## CryptoDB

### Christophe Petit

#### Publications

Year
Venue
Title
2019
PKC
Braid groups are infinite non-abelian groups naturally arising from geometric braids. For two decades they have been proposed for cryptographic use. In braid group cryptography public braids often contain secret braids as factors and it is hoped that rewriting the product of braid words hides individual factors. We provide experimental evidence that this is in general not the case and argue that under certain conditions parts of the Garside normal form of factors can be found in the Garside normal form of their product. This observation can be exploited to decompose products of braids of the form ABC when only B is known.Our decomposition algorithm yields a universal forgery attack on WalnutDSATM, which is one of the 20 proposed signature schemes that are being considered by NIST for standardization of quantum-resistant public-key cryptography. Our attack on WalnutDSATM can universally forge signatures within seconds for both the 128-bit and 256-bit security level, given one random message-signature pair. The attack worked on 99.8% and 100% of signatures for the 128-bit and 256-bit security levels in our experiments.Furthermore, we show that the decomposition algorithm can be used to solve instances of the conjugacy search problem and decomposition search problem in braid groups. These problems are at the heart of other cryptographic schemes based on braid groups.
2018
EUROCRYPT
2018
PKC
We present a practical cryptanalysis of WalnutDSA, a digital signature algorithm trademarked by SecureRF. WalnutDSA uses techniques from permutation groups, matrix groups and braid groups, and is designed to provide post-quantum security in lightweight IoT device contexts. The attack given in this paper bypasses the E-Multiplication$^{\text {TM}}$TM and cloaked conjugacy search problems at the heart of the algorithm and forges signatures for arbitrary messages in approximately two minutes. We also discuss potential countermeasures to the attack.
2017
ASIACRYPT
2017
ASIACRYPT
2016
EUROCRYPT
2016
PKC
2016
ASIACRYPT
2015
EPRINT
2015
EPRINT
2014
EPRINT
2014
EPRINT
2012
EUROCRYPT
2012
ASIACRYPT
2008
EPRINT
Collisions in the LPS cryptographic hash function of Charles, Goren and Lauter have been found by Zémor and Tillich, but it was not clear whether computing preimages was also easy for this hash function. We present a probabilistic polynomial time algorithm solving this problem. Subsequently, we study the Morgenstern hash, an interesting variant of LPS hash, and break this function as well. Our attacks build upon the ideas of Zémor and Tillich but are not straightforward extensions of it. Finally, we discuss fixes for the Morgenstern hash function and other applications of our results.
2007
EPRINT
We study the security of a block cipher-based pseudorandom number generator (PRNG), both in the black box world and in the physical world, separately. We first show that the construction is a secure PRNG in the black box world, relying on standard computational assumptions. Then, we demonstrate its security against a Bayesian side-channel key recovery adversary. As a main result, we show that our construction guarantees that the success rate of the adversary does not increase with the number of physical bservations, but in a limited and controlled way. Besides, we observe that, under common assumptions on side-channel attack strategies, increasing the security parameter (typically the block cipher key size) by a polynomial factor involves an increase of a side-channel attack complexity by an exponential factor, as usually expected for secure cryptographic primitives. Therefore, we believe this work provides a first interesting example of the way the algorithmic design of a cryptographic scheme influences its side-channel resistance.

PKC 2020
Crypto 2019
PKC 2017
PKC 2016