A cryptographic key generator for a first optical transceiver includes a photodetector that receives a continuous wave light beam received via an optical channel from a second optical transceiver. The generator samples and quantizes signals from the photodetector during a plurality of intervals to generate respective samples representing respective numbers of photons incident on the photodetector during each of the plurality of intervals. The generator creates a first cryptographic key from the plurality of digital values. The second optical transceiver receives a continuous wave light beam from the first transceiver and performs the same functions to create a second cryptographic key. Due to the reciprocal nature of the channels, the first and second cryptographic keys match.