In cryptography, RC4 (also known as ARC4 or ARCFOUR meaning Alleged RC4, see below) is the most widelyused software stream cipher and is used in popular protocols such as Secure Sockets Layer (SSL) (to protect Internet traffic) and WEP (to secure wireless networks). While remarkable for its simplicity and speed in software, RC4 has weaknesses that argue against its use in new systems.^{[1]} It is especially vulnerable when the beginning of the output keystream is not discarded, nonrandom or related keys are used, or a single keystream is used twice; some ways of using RC4 can lead to very insecure cryptosystems such as WEP.
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RC4 was designed by Ron Rivest of RSA Security in 1987. While it is officially termed "Rivest Cipher 4", the RC acronym is alternatively understood to stand for "Ron's Code"^{[2]} (see also 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^{[3]}. It was soon posted on the sci.crypt newsgroup, and from there 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, however. RC4 is often referred to as "ARCFOUR" or "ARC4" (meaning Alleged RC4, because RSA has never officially released the algorithm), to avoid possible trademark problems. It has become part of some commonly used encryption protocols and standards, including WEP and WPA for wireless cards and TLS.
The main factors which helped its deployment over such a wide range of applications consisted in its impressive speed and simplicity. Implementations in both software and hardware are very easy to develop.
RC4 generates a pseudorandom stream of bits (a keystream) which, for encryption, is combined with the plaintext using bitwise exclusiveor; decryption is performed the same way (since exclusiveor is a symmetric operation). (This is similar to the Vernam cipher except that pseudorandom bits, rather than random bits, are used.) To generate the keystream, the cipher makes use of a secret internal state which consists of two parts:
The permutation is initialized with a variable length key, typically between 40 and 256 bits, using the keyscheduling algorithm (KSA). Once this has been completed, the stream of bits is generated using the pseudorandom generation algorithm (PRGA).
The keyscheduling 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 algorithm, but also mixes in bytes of the key at the same time.
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(&S[i],&S[j]) endfor
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, adds the value of S pointed to by i to j, exchanges the values of S[i] and S[j], and then outputs the value of S at the location S[i] + S[j] (modulo 256). Each value of S is swapped at least once every 256 iterations.
i := 0 j := 0 while GeneratingOutput: i := (i + 1) mod 256 j := (j + S[i]) mod 256 swap(&S[i],&S[j]) byte_cipher := S[(S[i] + S[j]) mod 256] result_ciphered := byte_cipher XOR byte_message endwhile
Many stream ciphers are based on linear feedback shift registers (LFSRs), which while efficient in hardware are less so in software. The design of RC4 avoids the use of LFSRs, and is ideal for software implementation, as it requires only byte manipulations. It uses 256 bytes of memory for the state array, S[0] through S[255], k bytes of memory for the key, key[0] through key[k1], and integer variables, i, j, and y. Performing a modulus 256 can be done with a bitwise AND with 255 (or on most platforms, simple addition of bytes ignoring overflow).
Here is a simple implementation in C:
unsigned char S[256]; unsigned int i, j; void swap(unsigned char *s, unsigned int i, unsigned int j) { unsigned char temp = s[i]; s[i] = s[j]; s[j] = temp; } /* KSA */ void rc4_init(unsigned char *key, unsigned int key_length) { for (i = 0; i < 256; i++) S[i] = i; for (i = j = 0; i < 256; i++) { j = (j + key[i % key_length] + S[i]) & 255; swap(S, i, j); } i = j = 0; } /* PRGA */ unsigned char rc4_output() { i = (i + 1) & 255; j = (j + S[i]) & 255; swap(S, i, j); return S[(S[i] + S[j]) & 255]; } #include <stdio.h> #include <string.h> #include <stdlib.h> #define ARRAY_SIZE(a) (sizeof(a)/sizeof(a[0])) int main() { unsigned char *test_vectors[][2] = { {"Key", "Plaintext"}, {"Wiki", "pedia"}, {"Secret", "Attack at dawn"} }; int x; for (x = 0; x < ARRAY_SIZE(test_vectors); x++) { int y; rc4_init(test_vectors[x][0], strlen((char*)test_vectors[x][0])); for (y = 0; y < strlen((char*)test_vectors[x][1]); y++) printf("%02X", test_vectors[x][1][y] ^ rc4_output()); printf("\n"); } return 0; }
These test vectors are not official, but convenient for anyone testing their own RC4 program. The keys and plaintext are ASCII, the ciphertext is in hexadecimal.
Key  Keystream  Plaintext  Ciphertext 

Key 
eb9f7781b734ca72a719... 
Plaintext 
BBF316E8D940AF0AD3 
Wiki 
6044db6d41b7... 
pedia 
1021BF0420 
Secret 
04d46b053ca87b59... 
Attack at dawn 
45A01F645FC35B383552544B9BF5 
Unlike a modern stream cipher (such as those in eSTREAM), RC4 does not take a separate nonce alongside the key. This means that if a single longterm key is to be used to securely encrypt multiple streams, the cryptosystem must specify how to combine the nonce and the longterm key to generate the stream key for RC4. One approach to addressing this is to generate a "fresh" RC4 key by hashing a longterm key with a nonce. However, many applications that use RC4 simply concatenate key and nonce; RC4's weak key schedule then gives rise to a variety of serious problems.
The RC4 cipher is highly vulnerable to a Bitflipping attack if not implemented correctly,^{[4]} and for this reason has been deprecated by software companies such as Microsoft. The .Net framework runtime does not include an implementation of the cipher.
In 1995, Andrew Roos experimentally observed that the first byte of the keystream is correlated to the first three bytes of the key and the first few bytes of the permutation after the KSA are correlated to some linear combination of the key bytes. These biases remained unproved until 2007, when Paul, Rathi and Maitra^{[6]} proved the keystreamkey correlation and Paul and Maitra proved the permutationkey correlations.^{[7]} The latter work also used Roos' permutationkey correlations to design the first algorithm for complete key reconstruction from the final permutation after the KSA, without any assumption on the key or IV. This algorithm has a constant probability of success in a time which is the square root of the exhaustive key search complexity. Subsequently, many other works have been done on key reconstruction from RC4 internal states.^{[8]}^{[9]}^{[10]} In another work, Maitra and Paul^{[11]} showed that the Roos type biases still persist even when one considers nested permutation indices, like S[S[i]] or S[S[S[i]]]. These types of biases are used in some of the later key reconstruction methods for increasing the success probability.
The keystream generated by the RC4 is biased in varying degrees towards certain sequences. The best such attack is due to Itsik Mantin and Adi Shamir who showed that the second output byte of the cipher was biased toward zero with probability 1/128 (instead of 1/256). This is due to the fact that if the third byte of the original state is zero, and the second byte is not equal to 2, then the second output byte is always zero. Such bias can be detected by observing only 256 bytes.^{[12]}
Souradyuti Paul and Bart Preneel of COSIC showed that the first and the second bytes of the RC4 were also biased. The number of required samples to detect this bias is 2^{25} bytes.^{[13]}
Fluhrer and McGrew also showed such attacks which distinguished the keystream of the RC4 from a random stream given a gigabyte of output.^{[14]}
The complete characterization of a single step of RC4 PRGA was performed by Basu, Ganguly, Maitra and Paul.^{[15]} Considering all the permutations, they prove that the distribution of the output is not uniform given i and j, and as a consequence, information about j is always leaked from the output.
In 2001, a new and surprising discovery was made by Fluhrer, Mantin and Shamir: over all possible RC4 keys, the statistics for the first few bytes of output keystream are strongly nonrandom, leaking information about the key. If the longterm key and nonce are simply concatenated to generate the RC4 key, this longterm key can be discovered by analysing a large number of messages encrypted with this key.^{[16]} This and related effects were then used to break the WEP ("wired equivalent privacy") encryption used with 802.11 wireless networks. This caused a scramble for a standardsbased replacement for WEP in the 802.11 market, and led to the IEEE 802.11i effort and WPA.^{[17]}
Cryptosystems can defend against this attack by discarding the initial portion of the keystream. Such a modified algorithm is traditionally called "RC4drop[n]", where n is the number of initial keystream bytes that are dropped. The SCAN default is n = 768 bytes, but a conservative value would be n = 3072 bytes.^{[18]}
In 2005, Andreas Klein presented an analysis of the RC4 stream cipher showing more correlations between the RC4 keystream and the key.^{[19]} Erik Tews, RalfPhilipp Weinmann, and Andrei Pychkine used this analysis to create aircrackptw, a tool which cracks 104bit RC4 used in 128bit WEP in under a minute^{[20]} Whereas the Fluhrer, Mantin, and Shamir attack used around 10 million messages, aircrackptw can break 104bit keys in 40,000 frames with 50% probability, or in 85,000 frames with 95% probability.
A combinatorial problem related to the number of inputs and outputs of the RC4 cipher was first posed by Itsik Mantin and Adi Shamir in 2001, whereby, of the total 256 elements in the typical state of RC4, if x number of elements (x ≤ 256) are only known (all other elements can be assumed empty), then the maximum number of elements that can be produced deterministically is also x in the next 256 rounds. This conjecture was put to rest in 2004 with a formal proof given by Souradyuti Paul and Bart Preneel.^{[21]}
Where a cryptosystem is marked with "(optionally)", RC4 is one of several ciphers the system can be configured to use.
RC4
RC4 in WEP

In cryptography, RC4 (also known as ARC4 or ARCFOUR meaning Alleged RC4, see below) is one of the most common software stream ciphers. It is used in popular protocols like Secure Sockets Layer (SSL) (to protect Internet traffic) and WEP (to secure wireless networks).
RC4 is known for being simple and quick, but attacks are likely to happen when the start of the output keystream is not removed, or one keystream is used twice; some ways of using RC4 can turn into very insecure cryptosystems such as WEP.
RC4 was created by Ron Rivest of RSA Security in 1987. While its official name is "Rivest Cipher 4", the RC acronym is also known to stand for "Ron's Code"^{[1]} (see also RC2, RC5 and RC6).
RC4 was first created as a trade secret, but in September 1994 a description of it was posted to the Cypherpunks mailing list^{[2]}. It was soon posted on the sci.crypt newsgroup, and from there to many websites on the Internet. The code was confirmed to be genuine(not fake) as its output matched that of proprietary software using licensed RC4. Because the algorithm is known, it is no longer a trade secret. The name "RC4" is trademarked, however. RC4 is often referred to as "ARCFOUR" or "ARC4" (meaning Alleged RC4, because RSA has never officially released the algorithm), to avoid possible trademark problems. It has become part of some commonly used encryption protocols and standards, including WEP and WPA for wireless cards and TLS.
The two main reasons which helped its use over such a big range of applications are its speed and simplicity. Uses of RC4 in both software and hardware are extremely easy to develop.
The RC4 encryption algorithm is started with a different key length, usually between 40 and 256 bits, using the keyscheduling algorithm (KSA). Once this has been completed, the stream of encrypted bits is created using the pseudorandom generation algorithm (PRGA).
RC4 fails the standards set by cryptographers for a secure cipher in many ways, and is not recommended for use in new applications as there are a lot of methods of attacking RC4.
Contents 
Where a cryptosystem is marked with "(optionally)", RC4 is one of several ciphers the system can be set to use.
RC4
RC4 in WEP
