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.
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 to S. 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:
S[i] = i;
T[i] = K[i mod k - len];
we use T to produce the initial permutation of S. Starting with S to S, 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 = (j + S[i] + T[i])mod
- 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 the process continues, starting from S again
i, j =
i = (i +
j = (j + S[i])mod
t = (S[i] + S[j])mod
k = S[t];
- Encrypt using X-Or():
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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