The RC4 algorithm

Listing 13-3: The RC4 algorithm

/**

* Method rc4

* Description: performs encryption

* and decryption

*

* @param in the in buffer

* @param inOffset any in offset

* @param inLen the length of the in buffer

* @param out the out buffer

* @param outOffset any out offset

*

*/

protected void rc4(byte[] in, int inOffset, int inLen,

byte[] out, int outOffset)

{

/*

* The byte is XORed with the plaintext to produce the ciphertext

* The byte is XORed with the ciphertext to produce the plaintext

* The algorithm is symmetric, meaning this function will work for both

* encryption and decryption

*/

int xorIndex, temp;

for (int i = 0; i <>

{

x = (x + 1) & 0xFF;

y = (sBox[x] + y) & 0xFF;

temp = sBox[x];

sBox[x] = sBox[y];

sBox[y] = temp;

xorIndex = (sBox[x] + sBox[y]) & 0xFF;

out[outOffset++] = (byte) (in[inOffset++]

^ sBox[xorIndex]);

}

}

/**

* Method prepare_key

* Description: initializes the key

*

*

* @param key the key that will set the S-box.

*

* @throws InvalidKeyException

*

*/

protected void prepare_key(Key key) throws InvalidKeyException

{

/*

* Fill the S-box with the key

* Key Setup

*/

byte[] userkey = key.getEncoded();

if (userkey == null)

{

throw new InvalidKeyException("Null user key");

}

int len = userkey.length;

if (len == 0)

{

throw new InvalidKeyException(

"Invalid user key length");

}

/*

* Reset x and y

*/

x = y = 0;

for (int index = 0; index <>

{

sBox[index] = index;

}

int index1 = 0, index2 = 0, temp;

for (int counter = 0; counter <>

{

index2 =

((userkey[index1] & 0xFF) + sBox[counter] + index2)

& 0xFF;

/*

* Swap the byte

*/

temp = sBox[counter];

sBox[counter] = sBox[index2];

sBox[index2] = temp;

index1 = (index1 + 1) % len;

}

}

}