This work was devoted to the development of method for improving security to eavesdropping attacks in quantum direct communication protocols.

In this paper the non-quantum method of security amplification for the ping-pong protocol with many-qubit entangled Greenberger-Horne-Zeilinger states (GHZ-states) is proposed. This method can be used in quantum cryptography and consists in invertible hashing of classical message blocks. The necessary sizes of matrices for hashing of message blocks are calculated at some values of the protocol parameters. The proposed method does not require the presence from legitimate users of any pre-established keys. Thus, the main advantage of the quantum secure direct communication protocols, namely the lack of necessity to distribute of secret key remains valid at usage of the proposed method.