Pierre-Jean Spaenlehauer

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A fundamental problem in computer science is to find all the common zeroes of m quadratic polynomials in n unknowns over F2. The cryptanalysis of several modern ciphers reduces to this problem. Up to now, the best complexity bound was reached by an exhaustive search in 4log2 n2 n operations. We give an algorithm that reduces the problem to a combination of(More)
Solving multihomogeneous systems, as a wide range of structured algebraic systems occurring frequently in practical problems, is of first importance. Experimentally, solving these systems with Gröbner bases algorithms seems to be easier than solving homogeneous systems of the same degree. Nevertheless, the reasons of this behaviour are not clear. In this(More)
In this paper, we fully break the Algebraic Surface Cryptosystem (ASC for short) proposed at PKC’2009 [3]. This system is based on an unusual problem in multivariate cryptography: the Section Finding Problem. Given an algebraic surface X(x,y, t) ∈ Fp[x,y, t] such that degxy X(x,y, t) = w, the question is to find a pair of polynomials of degree d, ux(t) and(More)
Computing loci of rank defects of linear matrices (also called the MinRank problem) is a fundamental NP-hard problem of linear algebra which has applications in Cryptology, in Error Correcting Codes and in Geometry. Given a square linear matrix (i.e. a matrix whose entries are <i>k</i>-variate linear forms) of size <i>n</i> and an integer <i>r</i>, the(More)
Structured low-rank approximation is the problem of minimizing a weighted Frobenius distance to a given matrix among all matrices of fixed rank in a linear space of matrices. We study the critical points of this optimization problem using algebraic geometry. A particular focus lies on Hankel matrices, Sylvester matrices and generic linear spaces.
Toric (or sparse) elimination theory is a framework developped during the last decades to exploit monomial structures in systems of Laurent polynomials. Roughly speaking, this amounts to computing in a <i>semigroup algebra</i>, <i>i.e</i>. an algebra generated by a subset of Laurent monomials. In order to solve symbolically sparse systems, we introduce(More)
Algebraic cryptanalysis is as a general framework that permits to assess the security of a wide range of cryptographic schemes. However, the feasibility of algebraic cryptanalysis against block ciphers remains the source of speculation and especially in targeting modern block ciphers. The main problem is that the size of the corresponding algebraic system(More)
Computing the critical points of a polynomial function q ∈ Q[X1, . . . ,Xn] restricted to the vanishing locus V ⊂ Rn of polynomials f1, . . . , fp ∈ Q[X1, . . . ,Xn] is of first importance in several applications in optimization and in real algebraic geometry. These points are solutions of a highly structured system of multivariate polynomial equations(More)
Structured Low-Rank Approximation is a problem arising in a wide range of applications in Numerical Analysis and Engineering Sciences. Given an input matrix M , the goal is to compute a matrix M ′ of given rank r in a linear or affine subspace E of matrices (usually encoding a specific structure) such that the Frobenius distance ‖M −M ′‖ is small. We(More)