  We study information theoretical security for space links between a satellite
and a ground-station. Quantum key distribution (QKD) is a well established
method for information theoretical secure communication, giving the
eavesdropper unlimited access to the channel and technological resources only
limited by the laws of quantum physics. But QKD for space links is extremely
challenging, the achieved key rates are extremely low, and day-time operating
impossible. However, eavesdropping on a channel in free-space without being
noticed seems complicated, given the constraints imposed by orbital mechanics.
If we also exclude eavesdropper's presence in a given area around the emitter
and receiver, we can guarantee that he has only access to a fraction of the
optical signal. In this setting, quantum keyless private (direct) communication
based on the wiretap channel model is a valid alternative to provide
information theoretical security. Like for QKD, we assume the legitimate users
to be limited by state-of-the-art technology, while the potential eavesdropper
is only limited by physical laws: physical measurement (Helstrom detector) and
quantum electrodynamics (Holevo bound). Nevertheless, we demonstrate
information theoretical secure communication rates (positive keyless private
capacity) over a classical-quantum wiretap channel using on-off-keying of
coherent states. We present numerical results for a setting equivalent to the
recent experiments with the Micius satellite and compare them to the
fundamental limit for the secret key rate of QKD. We obtain much higher rates
compared with QKD with exclusion area of less than 13 meters for Low Earth
Orbit (LEO) satellites. Moreover, we show that the wiretap channel quantum
keyless privacy is much less sensitive to noise and signal dynamics and daytime
operation is possible.
