Security analysis and improvement on the (<i>T,N</i>) threshold semiquantum secret sharing

Seldom quantum secret sharing protocols with both the flexible (t, n) threshold and semi-quantum properties have been proposed. Recently, a novel idea of (t, n) threshold semiquantum secret sharing was proposed, which has both the (t, [Formula: see text] threshold and semi-quantum properties. Its idea can simplify the quantum secret sharing process such that many classical users with simple ability of quantum operations can realize the communication goal by cooperating with one quantum party. Furthermore, the protocol is very flexible since any t collaborators of the n classical users can reconstruct the full secret by Shamir’s threshold technology. Unfortunately, their protocol is vulnerable to the NOT-gate attack. This paper shows that an attacker can break the (t, n) threshold protocol by performing two rounds of NOT-gate attack. Then, an improved (t, n) threshold semiquantum secret sharing protocol is proposed. In the improvement scheme, each receiver can perform the eavesdropping check by measuring the Z-basis sample states. The improved protocol not only has semi-quantum properties but also can repair the security hole of the old version and has enhanced security against various quantum attacks.