In this paper, two vulnerabilities that allow an eavesdropper to extract data from an isolated user in a two-code-keying spectrally phase-coded optical-code-division-multiple-access (O-CDMA) system are experimentally demonstrated. One of these vulnerabilities stems from spectral dips that result from phase-to-amplitude conversion in the encoding process, which allows eavesdropping by using a narrowband tunable optical filter and a simple energy detector. A modified O-CDMA transmitter scheme that masks this vulnerability is demonstrated. A second, especially serious, vulnerability allows eavesdropping by using a differential-phase-shift-keying receiver. Both of these vulnerabilities arise from a structure in the coding and signaling schemes that allow an eavesdropper to recover data, with relatively simple hardware and without attempting to learn the codes.
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