RC4 Encryption Algorithm – GeeksforGeeks

May 24, 2022
RC4 is a pour nothing and variable-length cardinal algorithm. This algorithm encrypts one byte at a time ( or larger units at a clock time ).
A key input is pseudorandom piece generator that produces a pour 8-bit count that is unpredictable without cognition of input signal key, The output of the generator is called key-stream, is combined one byte at a time with the plaintext stream zero 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 bytes is used to initialize a 256-byte submit vector S, with elements S [ 0 ] to S [ 255 ]. For encoding and decoding, a byte thousand is generated from S by selecting one of the 255 entries in a systematic manner, 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 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];`

we use T to produce the initial substitution of S. Starting with S [ 0 ] to S [ 255 ], and for each S [ one ] algorithm trade it with another byte in S according to a schema dictated by T [ one ], 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]);` `    ` `}` Read more: Dual_EC_DRBG – Wikipedia

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 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];`

3. Encrypt using X-Or():

News :
In September 2015, Microsoft announced the end of using RC4 in Microsoft boundary and internet internet explorer 11. This video gives a absolved model of RC4 algorithm

My Personal Notes

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