On the matrix code of quadratic relationships for a Goppa code

Abstract: In this article, we continue the analysis started in \cite{CMT23} for the matrix code of quadratic relationships associated with a Goppa code. We provide new sparse and low-rank elements in the matrix code and categorize them according to their shape. Thanks to this description, we prove that the set of rank 2 matrices in the matrix codes associated with square-free binary Goppa codes, i.e. those used in Classic McEiece, is much larger than what is expected, at least in the case where the Goppa polynomial degree is 2. We build upon the algebraic determinantal modeling introduced in \cite{CMT23} to derive a structural attack on these instances. Our method can break in just a few seconds some recent challenges about key-recovery attacks on the McEliece cryptosystem, consistently reducing their estimated security level. We also provide a general method, valid for any Goppa polynomial degree, to transform a generic pair of support and multiplier into a pair of support and Goppa polynomial.