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A full-duplex analog speech-scrambling system is proposed for application to mobile communication systems and public switched telephone networks. The scrambling algorithm is based on the rearrangement of the fast Fourier transform (FFT) coefficients accompanied by adaptive dummy spectrum insertion and companding operation. The simulation results indicate that the scrambled speech has no residual intelligibility and the descrambled speech quality is satisfactory. The hardware unit is implemented by using seven advanced digital signal processor chips, including those for an adaptive equalizer and an echo canceller for full-duplex operation. The performance is proven to be satisfactory.