Abstract: Quantum secure direct communication does not need to prepare the quantum keys in advance, and secret information can be transmitted directly through quantums. In order to solve the problem of high cost of quantum communication equipment, a bidirectional three-party semi-quantum secure direct communication (SQSDC) scheme was proposed based on Greenberger-Horne-Zeilinger (GHZ) state and semi-quantum theory. The scheme can realize the secret communication between two classical parties and a quantum party. By adjusting the number of GHZ particles, the classical party can be extended to any number of parties according to the scheme, making the scheme particularly suitable for the communication between one superior unit and several subordinate units. The security analysis shows that, using the entanglement characteristics of GHZ state for eavesdropping detection can effectively resist the attack of eavesdroppers. In three-party communication, the communication efficiency of the scheme is up to 17.65%, showing a high communication efficiency.

Key words: quantum communication, semi-quantum secure direct communication, Greenberger–Horne– Zeilinger states, multi-party communication