## CryptoDB

### Pierre Briaud

#### Publications

Year
Venue
Title
2022
CRYPTO
The Support-Minors (SM) method has opened new routes to attack multivariate schemes with rank properties that were previously impossible to exploit, as shown by the recent attacks of [1] and [2] on the Round 3 NIST candidates GeMSS and Rainbow respectively. In this paper, we study this SM approach more in depth and we propose a greatly improved attack on GeMSS based on this Support-Minors method. Even though GeMSS was already affected by [1], our attack affects it even more and makes it completely unfeasible to repair the scheme by simply increasing the size of its parameters or even applying the recent projection technique from [3] whose purpose was to make GeMSS immune to [1]. For instance, our attack on the GeMSS128 parameter set has estimated time complexity $2^{72}$, and repairing the scheme by applying [3] would result in a signature with slower signing time by an impractical factor of $2^{14}$. Another contribution is to suggest optimizations that can reduce memory access costs for an XL strategy on a large SM system using the Block-Wiedemann algorithm as subroutine when these costs are a concern. In a memory cost model based on [4], we show that the rectangular MinRank attack from [2] may indeed reduce the security for all Round 3 Rainbow parameter sets below their targeted security strengths, contradicting the lower bound claimed by [5] using the same memory cost model. ***** [1] Improved Key Recovery of the HFEv- Signature Scheme, Chengdong Tao and Albrecht Petzoldt and Jintai Ding, CRYPTO 2021. [2] Improved Cryptanalysis of UOV and Rainbow, Ward Beullens, EUROCRYPT 2021. [3] On the Effect of Projection on Rank Attacks in Multivariate Cryptography, Morten Øygarden and Daniel Smith-Tone and Javier Verbel, PQCrypto 2021. [4] NTRU Prime: Round 3 submission. [5] Rainbow Team: Response to recent paper by Ward Beullens. https://troll.iis. sinica.edu.tw/by-publ/recent/response-ward.pdf
2020
EUROCRYPT
The Rank metric decoding problem is the main problem considered in cryptography based on codes in the rank metric. Very efficient schemes based on this problem or quasi-cyclic versions of it have been proposed recently, such as those in the submissions ROLLO and RQC currently at the second round of the NIST Post-Quantum Cryptography Standardization Process. While combinatorial attacks on this problem have been extensively studied and seem now well understood, the situation is not as satisfactory for algebraic attacks, for which previous work essentially suggested that they were ineffective for cryptographic parameters. In this paper, starting from Ourivski and Johansson's algebraic modelling of the problem into a system of polynomial equations, we show how to augment this system with easily computed equations so that the augmented system is solved much faster via Gröbner bases. This happens because the augmented system has solving degree $r$, $r+1$ or $r+2$ depending on the parameters, where $r$ is the rank weight, which we show by extending results from Verbel \emph{et al.} (PQCrypto 2019) on systems arising from the MinRank problem; with target rank $r$, Verbel \emph{et al.} lower the solving degree to $r+2$, and even less for some favorable instances that they call superdetermined''. We give complexity bounds for this approach as well as practical timings of an implementation using \texttt{magma}. This improves upon the previously known complexity estimates for both Gröbner basis and (non-quantum) combinatorial approaches, and for example leads to an attack in 200 bits on ROLLO-I-256 whose claimed security was 256 bits.