Security analyses and improvement of arbitrated quantum signature with an untrusted arbitrator

Very recently, an arbitrated quantum signature (AQS) scheme of classical message with an untrusted arbitrator was presented[Eur. Phys. J. D 61(3), 773 (2011)]. In this paper, the security of the AQS scheme with an untrusted arbitrator is analyzed. An AQS scheme with an untrusted arbitrator should satisfy the unforgeable property and undeniable property. In particular, the malicious verifier can not modify a message and its signature to produce a new message with a valid signature, and the dishonest signer who really has sent the message to the verifier which the verifier accepted as an authentic one cannot later deny having sent this message. However, we show that, in the AQS scheme with an untrusted arbitrator, the dishonest signer can successfully disavow his/her signature and the malicious verifier can counterfeit a valued signature for any message by known message attack when he has received a message-signature pair. Then, we suggest an improved AQS scheme of classical message with an untrusted arbitrator that can solve effectively the two problems raised above. Finally, we prove the security of the improved scheme.