Enhanced circular quantum secret sharing for remote agents

A new circular quantum secret sharing (QSS) scheme using the controlled-NOT (CNOT) gate was put forward by Lin et al. [Quantum Information Processing 12, 685 (2013)] recently. However, Zhu et al. [Quantum Information Processing 12, 1173 (2013)] found it is insecure against the head agent and the tail agent´s collusion attack without giving the corresponding improvement. In this study, we further proposed another collusion attack in which ⌊(n+1)/2⌋ dishonest participants can collaborate to obtain the dealer´s entire secret without introducing any error. Furthermore, an enhanced QSS scheme was also presented and proved to be secure against various kinds of attack strategies while retaining some merits of the original scheme.