A polynomial-time attack on the BBCRS scheme

Abstract

The BBCRS scheme is a variant of the McEliece public-key encryption scheme where the hiding phase is performed by taking the inverse of a matrix which is of the form T + R where T is a sparse matrix with average row/column weight equal to a very small quantity m, usually m < 2, and R is a matrix of small rank z ≥ 1. The rationale of this new transformation is the reintroduction of families of codes, like generalized Reed-Solomon codes, that are famously known for representin insecure choices. We present a key-recovery attack when z = 1 and m is chosen between 1 and 1+R+O(1/√n) where R denotes the code rate. This attack has complexity O(n6) and breaks all the parameters suggested in the literature.