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This letter proposes a secure orthogonal frequency division multiplexing (OFDM) waveform based on two physical-layer secrecy enhancing methods. The first method is time-domain pre-coding that involves interleaving and scaling based on a key shared between two legitimate parties adopting the OFDM waveform. The second method is a random change of the cyclic prefix (CP) size resulting in the randomization of the OFDM symbol time. The CP size is also changed based on a key agreed upon between the legitimate parties. The proposed methods not only increase the detection complexity at the eavesdropping node, but also suppress any cyclic features that distinguish OFDM waveforms. The evaluation of the proposed methods is carried out by deriving a closed-form expression for the ergodic secrecy rate of the system. Then, the effect of pre-coding on the quality-of-service performance is analyzed. The …