**RC4 **is a stream cipher and variable length key algorithm. This algorithm encrypts 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 the generator is called key-stream, is combined one byte at a time with the plaintext stream cipher using X-OR operation.

**Example:**

RC4 Encryption 10011000 ? 01010000 = 11001000 RC4 Decryption 11001000 ? 01010000 = 10011000

**Key-Generation Algorithm –**

A variable-length key 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 the 255 entries in a systematic fashion, then the entries in S are permuted again.

**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 the 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];`

*chevron_right**filter_none*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]);`

`}`

*chevron_right**filter_none***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 the 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];`

*chevron_right**filter_none***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 a clear example of RC4 algorithm

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