  We propose a variational quantum attack algorithm (VQAA) for classical
AES-like symmetric cryptography, as exemplified the simplified-data encryption
standard (S-DES). In the VQAA, the known ciphertext is encoded as the ground
state of a Hamiltonian that is constructed through a regular graph, and the
ground state can be found using a variational approach. We designed the ansatz
and cost function for the S-DES's variational quantum attack. It is surprising
that sometimes the VQAA is even faster than Grove's algorithm as demonstrated
by our simulation results. The relationships of the entanglement entropy,
concurrence and the cost function are investigated, which indicate that
entanglement plays a crucial role in the speedup.
