Intercept-Resend Attacks on Semi-quantum Secret Sharing and the Improvements

Recently, Li et al. [Phys. Rev. A 82(2):022303, 2010] presented two semi-quantum secret sharing (SQSS) protocols using Greenberger-Horne-Zeilinger-like states. The proposed schemes are quite practical because only the secret dealer needs to be equipped with advanced quantum devices such as quantum memory, whereas the other agents can merely perform classical operations to complete the secret sharing. However, the present study demonstrates the existence of a security pitfall in the eavesdropping check phase of both the schemes, which can lead to an intercept-resend attack and a Trojan horse attack on the two schemes by a dishonest agent, to determine the other agent's shadow and consequently derive the master key of the SQSS. This contradicts the security requirement of QSS. Fortunately, two possible solutions are proposed herein to eliminate this security pitfall.