M ar 2 01 9 SATELLITE QUANTUM COMMUNICATIONS WHEN MAN-INTHE-MIDDLE ATTACKS ARE EXCLUDED

Satellites can enable global delivery of encryption keys using Quantum Key Distribution (QKD). Traditional QKDmethods are designed to detect the presence of eavesdroppers who have have almost limitless technical abilities. Any eavesdropping attack permitted by the laws of physics is considered as a threat, including attempts to covertly intercept (and manipulate) the key delivery channel, but this comes at the expense of efficiency. We consider that in the scenario of satellite-based QKD a relaxation of the threat model may be justified as the key delivery channel is often monitored by accompanying methods such as radar and optical beacons. This would allow for improved efficiency in key generation and thus higher key rates compared to conventional QKD. If the channel is assessed to be free of any active man-in-the-middle attack, then the most relevant consideration would be the channel losses, the worst case being where all lost photons are collected by an adversary. We investigate three possible QKD implementation technologies – weak coherent pulses, parametric down conversion photon pair sources, and true single photon sources – and calculate how key rates and distance limits (loss limitations) can be improved if man-in-the-middle attacks are ruled out by assumption. We term these relaxed security assumption protocols as photon key distribution (PKD) to differentiate them from the standard QKD protocols.