Fig. 1
Illustration of the most common satellite QKD scheme: the flying trusted-node. In step a, the satellite establishes a shared secret key K A with station A by running a QKD protocol, which requires both classical and quantum communication. This step is repeated in b to establish a shared secret key K B , this time with station B located further away. At the end of these steps, the satellite holds both keys, while each station knows only their own. Finally, in step c, the satellite publicly announces the parity of both keys K A ⊕ K B . This allows station B to determine key K A , which can then be used to encrypt private communications to A and vice versa
