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We propose a noninvasive optical encryption technique, taking advantage of the specificities of the physical layer of a backbone transmission network, to secure optical transmissions at high data rates ( >10 Gb/s). Our goal is to secure data transmission without impacting the physical layer by guaranteeing the transparency of the encryption technique with respect to the transmission equipment. The proposed method belongs to the optical code-division multiple-access technique using spectral-phase encoding, based on pulse-overlapping sequence. The goal is to scale the protection levels to different threat and attack scenarios and to make them resilient to intrusions and robust to fiber-propagation parameters. In this paper, linear and nonlinear transmission effects (where self- and cross-phase modulations become important) are considered to maintain the quality of transmission. Finally, particular attention is paid to the ciphering key transmission for which we propose an adapted solution.