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Section: New Results

Keywords : symmetric cryptography, stream ciphers, hash functions, cryptanalysis.

Security analysis of symmetric cryptosystems

Participants : Céline Blondeau, Anne Canteaut, Pascale Charpin, Benoît Gérard, Stéphane Jacob, Yann Laigle-Chapuy, Stéphane Manuel, María Naya Plasencia, Andrea Röck, Jean-Pierre Tillich.

From outside, it might appear that symmetric techniques become obsolete after the invention of public-key cryptography in the mid 1970's. However, they are still widely used because they are the only ones that can achieve some major features as high-speed or low-cost encryption, fast authentication, and efficient hashing. Today, we find symmetric algorithms in GSM mobile phones, in credit cards, in WLAN connections. Symmetric cryptology is a very active research area which is stimulated by a pressing industrial demand for low-cost implementations (in terms of power consumption, gate complexity...). These extremely restricting implementation requirements are crucial when designing secure symmetric primitives and they might be at the origin of some weaknesses. Actually, these constraints seem quite incompatible with the rather complex mathematical tools needed for constructing a provably secure system.

The specificity of our research work is that it considers all aspects of the field, from the practical ones (new attacks, concrete specifications of new systems) to the most theoretical ones (study of the algebraic structure of underlying mathematical objects, definition of optimal objects). But, our purpose is to study these aspects not separately but as several sides of the same domain. Our approach mainly relies on the idea that, in order to guarantee a provable resistance to the known attacks and to achieve extremely good performance, a symmetric cipher must use very particular building blocks, whose algebraic structures may introduce unintended weaknesses. Our research work captures this conflict for all families of symmetric ciphers. It includes new attacks and the search for new building blocks which ensure both a high resistance to the known attacks and a low implementation cost. This work, which combines cryptanalysis and the theoretical study of discrete mathematical objects, is essential to progress in the formal analysis of the security of symmetric systems.

In this context, two very important challenges are the designs of low-cost stream ciphers and of secure hash functions. Most teams in the research community are actually working on the design and on the analysis (cryptanalysis and optimization of the performance) of such primitives.

Stream ciphers.

Our research work on stream ciphers is a long-term work which is currently developed within the 4-year ANR RAPIDE project. The project-team is involved in some concrete realizations through the international call for proposals eSTREAM. Some researchers from the project-team are actually co-authors of three stream cipher proposals which have been submitted to the eSTREAM project: Sosemanuk , DECIM and F-FCSR. Sosemanuk and F-FCSR belong to the 8 recommended ciphers which have been included in the final portfolio of eSTREAM in April 2008 (among 34 submissions). Our work within the eSTREAM project also includes an important cryptanalytic effort on stream ciphers.

Recent results:

Hash functions.

Following the recent attacks against almost all existing hash functions (MD5, SHA-0, SHA-1...), we have initiated a research work in this area, especially within the EDHASH ANR Project and with S. Manuel's PhD thesis. Our work on hash functions is two-fold: we have designed two new hash functions, named FSB and Shabal, which have been submitted to the SHA-3 competition, and we have investigated the security of several hash functions, including the previous standards (SHA-0, SHA-1...) and some other SHA-3 candidates.

Recent results:

Cryptographic properties and construction of appropriate building blocks.

The construction of building blocks which guarantee a high resistance to the known attacks is a major topic within our project-team, for stream ciphers, block ciphers and hash functions. The use of such optimal objects actually leads to some mathematical structures which may be the origin of new attacks. This work involves fundamental aspects related to discrete mathematics, cryptanalysis and implementation aspects. Actually, characterizing the structures of the building blocks which are optimal regarding to some attacks is very important for finding appropriate constructions and also for determining whether the underlying structure induces some weaknesses or not.

For these reasons, we have investigated several families of filtering functions and of S-boxes which are well-suited for their cryptographic properties or for their implementation characteristics. For instance, bent functions, which are the Boolean functions which achieve the highest possible nonlinearity, have been extensively studied in order to provide some elements for a classification, or to adapt these functions to practical cryptographic constructions. We have also been interested in APN functions, which are the S-boxes ensuring an optimal resistance to differential cryptanalysis.

Recent results:


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