On the security of arbitrated quantum signature schemes
  Due to potential capability of providing unconditional security, arbitrated quantum signature (AQS) schemes, whose implementation depends on the participation of a trusted third party, received intense attention in the past decade. Recently, some typical AQS schemes were cryptanalyzed and improved. In this paper, we analyze security property of some AQS schemes and show that all the previous AQS schemes, no matter original or improved, are still insecure in the sense that the messages and the corresponding signatures can be exchanged among different receivers, allowing the receivers to deny accepting the signature of an appointed message. Some further improvement methods on the AQS schemes are also discussed. 
Qin Li, Chengqing Li, Zhonghua Wen, Weizhong Zhao, W. H. Chan
quant-ph