Security of Covert Quantum Key Distribution under a Class of Coherent Attacks
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We show that covert secret key expansion is possible against a restricted class of coherent attacks, which we characterize. Specifically, we propose a protocol based on pulse-position modulation and multi-level coding that allows one to use traditional quantum key distribution (QKD) protocols while ensuring covertness, in the sense that no statistical test by the adversary can detect the presence of communication better than a random guess. Covertness holds as long as there exists some noise in the channel that is not under the control of the adversary, which we model as a quantum channel preceding the adversary's apparatus.
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