  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.
