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.

Example: dCode (64,43,6F,64,65 in hexadecimal) encrypted with the key RC4 (52,43,34 in hexadecimal) is coded 2B,7F,DA,B6,1D (hexadecimal)
Identically 2B,7F,DA,B6,1D (in hexadecimal) decrypted with the same key RC4 (52,43,34 in hex) becomes 64,43,6F,64,65 (dCode in ASCII)

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, ARC4, Alleged RC4 or Ron's Code 4.

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