Security on the quantum-type Even-Mansour cipher

Quantum cryptography such as BB84 is a quantum protocol for sharing classical information, but is not a scheme for encrypting quantum information itself. This paper considers that quantum information is encrypted with the quantum circuit of the Even-Mansour cipher. It has been proved that breaking the Even-Mansour cipher requires exponential time in the key length using any classical algorithm. This paper shows that the quantum version of the Even-Mansour cipher is insecure, that is, a key can be found in polynomial time in the key length. This is an example that the quantum version of a secure classical cipher is not always secure.