  Until now, there have been developed many arbitrated quantum signature
schemes implemented with a help of a trusted third party. In order to guarantee
the unconditional security, most of them take advantage of the optimal quantum
one-time encryption method based on Pauli operators. However, we in this paper
point out that the previous schemes only provides a security against total
break and actually show that there exists a simple existential forgery attack
to validly modify the transmitted pair of message and signature. In addition,
we also provide a simple method to recover the security against the proposed
attack.
