RC4 Cipher

RC4 (Rivest Cipher 4) is a symmetric encryption algorithm, designed by Ronald Rivest in 1987. RC4 has been used in protocols like TLS and WEP, but it is now considered cryptographically weak.

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.

An RC4 cipher produces bytes between 0 and 255, often represented in hexadecimal.

The generated stream is pseudo-random, but RC4 exhibits known statistical biases (particularly in the first few bytes of the stream). These biases are difficult to detect in small messages but exploitable on a large scale.

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.

The algorithm remained secret for several years before being anonymously leaked on the internet in 1994. This leak contributed to its widespread adoption in numerous software programs and protocols.