Unitary operation attack and the improvement on probabilistic quantum key distribution

This study points out that a malicious communicant in Hwang et al.’s probabilistic quantum key distribution (PQKD) protocol can manipulate the secret key without being detected by using the unitary operation attack. Accordingly, the security requirements of a PQKD protocol, i.e., unpredictability and fairness, cannot be satisfied in their protocol. A possible solution is also provided to solve the problem.