# Satellite Quantum Communications When Man-in-the-Middle Attacks Are Excluded
## 1 Introduction  
Cryptographic key distribution is a major application of quantum communication. Such schemes typically use measurements of quantum states of photons shared between two remote parties to allow both sides to derive shared entropy that can be quantitatively assessed to be private. This shared entropy may be used as keying material for use as one time pads [1] or as seed keys for symmetric encryption [2]. Quantum key distribution relies on the transmission of single photons so many photons must be distributed to generate a key over lossy channels. For shorter distances and metropolitan regions photons can be distributed between parties using optical fibres, but for global distances satellite-based nodes become more practical [3]. For quantum communication from satellites, and other moving platforms, photons are distributed using free space optics (FSO). Quantum key distribution schemes [4; 5] have extremely strong security guarantees due to minimal assumptions on the capabilities of the technology available to potential eavesdroppers--essentially any attack permitted by the laws of physics is deemed possible. Although typically the two communicating parties must trust the quantum key distribution (QKD) hardware within their control, they need not trust the optical channel between them because they can detect any attempt at eavesdropping and any man-in-the-middle attacks on the channel using statistical tests that are inherent to the QKD process. These tests require a fraction of the received photons to be discarded such that they cannot be used in the final keying material. This discarding arises in processes such as basis reconciliation, parameterestimation and privacy amplification. In many situations these discarded photons and slower key rates are a necessity for security e.g., in a metropolitan environment where QKD is performed over optical fibres which pass through many ducts and underground passages where, in principle, eavesdropping can take place. For the case of satellite QKD (and other FSO delivery methods), where there is a direct line of sight, these measures seem to be harder to justify. To compromise the security of the link, other than disrupting it through denial-of-service attacks, an adversary would have to act as a man-in-the-middle. We argue that performing this attack for an optical link between a low earth orbit (LEO) satellite and ground station would be physically possible, but technically not feasible for most adversaries. It requires intercepting and re-sending a beam that is only a few metres across and rapidly tracking across the sky without being detected. Practical satellite-ground links employ dual tracking beacons to establish the required high accuracy pointing link, effectively providing a channel monitoring system. Additional hardware may be deployed to monitor the channel in other wavelengths, e.g., radar systems or thermal imaging cameras. Furthermore, space-situational awareness has led to publicly accessible catalogues that provide ephemeris data of orbiting space objects which are frequently updated, making it possible to assess if any (publicly known) satellites can threaten the link in space. If users fear that their adversaries could have technologies which can covertly intercept the link they could of course use QKD; other users can relax their threat assumptions. In light of the above we thus focus only on passive eavesdroppers, attackers which cannot be detected by channel monitoring techniques. As we will show, using this updated threat model simplifies the key delivery protocol. In this paper we term the simplified quantum communication schemes that address this scenario as 'photon key distribution' (PKD) so as to separate them from conventional QKD protocols. Some more recent QKD schemes, where the security proofs may not yet be completely agreed, may also prove to be somewhere on the PKD to QKD spectrum depending on the security assumptions they require.