A practical secret-key-distribution protocol using general phase fluctuations is described. A preshared key is assumed and is used only inside a transmitter and a receiver. The probabilistic property of signal-light fluctuations works with the preshared key to produce a difference in bit-error rate between a receiver and an eavesdropper. Using the difference, new secret keys are generated information theoretically although a preshared key is used. The probabilistic property of signal-light

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... uations is essential in this protocol, and classical fluctuations as well as quantum fluctuations are applicable. Because signal states are classical, the system becomes tolerant of loss and amplification.