In this paper the non-quantum method of security amplification for the ping-pong protocol with manyqubit entangled Greenberger-Horne-Zeilinger states (GHZ-states) is proposed. This method can be used in quantum  Ukrainian Scientific Journal of Information Security, 2012 №2 (18)  85 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

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... d does not require the presence from legitimate users of any pre-established keys. Thus, the main advantage of the quantum secure direct protocols, namely the lack of necessity to distribute of secret key remains valid at usage of the proposed method.