Description
In November 2004, the European Network of Excellence for Cryptology (ECRYPT) launched a call for new stream cipher primitives. Authors from academia as well as industry submitted 34 designs, and in May 2008, 8 ciphers were chosen for the eSTREAM final portfolio. In this talk, we look back at the history of stream ciphers to understand the design choices made by cryptographers today. We start by reviewing "historical" designs based on linear feedback shift registers, such as the GSM A5/1 cipher. We explain basic cryptanalytic techniques - such as guess-and-determine attacks and correlation attacks - used to break LFSR-based ciphers. We also stop briefly to examine the importance of state size and key/IV length w.r.t. time-memory trade-offs. We then move on to the beginning of 2000s, which brought us new designs from the NESSIE research project, including SNOW 2.0, later to become the 3G mobile standard SNOW 3G. We discuss how new cryptanalytic tools, most notably algebraic cryptanalysis, reshaped design principles in modern ciphers, and review some of the eSTREAM submissions.<br/> In the second part of this talk, we focus on the cryptanalysis of one of the eSTREAM finalists, Moustique. We give a step-by-step overview of our attack that allows to recover the full 96-bit key in 2^{38} steps, using related keys, and allows to speed up exhaustive search in the standard case (without related keys) by a factor 28. Here, we invite the audience to interact, identify weaknesses that lead to the break and propose tweaks to thwart the attack. Cryptanalysis of Moustique is joint work with Vincent Rijmen, Tor E. Bjorstad, Christian Rechberger, Matt Robshaw and Gautham Sekar.
Prochains exposés
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Efficient zero-knowledge proofs and arguments in the CL framework
Orateur : Agathe Beaugrand - Institut de Mathématiques de Bordeaux
The CL encryption scheme, proposed in 2015 by Castagnos and Laguillaumie, is a linearly homomorphic encryption scheme, based on class groups of imaginary quadratic fields. The specificity of these groups is that their order is hard to compute, which means it can be considered unknown. This particularity, while being key in the security of the scheme, brings technical challenges in working with CL,[…] -
Constant-time lattice reduction for SQIsign
Orateur : Sina Schaeffler - IBM Research
SQIsign is an isogeny-based signature scheme which has recently advanced to round 2 of NIST's call for additional post-quantum signatures. A central operation in SQIsign is lattice reduction of special full-rank lattices in dimension 4. As these input lattices are secret, this computation must be protected against side-channel attacks. However, known lattice reduction algorithms like the famous[…] -
Circuit optimisation problems in the context of homomorphic encryption
Orateur : Sergiu Carpov - Arcium
Fully homomorphic encryption (FHE) is an encryption scheme that enables the direct execution of arbitrary computations on encrypted data. The first generation of FHE schemes began with Gentry's groundbreaking work in 2019. It relies on a technique called bootstrapping, which reduces noise in FHE ciphertexts. This construction theoretically enables the execution of any arithmetic circuit, but[…] -
Cycles of pairing-friendly abelian varieties
Orateur : Maria Corte-Real Santos - ENS Lyon
A promising avenue for realising scalable proof systems relies on the existence of 2-cycles of pairing-friendly elliptic curves. More specifically, such a cycle consists of two elliptic curves E/Fp and E’/Fq that both have a low embedding degree and also satisfy q = #E(Fp) and p = #E’(Fq). These constraints turn out to be rather restrictive; in the decade that has passed since 2-cycles were first[…]-
Cryptography
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