Cryptanalysis of the secure quantum private comparison protocol

Recently, Yang et al [2009 Phys. Scr. 80 065002] presented the first quantum private comparison protocol that involves single photons only. This protocol is very promising in practice. However, in this paper, we analyze the security of the protocol and point out a subtle loophole. It is shown that both Eve and Charlie could obtain Bob's secret by launching a special dense-coding attack. Moreover, details of this attack and its correctness are given, and also two solutions are demonstrated to improve the protocol in order to overcome the fatal flaw.