RC4 Cipher

The RC4 digit uses a key that can initialize an array of 256 boxes.

The algorithm that allows to initialize the array with the key key is:

// Pseudocode
for i = 0 ... 255 {
t[i] = i
}
j = 0
k = length(cle)
for i = 0 ... 255 {
j = (j + t[i] + key[i % k]) % 256
swap t[i] <-> t[j]
}

The array t can then be used to generate a stream by moving values and XOR operation.

The RC4 algorithm is then: // Pseudocode
a = b = 0
j = length(string)
codes = []
for i = 0 ... j {
a = (a + 1) % 256
b = (b + t[a]) % 256
swap t[a] <-> t[b]
codes []= ( t[ (t[a] + t[b]) % 256] ) XOR string[i]
}
print codes

The codes are values between 0 and 255.

Decryption is exactly the same as encryption.

The codes generated by RC4 are between 0 and 255, usually represented in hexadecimal.

RC4 is pseudo-random, there is no easily detectable bias.

The code is also called RCfour, ARCFour, ARC4, Alleged RC4 or Ron's Code 4.

Any reference to WEP or TLS protocols is a clue.

RC4 was invented by Ronald Rivest (one of the inventors of RSA encryption) in 1987.