Description
Time-Memory Tradeoff (TMTO) attacks on stream ciphers are a serious security threat and the resistance to this class of attacks is an important criterion in the design of a modern stream cipher. TMTO attacks are especially effective against stream ciphers where a variant of the TMTO attack can make use of multiple data to reduce the off-line and the on-line time complexities of the attack (given a fixed amount of memory).<br/> In this talk we present a new approach to TMTO attacks against stream ciphers using a publicly known initial value (IV): We suggest not to treat the IV as part of the secret key material (as done in current attacks), but rather to choose in advance some IVs and apply a TMTO attack to streams produced using these IVs. We show that while the obtained tradeoff curve is identical to the curve obtained by the current approach, the new technique allows to mount the TMTO attack in a larger variety of settings. For example, if both the secret key and the IV are of length $n$, it is possible to mount an attack with data, time, and memory complexities of 2^{4n/5}, while in the current approach, either the time complexity or the memory complexity is not less than 2^n. We conclude that if the IV length of a stream cipher is less than 1.5 times the key length, there exists an attack on the cipher with data, time, and memory complexities less than the complexity of exhaustive key search.<br/> This is a joint work with Nathan Keller.
Prochains exposés
-
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
-