Mécanisme de chiffrement et de déchiffrement Le WEP utilise un algorithme à clé Utiliser le RC4 pour faire du chiffrement est considéré comme sûr dès lors. Celles-ci concernent principalement la façon dont l’algorithme de chiffrement RC4 et la clé associée ou vecteur d’initialisation (IV) sont utilisés. Elles rendent. Research paper on an effective RC4 stream ciher. In this paper, a new effective RC4 cipher is proposed and the security analysis has been done using Shannon’s Secrecy .. Algorithme de chiffrement RC4, A5/1 & A5/2.

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While remarkable for its simplicity and speed in software, multiple vulnerabilities have been discovered in RC4, rendering it insecure.

## Stream cipher

As of [update]there is speculation that some state cryptologic agencies may possess the capability to break RC4 when used in the TLS protocol. RC4 was initially a trade secretbut in September a description of it was anonymously posted to the Cypherpunks mailing list.

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. 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 [13] and confirmed the history of RC4 and its code in a paper by him.

The main factors in RC4’s success over such a wide range of applications have been its speed and simplicity: 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 bit-wise 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 bitsrather than a prepared stream, 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 keytypically between 40 and 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. The key-scheduling algorithm is used to initialize the permutation in the array “S”. First, the array “S” is initialized to the identity permutation. S is then processed for iterations in a similar way to the main PRGA, but also mixes in bytes of the key at the same time. For as many iterations as are needed, the PRGA modifies the state and outputs a byte of the keystream.

In each iteration, the PRGA:. In the release of its desktop and mobile operating systems, Apple replaced RC4 with AES in its implementation of arc4random. Proposed new random number generators are often compared to the RC4 random number generator. Several attacks on RC4 are able to distinguish its output from a random sequence.

Many stream ciphers are based on linear-feedback shift registers LFSRswhich, 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 bytes of memory for the state array, S[0] through S[], k bytes of memory for the key, key[0] through key[k-1], and integer variables, i, j, and K.

Performing a modular reduction of some chifftement modulo can be done with a bitwise AND with which is equivalent to taking the low-order byte of the value in question. These test vectors are chiffrementt official, but convenient for anyone testing their own RC4 program.

This means that if a single long-term key is to be used to securely encrypt multiple streams, the protocol must specify how to combine the nonce and the long-term key to generate the stream key for RC4. One approach to addressing this is to generate a “fresh” RC4 key by hashing a long-term key with a nonce. However, many applications that use RC4 simply concatenate key and nonce; RC4’s weak key schedule then gives rise to related key attackslike the Fluhrer, Mantin and Shamir attack which is famous for breaking the WEP standard.

Because RC4 is a stream cipherit is more malleable than common block ciphers. If not used together with a strong message authentication code MACthen encryption is vulnerable to a bit-flipping attack. The cipher is also vulnerable to a stream cipher attack if not implemented correctly.

The attack exploits a known weakness in the way cipher block chaining mode is used with all of the other ciphers supported by TLS 1. InAndrew 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.

The latter work also used the permutation—key correlations to design the first algorithm for complete key reconstruction from the final permutation after the KSA, without any assumption on the key or initialization vector.

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 performed on key reconstruction from RC4 internal states. 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 making it vulnerable to distinguishing attacks.

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 bytes. The number of required samples to detect this bias is 2 25 bytes. Scott Fluhrer and David McGrew also showed such attacks which distinguished the keystream of the RC4 from a random stream given a gigabyte of output.

Ina new and surprising discovery was made by FluhrerMantin and Shamir: If the nonce and long-term key are simply concatenated to generate the RC4 key, this long-term key can be discovered by analysing a large number of messages encrypted with this key. This caused a scramble for a standards-based replacement for WEP in the Protocols can defend against this attack by discarding the initial portion of the keystream. Such a modified algorithm is traditionally called “RC4-drop[n]”, where n is the number of initial keystream bytes that are dropped.

InAndreas Klein presented an analysis of the RC4 stream cipher showing more correlations between the RC4 keystream and the key. This conjecture was put to rest in with a formal proof given by Souradyuti Paul and Bart Preneel.

Ina group of security researchers at the Information Security Group at Royal Holloway, University of London reported an attack that can become effective using only 2 34 encrypted messages.

In March researcher to Royal Holloway announced improvements to their attack, providing a 2 26 attack against passwords encrypted with RC4, as used in TLS.

As mentioned above, the most important weakness of RC4 comes from the chiffrenent key schedule; the first bytes of output reveal information about the key. This can be corrected by simply discarding some initial portion of the output stream.

RC4A uses two state arrays S1 r4 S2and two indexes j1 and j2.

### Azure Services SSL/TLS cipher suite update and removal of RC4 | Blog | Microsoft Azure

Each time i is incremented, two bytes are generated:. Although the algorithm required the same number of operations per output byte, there is greater parallelism than RC4, providing a possible speed improvement.

Although stronger than RC4, this algorithm has also been attacked, with Alexander Maximov [57] and a team from NEC [58] developing ways to distinguish its output from a truly random sequence. The output generation function operates as follows:.

This was attacked in the same papers as RC4A, and can be distinguished within 2 38 output bytes. InRonald Rivest gave a talk and co-wrote a paper [14] on an updated redesign called Spritz. A hardware accelerator of Spritz was published in Secrypt, The value wis relatively prime to the size of the S array. So after iterations of chifffrement inner loop, the value i incremented by w every iteration has taken on all possible values Like other sponge functionsSpritz can be used to build a cryptographic hash function, a deterministic random bit generator DRBGan encryption algorithm that supports authenticated encryption with associated fhiffrement AEADetc.

Spritz was broken by Banik and Isobe. Where a protocol is marked with ” optionally “, RC4 is one of multiple ciphers the system can be configured to use. From Wikipedia, the free encyclopedia. This chiffdement is about the stream cipher. For other uses, rc RC4 disambiguation.

Fluhrer, Mantin and Shamir attack. Variably Modified Permutation Composition. Archived from the original PDF on 3 December Retrieved 22 September Prohibiting RC4 Cipher Suites. Have spooks smashed RC4? Recommendation to disable RC4″.

Archived from the original on hciffrement July Retrieved 2 February Retrieved 26 October Retrieved 21 September Retrieved 6 January Lecture Notes in Computer Science. RC4 is kind of broken in TLS”. Retrieved 12 March Royal Holloway University of London.

Retrieved 13 March Two posts in sci. SACpages —, vol. FSEpages —, vol. Archived from the original PDF on 2 May Journal of Mathematical Cryptology. Breaking bit WEP in under a minute.

Retrieved November 19, Archived from the original on 1 October Retrieved 4 November Retrieved 29 July Cryptanalysis of the Full Spritz Stream Cipher.

Archived from the original on 11 July