  As a new model for signing quantum message, arbitrated quantum signature
(AQS) has recently received a lot of attention. In this paper we study the
cryptanalysis of previous AQS protocols from the aspects of forgery and
disavowal. We show that in these protocols the receiver Bob can realize
existential forgery of the sender's signature under known message attack. Bob
can even achieve universal forgery when the protocols are used to sign a
classical message. Furthermore, the sender Alice can successfully disavow any
of her signatures by simple attack. The attack strategies are described in
detail and some discussions about the potential improvements of the protocols
are given. Finally we also present several interesting topics in future study
on AQS protocols.
