  We investigate the quantum signature scheme proposed by Zeng and Keitel
[Phys. Rev. A 65, 042312 (2002)]. It uses Greenberger-Horne-Zeilinger (GHZ)
states and the availability of a trusted arbitrator. However, in our opinion
the protocol is not clearly operationally defined and several steps are
ambiguous. Moreover, we argue that the security statements claimed by the
authors are incorrect.
