We show how fixed-unitary quantum encryption schemes can be attacked in a black-box setting. We use an efficient technique to invert a unitary transformation on a quantum computer to retrieve an encrypted secret quantum state Psi. This attack has a success rate of 100% and can be executed in constant time. We name a vulnerable scheme which security is fully broken by our attack and suggest how to improve the scheme to invalidate this attack. The proposed attack highlights the importance of carefully designing quantum encryption schemes to ensure their security against quantum adversaries, even in a black-box setting. We point to the faulty assumption and name a criterion for future quantum cipher design to prevent similar vulnerabilities.
