News Description

The Crypto Group and the Information Security Group of the UCL are pleased to announce a double-seminar about block ciphers.

Location: Room BARB-20, Place Sainte Barbe, Louvain-La-Neuve, Belgium
Date: Thursday 5 February, 2009, 10:30 to 12:00.
Speakers: Thomas Baignères (EPFL, Switzerland) and Baudoin Collard (UCL, Belgium).

First Part : Quantitative security of block ciphers : designs and cryptanalysis tools.

Speaker: Thomas Baignères, EPFL, Switzerland.
Time :  10:30-11:30

Abstract:
Block ciphers probably figure in the list of the most important cryptographic primitives. Although they are used for many different  purposes, their essential goal is to ensure confidentiality. In this talk, we are concerned by their quantitative security, that is, by measurable attributes that reflect their ability to guarantee this confidentiality.

We will first consider the (in)security of block ciphers against statistical cryptanalytic attacks and develop some tools to perform optimal attacks and quantify their efficiency. We start by studying a simple setting in which the adversary has to distinguish between two sources of randomness and show how an optimal strategy can be derived in certain cases. We show that in practice the cardinality of the sample space is too large for the optimal strategy to be implemented and how this naturally leads to the concept of projection-based distinguishers.  We show how these distinguishers between random sources can be turned into distinguishers between random oracles (or block ciphers) and how, in this setting, one can generalize linear cryptanalysis to Abelian groups.

In the second part of this talk, we introduce two new constructions. We start by recalling some essential notions about provable security for block ciphers and about Serge Vaudenay's Decorrelation Theory, and introduce new simple modules for which we prove essential properties that we will later use in our designs. We then present the block cipher C and prove that it is immune against a wide range of cryptanalytic attacks. In particular, we compute its exact security against linear and differential cryptanalysis, taking into account the cumulative effect of the linear hull and of differentials. We then introduce the main ideas underlying the design of KFC, a block cipher which builds upon the same foundations as C but for which we can prove results for higher order adversaries.

Bio:
Thomas Baignères started his PhD in 2003 at EPFL, under the supervision of Prof. Serge Vaudenay. He successfully defended his thesis on November 2008. His research covers block ciphers and the foundations of their cryptanalysis. Apart from research, Thomas was one of the general chairs of FSE'08, co-authored an exercise book on cryptography published by Springer and, together with Matthieu Finiasz (ENSTA), he developed iChair, a submission/review server software.

Second Part :  A Statistical Saturation Attack against the Block Cipher PRESENT.

Speaker: Baudoin Collard, UCL, Belgium.
Time : TIME: 11:30-12:00

Abstract:
In this talk, we present a statistical saturation attack that combines previously introduced cryptanalysis techniques against block ciphers. As the name suggests, the attack is statistical and can be seen as a particular example of partitioning cryptanalysis. It can also be seen as a dual to saturation attacks in the sense that it exploits the diffusion properties in block ciphers and a combination of active and passive multisets of bits in the plaintexts. The attack is chosen-plaintext in its basic version but can be easily extended to a known-plaintext scenario. As an illustration, it is applied to the block cipher PRESENT proposed by Bogdanov et al. at CHES 2007. We provide theoretical arguments to predict the attack efficiency and show that it improves previous (linear, differential) cryptanalysis results. We also provide experimental evidence that we can break up to 15 rounds  of PRESENT with 235.6 plaintext-ciphertext pairs. Eventually, we discuss the attack specificities and possible countermeasures.

Bio:
Baudoin Collard started a PhD under the direction of Jean-Jacques Quisquater in September 2006 after he graduated as an engineer in Applied Mathematics. He is now a member of the UCL Crypto Group. He is working on the Walloon project Cosmos, which is about secured wireless sensor networks, and the cryptanalysis of block ciphers. His previous work relates to the linear cryptanalysis of the block cipher Serpent, and on improving the time complexity of the linear cryptanalysis.