We introduce a scheme for steganographic communication based on a channel hidden within the quantum key distribution protocol of Bennett and Brassard. An outside observer cannot establish evidence that this communication is taking place for the simple reason that no correlations between public data and hidden information exist. Assuming an attacker guesses hidden communication is underway, we obtain a precise quantitative bound on the amount of hidden information they can acquire, and find that it is very small, less than 10~(-7) bits per channel use in typical experimental settings. We also calculate the capacity of the steganographic channel, including an analysis based on data available from experimental realizations of quantum protocols.
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