  Semi-quantum private comparison (SQPC) allows two participants with limited
quantum ability to securely compare the equality of their secrets with the help
of a semi-dishonest third party (TP). Recently, Jiang proposed a SQPC protocol
based on Bell states (Quantum Inf Process 19(6): 180, 2020) and claimed it is
secure. In this paper, we present two types of attack on Jiang's SQPC protocol.
In the first type of attack, an outside eavesdropper will make participants
accept a wrong result. In the second type of attack, a malicious participant
will not only make the other participant accept a wrong result, but also learn
the secret of the honest participant. Neither type of attack will be detected.
In addition, we propose an improved SQPC protocol that can resist these two
types of attack.
