Vlad Drăgoi

Ferucio Laurentiu Tiplea, Vlad-Florin Dragoi

The concept of Generalized Inverse based Decoding (GID) is introduced, as an algebraic framework for the syndrome decoding problem (SDP) and low weight codeword problem (LWP). The framework has ground on two characterizations by generalized inverses (GIs), one for the null space of a matrix and the other for the solution space of a system of linear equations over a finite field. Generic GID solvers are proposed for SDP and LWP. It is shown that information set decoding (ISD) algorithms, such as Prange, Lee-Brickell, Leon, and Stern's algorithms, are particular cases of GID solvers. All of them search GIs or elements of the null space under various specific strategies. However, as the paper shows the ISD variants do not search through the entire space, while our solvers do even when they use just one Gaussian elimination. Apart from these, our GID framework clearly shows how each ISD algorithm, except for Prange's solution, can be used as an SDP or LWP solver. A tight reduction from our problems, viewed as optimization problems, to the MIN-SAT problem is also provided. Experimental results show a very good behavior of the GID solvers. The domain of easy weights can be reached by a very few iterations and even enlarged.

Vlad Dragoi, Hervé Talé Kalachi

This letter presents a cryptanalysis of the modified McEliece cryptosystem recently proposed by Moufek, Guenda and Gulliver [24]. The system is based on the juxtaposition of quasi-cyclic LDPC and quasi-cyclic MDPC codes. The idea of our attack is to find an alternative permutation matrix together with an equivalent LDPC code which allow the decoding of any cipher-text with a very high probability. We also apply a recent technique to determine weak keys [4] for this scheme. The results show that the probability of weak keys is high enough that this variant can be ruled out as a possible secure encryption scheme.