Rc4 Algorithm using Matlab without functions

RC4 is a symmetric key stream cipher.

RC4 creates a pseudo-random bit stream (a keystream). These, like any other stream cipher, can be used for encryption by utilizing bit-wise exclusive or to combine it with the plaintext. The same procedure is used for decryption (since exclusive-OR is a symmetric operation).

The cipher uses a secret internal state that is divided into two sections to generate the keystream −

Each of the 256 available bytes is permuted.

Two index pointers (8 bits each).

The key-scheduling algorithm is known to initialize the permutation using a variable-length key, typically between 40 and 256 bits (KSA). A pseudo-random generating technique then generates the stream of bits.

Flow Diagram:

Key-scheduling algorithm (KSA)

The key-scheduling algorithm is used to initialize the permutation in the array "S". "keylength" is defined as the number of bytes in the key and can be in the range 1 ≤ keylength ≤ 256, typically between 5 and 16, corresponding to a key length of 40–128 bits. First, the array "S" is initialized to the identity permutation. S is then processed for 256 iterations in a similar way to the main PRGA, but also mixes in bytes of the key at the same time.

for i from 0 to 255

S[i] := i

end

j := 0

for i from 0 to 255

j := (j + S[i] + key[i mod keylength]) mod 256

swap values of S[i] and S[j]

end

Pseudo-random generation algorithm (PRGA)

The lookup stage of RC4. The output byte is selected by looking up the values of S[i] and S[j], adding them together modulo 256, and then using the sum as an index into S; S(S[i] + S[j]) is used as a byte of the key stream K.

For as many iterations as are needed, the PRGA modifies the state and outputs a byte of the keystream. In each iteration, the PRGA:

increments i;

looks up the ith element of S, S[i], and adds that to j;

exchanges the values of S[i] and S[j], then uses the sum S[i] + S[j] (modulo 256) as an index to fetch a third element of S (the keystream value K below);

then bitwise exclusive ORed (XORed) with the next byte of the message to produce the next byte of either ciphertext or plaintext.

Each element of S is swapped with another element at least once every 256 iterations.

i := 0

j := 0

while GeneratingOutput:

i := (i + 1) mod 256

j := (j + S[i]) mod 256

swap values of S[i] and S[j]

K := S[(S[i] + S[j]) mod 256]

output K endwhile

For encryption −

The user enters the Plaintext and a secret key.

For the secret key entered, the encryption engine creates the keystream using the KSA and PRGA algorithms.

Plaintext is XORed with the generated keystream. Because RC4 is a stream cipher, byte-by-byte XORing is used to generate the encrypted text.

This encrypted text is now sent in encrypted form to the intended recipient.

For Decryption −

The same byte-wise X-OR technique is used on the ciphertext to decrypt it.