In cryptography, RC4 (also known as ARC4 or ARCFOUR, meaning Alleged RC4, see below) is a stream cipher. While it is remarkable for its simplicity and speed in software, multiple vulnerabilities have been discovered in RC4, rendering it insecure. It is especially vulnerable when the beginning of the output keystream is not discarded, or when nonrandom or related keys are used. Particularly problematic uses of RC4 have led to insecure protocols such as the obsolete WEP protocol historically used to secure WiFi networks. There has long been speculation that some state cryptologic agencies may possess the capability to break RC4 when used in the TLS protocol. In response, the IETF published RFC 7465 to prohibit the use of RC4 in TLS; Mozilla and Microsoft have issued similar recommendations. A number of attempts have been made to strengthen RC4, notably Spritz, RC4A, VMPC, and RC4+.
History RC4 is a stream cipher designed by Ronald Rivest of RSA Security in 1987. According to Rivest, the letters RC stand for "Ron's Code", though in general it is simply referred to as RC4. The same naming convention applies to RC2, RC5, and RC6. RC4 was initially a trade secret, but in September 1994, a description of it was anonymously posted to the Cypherpunks mailing list. It was soon posted on the sci.crypt newsgroup, where it was broken within days by Bob Jenkins. From there, it spread to many sites on the Internet. The leaked code was confirmed to be genuine, as its output was found to match that of proprietary software using licensed RC4. Because the algorithm is known, it is no longer a trade secret. The name RC4 is trademarked, so RC4 is often referred to as ARCFOUR or ARC4 (meaning alleged RC4) to avoid trademark problems. RSA Security has never officially released the algorithm; Rivest has, however, linked to the English Wikipedia article on RC4 in his own course notes in 2008 and confirmed the history of RC4 and its code in a 2014 paper. RC4 became part of some commonly used encryption protocols and standards, such as WEP in 1997 and WPA in 2003/2004 for wireless cards; and SSL in 1995 and its successor TLS in 1999, until it was prohibited for all versions of TLS in 2015 by RFC 7465, due to the RC4 attacks weakening or breaking RC4 used in SSL/TLS. The main factors in RC4's success over such a wide range of applications have been its speed and simplicity: efficient implementations in both software and hardware were very easy to develop.
Description RC4 generates a pseudorandom stream of bits (a keystream). As with any stream cipher, these can be used for encryption by combining it with the plaintext using bitwise exclusive or; decryption is performed the same way (since exclusive or with given data is an involution). This is similar to the one-time pad, except that generated pseudorandom bits, rather than a prepared stream, are used. To generate the keystream, the cipher makes use of a secret internal state which consists of two parts:
A permutation of all 256 possible bytes (denoted "S" below). Two 8-bit index-pointers (denoted "i" and "j"). The permutation is initialized with a variable-length key, typically between 40 and 2048 bits, using the key-scheduling algorithm (KSA). Once this has been completed, the stream of bits is generated using the pseudo-random generation algorithm (PRGA).
Key-scheduling algorithm (KSA) The key-scheduling algorithm is used to initialize the permutation in the array "S". "keylength" is defined as the number of bytes in the key and can be in the range 1 ≤ keylength ≤ 256, typically between 5 and 16, corresponding to a key length of 40–128 bits. First, the array "S" is initialized to the identity permutation. S is then processed for 256 iterations in a similar way to the main PRGA, but also mixes in bytes of the key at the same time. Note that many different keys like 'Text' and 'TextText' lead to the same cipher.
for i from 0 to 255 S[i] := i endfor j := 0 for i from 0 to 255 j := (j + S[i] + key[i mod keylength]) mod 256 swap values of S[i] and S[j] endfor
Pseudo-random generation algorithm (PRGA)
For as many iterations as are needed, the PRGA modifies the state and outputs a byte of the keystream. In each iteration, the PRGA:
increments i; looks up the ith element of S, S[i], and adds that to j; exchanges the values of S[i] and S[j], then uses the sum S[i] + S[j] (modulo 256) as an index to fetch a third element of S (the keystream value K below); then bitwise exclusive ORed (XORed) with the next byte of the message to produce the next byte of either ciphertext or plaintext. Each element of S is swapped with another element at least once every 256 iterations.
i := 0 j := 0 while GeneratingOutput: i := (i + 1) mod 256 j := (j + S[i]) mod 256 swap values of S[i] and S[j] t := (S[i] + S[j]) mod 256 K := S[t] output K endwhile
Thus, this produces a stream of K[0], K[1], ... which are XORed with the plaintext to obtain the ciphertext. So ciphertext[l] = plaintext[l] ⊕ K[l].
RC4-based random number generators Several operating systems include arc4random, an API originating in OpenBSD providing access to a random number generator originally based on RC4. The API allows no seeding, as the function initializes itself using /dev/random. The use of RC4 has been phased out in most systems implementing this API. Man pages for the new arc4random include the backronym "A Replacement Call for Random" for ARC4 as a mnemonic, as it provided better random data than the original and highly insecure rand() function based on a linear congruential pseudo-random number generator with a 32-bit internal state. Several attacks on RC4 are able to distinguish its output from a random sequence. As a result the use of ARC4 in arc4random was eventually replaced with better pseudo-random number generators:
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