  Global System for Mobile Communications (GSM) is one of the most commonly
used cellular technologies in the world. One of the objectives in mobile
communication systems is the security of the exchanged data. GSM employs many
cryptographic algorithms for security like A5/1, A5/2 and A5/3. Even so, these
algorithms do not provide sufficient level of security for protecting the
confidentiality of GSM. Therefore, it is desirable to increase security by
additional encryption methods. This paper presents a voice encryption method
called: "DES with Random permutation and Inversion", based on current voice
channel, which overcomes data channel's insufficiencies and solves the problem
of penetrating the RPE-LTP vocoder by the encrypted voice. The proposed method
fulfils an end-to-end secured communication in the GSM; insure a good
compatibility to all GSM networks, and easy implementation without any
modification in these systems.
