Abstract. Recently, Liu et al. [Commun. Theor. Phys. 57, 583, 2012] proposed a quantum private comparison protocol based on entanglement swapping of Bell states, which aims to securely compare the equality of two participants' information with the help of a semi-honest third party (TP). However, this study points out there is a fatal loophole in this protocol, i.e., TP can obtain all of the two participants secret inputs without being detected through making a specific Bell-basis measurement. To fix the problem, a simple solution, which uses one-time eavesdropper checking with decoy photons instead of twice eavesdropper checking with Bell states, is demonstrated. Compared with the original protocol, it also reduces the Bell states consumption and simplifies the steps in the protocol.PACS numbers: 03.65. Ta, 03.67.Dd, 03.67.Lx Keywords: Quantum private comparison, Bell states, Entanglement swapping, TP's measurement attack, ImprovementCryptanalysis and improvement of the quantum private comparison protocol based on Bell entangled states2

In Yang et al. 's literatures [J. Phys. A: Math. 42, 055305, 2009; J. Phys. A: Math. 43, 209801, 2010], a quantum private comparison protocol based on Bell states and hash function is proposed, which aims to securely compare the equality of two participants' information with the help of a dishonest third party (TP). However, this study will point out their protocol cannot resist a special kind of attack, TP's same initial states attack, which is presented in this paper. That is, the dishonest TP can disturb the comparison result without being detected through preparing the same initial states. Finally, a simple improvement is given to avoid the attack.Keywords Quantum cryptography · Quantum computation · Quantum private comparison · Same initial states attack IntroductionThe principles of quantum mechanics, such as no-cloning theorem, uncertainty principle, and entanglement characteristics, provide some interesting ways for cryptography communication and secure computation. During the past thirty years, quantum communication has developed in a variety of directions, including quantum key distribution (QKD) [1,2], quantum secret sharing (QSS) [3,4], quantum direct communication (QDC) [5][6][7], quantum

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