)
Once the vector S is initialized, the input
key will not be used. In this step, for each S[i] algorithm swap it with
another byte in S according to a scheme dictated by current configuration of S.
After reaching S[255] the process continues, starting from S[0] again
i, j = 0;
while (true)
i = (i + 1) mod 256;
j = (j + S[i]) mod 256;
Swap (S[i], S[j]);
t = (S[i] + S[j]) mod 256;
k = S[t];
while (true)
i = (i + 1) mod 256;
j = (j + S[i]) mod 256;
Swap (S[i], S[j]);
t = (S[i] + S[j]) mod 256;
k = S[t];
3- Encrypt using X-Or
News:
In September 2015, Microsoft announced the end of using RC4
in Microsoft edge and internet explorer 11.
This video gives clear example of RC4 algorithm
