  We analyze the security of the efficient two-party quantum private comparison
protocol with decoy photons and two-photon entanglement. It is shown that the
compromised third party (TP) can obtain the final comparison result without
introducing any detectable errors in the standard semi-honest model. The attack
strategy is discussed in detail and an improvement of this protocol is
demonstrated. The idea of our attack might be instructive for the cryptanalysis
of quantum cryptographic schemes.
