  Sometimes it is possible to embed an algebraic trapdoor into a block cipher.
Building on previous research, in this paper we investigate an especially
dangerous algebraic structure, which is called a hidden sum and which is
related to some regular subgroups of the affine group. Mixing group theory
arguments and cryptographic tools, we pass from characterizing our hidden sums
to designing an efficient algorithm to perform the necessary preprocessing for
the exploitation of the trapdoor.
