Comment on "security analysis and improvements of arbitrated quantum signature schemes"

Recently, Zou, and Qiu demonstrated that two arbitrated quantum signature (AQS) schemes are not secure because an arbitrator cannot arbitrate the dispute between two users when a receiver repudiates the integrity of a signature. By using a public board, Zou and Qiu's proposed two AQS schemes to solve the problem. This work shows that the same security problem may exist in Zou and Qiu's schemes. Moreover, a malicious verifier, Bob, can actively negate a signed order if he wants to. This attack, a special case of the denial-of-service (DoS) attack mentioned by Cai, is important in quantum cryptography. Bob may get some benefits from this DoS attack since he can actively deny Alice's signed order without being detected. This work also shows that a malicious signer can reveal the verifier's secret key without being detected by using Trojan-horse attacks.