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We propose and experimentally demonstrate a dynamic encryption method to realize physical layer security for orthogonal frequency division multiplexing passive optical network (OFDM-PON). In our scheme, encryption of the downstream signal is obtained by applying exclusive or (xor) operation between optical network units' (ONUs') downstream signals and received upstream signals at the optical line terminal side. The upstream signals are used as secure keys for corresponding ONUs. Then the encrypted downstream signals are sent to the ONU sides, where the downstream signal can be retrieved by applying xor operation again between the encrypted downstream signal and the stored upstream signal. Since each ONU cannot obtain the upstream signals of other ONUs, only the ONU itself can recover its downstream signal from the encrypted downstream signal. Moreover, the secure key is dynamically changing along with the upstream signal, significantly improving the security of the downstream signal for the OFDM-PON system. A 5-Gb/s 16-quadrature amplitude modulation OFDM signal with xor-based encryption has been successfully implemented over a 25-km standard single-mode fiber. Experimental results verify that the encryption scheme can effectively prevent eavesdropping by malicious users.