  Biggs proposed the sandpile group of certain modified wheel graphs for
cryptosystems relying on the difficulty of the discrete logarithm problem.
Blackburn and independently Shokrieh showed that the discrete logarithm problem
is efficiently solvable. We study Shokrieh's method in cases of graphs such
that the sandpile group is not cyclic, namely the square cycle graphs and the
wheel graphs. Knowing generators of the group or the form of the pseudoinverse
of the Laplacian matrix makes the problem more vulnerable. We also consider the
discrete logarithm problem in case of the so-called subdivided banana graphs.
In certain cases the sandpile group is cyclic and a generator is known and one
can solve the discrete logarithm problem without computing the pseudoinverse of
the Laplacian matrix.
