  We present an attack against a code-based signature scheme based on the
Lyubashevsky protocol that was recently proposed by Song, Huang, Mu, Wu and
Wang (SHMWW). The private key in the SHMWW scheme contains columns coming in
part from an identity matrix and in part from a random matrix. The existence of
two types of columns leads to a strong bias in the distribution of set bits in
produced signatures. Our attack exploits such a bias to recover the private key
from a bunch of collected signatures. We provide a theoretical analysis of the
attack along with experimental evaluations, and we show that as few as 10
signatures are enough to be collected for successfully recovering the private
key. As for previous attempts of adapting Lyubashevsky's protocol to the case
of code-based cryptography, the SHMWW scheme is thus proved unable to provide
acceptable security. This confirms that devising secure code-based signature
schemes with efficiency comparable to that of other post-quantum solutions
(e.g., based on lattices) is still a challenging task.
