XSL attack
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In cryptography, the XSL attack is a method of cryptanalysis for block ciphers. Block ciphers are a specific type of cipher (a "secret code" in more common speech). Cryptanalysis is the study of methods for obtaining the meaning of encrypted information without access to the secret information (the "key") which is normally required to do so. Cryptanalysis can loosely be thought of as "codebreaking", though that term is not entirely accurate.
The attack was first published in 2002 by researchers Nicolas Courtois and Josef Pieprzyk. It has caused some controversy as it is claimed to have the potential to break the Advanced Encryption Standard (AES) cipher — also known as Rijndael — faster than an exhaustive search. Since AES is already widely used in commerce and government for the transmission of secret information, finding a technique that can shorten the amount of time it takes to retrieve the secret message without having the key would have wide implications. Opinions differ on whether the attack works because the method is heuristic and very technical, and so it has proved difficult to evaluate its complexity. In addition, the method is expected to have a high workfactor, which unless lessened, means the technique would not reduce the effort to break AES very much in comparison to an exhaustive search. Therefore, even if the attack has been analysed correctly, it is unlikely to affect the realworld security of block ciphers in the near future. Nonetheless, the attack has caused some experts to express greater unease at the algebraic simplicity of the current AES.
In overview, the XSL attack relies on first analysing the internals of a cipher and deriving a system of quadratic simultaneous equations. These systems of equations are typically very large, for example 8000 equations with 1600 variables for the 128bit AES. Several methods for solving such systems are known. In the XSL attack, a specialised algorithm, termed XSL (eXtended Sparse Linearisation), is then applied to solve these equations and recover the key.
The attack is notable for requiring only a handful of known plaintexts to perform; previous methods of cryptanalysis, such as linear and differential cryptanalysis, often require unrealistically large numbers of known or chosen plaintexts.
Contents 
Solving multivariate quadratic equations
Solving multivariate quadratic equations (MQ) is an NPhard problem (in the general case) with several applications in cryptography. The XSL attack requires an efficient algorithm for tackling MQ. In 1999, Kipnis and Shamir showed that a particular public key algorithm — known as the Hidden Field Equations scheme (HFE) — could be reduced to a system of overdefined quadratic equations ("overdefined" means that there are more equations than unknowns). One technique for solving such systems is linearisation, which involves replacing each quadratic term with an independent variable and solving the resultant linear system using an algorithm such as Gaussian elimination. To succeed, linearisation requires enough linearly independent equations (approximately as many as the number of terms). However, for the cryptanalysis of HFE there were too few equations, so Kipnis and Shamir proposed relinearisation, a technique where extra nonlinear equations are added after linearisation, and the resultant system is solved by a second application of linearisation. Relinearisation proved general enough to be applicable to other schemes.
In 2000, Courtois et al. proposed an improved algorithm for MQ known as XL (for eXtended Linearisation), which increases the number of equations by multiplying them with all monomials of a certain degree. Complexity estimates showed that the XL attack would not work against the equations derived from block ciphers such as AES. However, the systems of equations produced had a special structure, and the XSL algorithm was developed as a refinement of XL which could take advantage of this structure. In XSL, the equations are multiplied only by carefully selected monomials, and several variants have been proposed.
As of 2004, research into the efficiency of XL and its derivative algorithms remains ongoing (Yang and Chen, 2004).
Application to block ciphers
Courtois and Pieprzyk (2002) observed that Rijndael and Serpent could be expressed as a system of quadratic equations. The variables represent not just the plaintext, ciphertext and key bits, but also various intermediate values within the algorithm. The Sbox of Rijndael seems especially vulnerable to this type of analysis, as it is based on the algebraically simple inverse function. Subsequently, other ciphers have been studied to see what of systems of equations can be produced (Biryukov and De Cannière, 2003), including Camellia, KHAZAD, MISTY1 and KASUMI. Unlike other forms of cryptanalysis, such as differential and linear cryptanalysis, only one or two known plaintexts are required.
The XSL algorithm is tailored to solve the type of equation systems that are produced. Courtois and Pieprzyk estimate that an "optimistic evaluation shows that the XSL attack might be able to break Rijndael [with] 256 bits and Serpent for key lengths [of] 192 and 256 bits." Their analysis, however, is not universally accepted. For example:
 "I believe that the CourtoisPieprzyk work is flawed. They overcount the number of linearly independent equations. The result is that they do not in fact have enough linear equations to solve the system, and the method does not break Rijndael...The method has some merit, and is worth investigating, but it does not break Rijndael as it stands." — Don Coppersmith, [1] (http://www.schneier.com/cryptogram0210.html#8).
One of the inventors of Rijndael, Vincent Rijmen, commented, "The XSL attack is not an attack. It is a dream." [2] (http://www.cosic.esat.kuleuven.ac.be/nessie/forum/read.php?f=1&i=82&t=82)
In 2003, Murphy and Robshaw discovered an alternative description of AES, embedding it in a larger cipher called "BES", which can be described using very simple operations over a single field, GF(2^{8}). An XSL attack mounted on this system yields a simpler set of equations which would break AES with complexity of around 2^{100}, if the Courtois and Pieprzyk analysis is correct.
Even if XSL works against some modern algorithms, the attack currently poses little danger in terms of practical security. Like many modern cryptanalytic results, it would be a socalled "certificational weakness": while faster than a brute force attack, the resources required are still huge, and it is very unlikely that realworld systems could be compromised by using it. Future improvements could increase the practicality of an attack, however. Because this type of attack is new and unexpected, some cryptographers have expressed unease at the algebraic simplicity of ciphers like Rijndael. Bruce Schneier and Niels Ferguson write, "We have one criticism of AES: we don't quite trust the security...What concerns us the most about AES is its simple algebraic structure....No other block cipher we know of has such a simple algebraic representation. We have no idea whether this leads to an attack or not, but not knowing is reason enough to be skeptical about the use of AES." (Practical Cryptography, 2003, pp5657)
References
 Alex Biryukov, Christophe De Cannière: Block Ciphers and Systems of Quadratic Equations. FSE 2003: 274–289
 Nicolas Courtois, Alexander Klimov, Jacques Patarin, Adi Shamir: Efficient Algorithms for Solving Overdefined Systems of Multivariate Polynomial Equations. EUROCRYPT 2000: 392–407
 Nicolas Courtois, Josef Pieprzyk, "Cryptanalysis of Block Ciphers with Overdefined Systems of Equations". pp267–287, ASIACRYPT 2002.
 Aviad Kipnis, Adi Shamir: Cryptanalysis of the HFE Public Key Cryptosystem by Relinearization. CRYPTO 1999: 19–30
 Dana Mackenzie, "A game of chance", New Scientist vol 178, issue 2398, 7th June 2003, p36.
 Sean Murphy, Matthew J. B. Robshaw: Essential Algebraic Structure within the AES. CRYPTO 2002: 1–16
 S. Murphy, M. Robshaw, "Comments on the Security of the AES and the XSL Technique", [3] (http://www.isg.rhul.ac.uk/~sean/Xslbes8.ps) (Postscript format)
 BoYin Yang, JiunMing Chen, "Theoretical Analysis of XL over Small Fields", ACISP 2004, (Lecture Notes in Computer Science vol. 3108, p.277288).
External links
 Courtois' page on AES (http://www.cryptosystem.net/aes/)
 Quadratic cryptanalysis  an explanation of the XSL attack (http://home.ecn.ab.ca/~jsavard/crypto/co4514.htm)
 "AES is NOT broken" by T. Moh (http://www.usdsi.com/aes.html)
 Courtois and Piepryzk paper on ePrint (http://eprint.iacr.org/2002/044/)
 Commentary in the Cryptogram newsletter: [4] (http://www.schneier.com/cryptogram0209.html#1), [5] (http://www.schneier.com/cryptogram0210.html#2), [6] (http://www.schneier.com/cryptogram0211.html#7).
 An overview of AES and XSL (http://www.securityforums.com/forum/viewtopic.php?p=47765)
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