Description
Following the famous 1949 paper of Shannon, breaking a "good" cipher should require: "as much work as solving a system of simultaneous equations in a large number of unknowns of a complex type". For most practical cryptosystems, the problem of recovering the key can indeed can be seen as solving a huge system of binary nonlinear equations. In general, solving such a problem is known to be NP-hard, and chances to have a general and efficient method to break ciphers this way are extremely low. However, what makes this problem hard, is not the size of the system, and not the number of variables involved, but the difference between the number of equations and the number of monomials. In many interesting cases, even very big systems of multivariate equations may be solved efficiently. For example, systems that are overdefined, sparse, or both turn out to be solved much easier than expected, by the XL method [due to Shamir, Patarin, Courtois, Klimov, Eurocrypt'2000] and the recent XSL variant [Courtois, Pieprzyk, Asiacrypt'2002].<br/> We will explain that, the new encryption standard AES, designed to resist all previously known attacks on block ciphers, turns out to be an extremely weak cipher with regard to these new algebraic attacks (it is in fact difficult to imagine a worse cipher). One version of this attack, due to Courtois, Pieprzyk, Murphy and Robshaw, seems to be able to achieve an "academic" break of AES: recover the key a bit faster than trying all possible keys.<br/> These attacks are very new, based on some unproven heuristics, and evaluating their exact complexity remains an open problem. In fact, many people naively believe that they cannot break on AES. For example the American government NIST and the European consortium Nessie, still fully back up and recommend AES. Yet, to the best of our knowledge, there is nothing that allows to say they will not work. We will also explain how to design better ciphers, for which there will be no algebraic attacks.<br/> Another area of application of algebraic attacks are stream ciphers. For reasons that will be explained the situation is much worse here, and we will exhibit several general classes of stream ciphers that resist to all known attacks, and yet can be broken in polynomial time by an algebraic attack. Surprisingly, the resulting design criterion on Boolean functions and S-boxes to be used in stream ciphers in order to resist algebraic attacks, applies equally to the components of block ciphers, and can be seen as an application of the security criterion proposed by Courtois for multivariate trapdoor functions (motivated by the security of the multivariate public key cryptosystem HFE). Finally, it can also be seen as an interpretation of Shannon's prescription quoted above.
Prochains exposés
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SoK: Security of the Ascon Modes
Orateur : Charlotte Lefevre - Radboud University
The Ascon authenticated encryption scheme and hash function of Dobraunig et al (Journal of Cryptology 2021) were recently selected as winner of the NIST lightweight cryptography competition. The mode underlying Ascon authenticated encryption (Ascon-AE) resembles ideas of SpongeWrap, but not quite, and various works have investigated the generic security of Ascon-AE, all covering different attack[…] -
Comprehensive Modelling of Power Noise via Gaussian Processes with Applications to True Random Number Generators
Orateur : Maciej Skorski - Laboratoire Hubert Curien
The talk examines power noise modelling through Gaussian Processes for secure True Random Number Generators. While revisiting one-sided fractional Brownian motion, we obtain novel contributions by quantifying posterior uncertainty in exact analytical form, establishing quasi-stationary properties, and developing rigorous time-frequency analysis. These results are applied to model oscillator[…]-
Cryptography
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TRNG
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CryptoVerif: a computationally-sound security protocol verifier
Orateur : Bruno Blanchet - Inria
CryptoVerif is a security protocol verifier sound in the computational model of cryptography. It produces proofs by sequences of games, like those done manually by cryptographers. It has an automatic proof strategy and can also be guided by the user. It provides a generic method for specifying security assumptions on many cryptographic primitives, and can prove secrecy, authentication, and[…]-
Cryptography
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