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
] 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, parameter estimation 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. While most QKD schemes use two bases for encoding, in our PKD schemes only a single encoding basis is used since mixed bases are primarily a mechanism to detect man-in-the-middle attacks. This avoids the basis reconciliation sifting stage and so is more efficient in its use of photons.
Sasaki et al. have previously discussed the opportunity for simplified cryptography schemes that fall between QKD and laser communications [6
]. As they explain QKD (and PKD) fit within the wider field of physical layer security methods. These encompass such techniques as wiretap channels [7
] and radio frequency equivalents [8
], to laser-based methods that actively quantify the amount of information an eavesdropper can receive [9
]. Our exploration of PKD here builds on the ideas put forward by Sasaki et al.
In this paper we look at the asymptotic secret key rates at high losses (i.e., long distance free space links) for three QKD schemes using three different kinds of transmitter hardware, and compare this to three simplified PKD schemes that use similar hardware. The simplified schemes are primarily targeted at satellite applications and all assume that a covert man-in-the-middle attack has been assessed to be non-existent. As expected, they are all found to remain effective at higher losses than their QKD counterparts.
The results are discussed in the sections below based on the different hardware types. General assumptions were that space-based detectors have 15,000 dark counts per second while ground-based detectors have 2500 dark counts per second. This was based on pessimistic cases for single photon detectors before and after radiation damage [10
]. Furthermore, ground-based detectors were assumed to have an additional 1000 background counts per second due to scattered light entering the receiver. This was slightly more conservative than noise counts estimated in recent literature [11
]. All sources were assumed to have perfect visibility, e.g., for a SPDC source this means it was assumed to produce perfectly entangled states.
3.1. Weak Coherent Pulse (WCP) Source
Satellite-to-ground decoy state QKD using a weak coherent pulse (WCP) source has conclusively been demonstrated on the Micius satellite in 2016 [12
] and the QUBE mission aims to demonstrate a WCP QKD system on a nanosatellite [13
]. WCP sources are highly attenuated lasers which approximate a single photon state although multi-photon emissions are possible. We used the Micius source parameters as a realistic assumption in our model; on average our source emitted 0.8 photons per pulse. For QKD it performed the decoy state BB84 protocol [14
] where it also emitted decoy states of mean photon number 0.1, and operated at a ratio of 0.5:0.25:0.25 for signal, decoy and vacuum states.
For WCP PKD, no decoy states were transmitted, only signal states with an optimized mean photon number of between 0.4701 and 0.4583 for losses of 20–70 dB. These were assumed to be coupled to a modulator that enables pulse position modulation (PPM) to encode bits as early (bit 0) or late pulses (bit 1). Compared to polarization encoding this also removes the requirement to synchronize polarization reference frames which is especially convenient for then communicating to satellites.
3.2. Spontaneous Parametric Down Conversion (SPDC) Photon Pair Source
For the QKD protocols considered here the SPDC photon pair source was assumed to be an entangled photon pair source performing the BBM92 protocol [15
]. The Micius satellite demonstrated an entangled photon pair source in orbit [17
] and the SpooQy-1 nanosatellite will demonstrate a miniaturized device in orbit in 2019 [19
]. The PKD protocols with this setup can use the same hardware or a simpler correlated (heralded) photon pair source, with unit visibility, such as the miniaturized device flown on the Galassia nanosatellite [20
]. To avoid synchronising reference frames the two-state encoding could be in left and right hand circular polarization.
Protocols using pair sources require two detections i.e., one of the pair must be detected at the source and the other of the pair detected at the receiver. The total detection efficiency at the source was assumed to be 25% (i.e., there was a system loss of
dB). The main effect of this approach is that the quantum bit error rate (QBER) partly becomes a function of the accidental rate of coincident detection between the two detectors. This accidentals rate is given by
are the singles rates observed at the detectors, and
the timing coincidence window.
In this study, we assume that the source was capable of producing photon pairs at a raw rate of
per second into a single spatial mode, and that the timing resolution was 1 ns, which are readily accessible [23
]. This leads to a QBER that is between 3% and 4%; this QBER has a linear relationship with the timing window—faster electronics will lead to smaller QBER. An advantage of using photon pairs is that the probability of multi-photon emission is insignificant (similar to single photon sources), leading to a smaller percentage of the raw key being discarded in privacy amplification.
3.3. Single Photon Source (SPS)
The SPS was assumed to be a perfect device that produces single photons on demand and never multi-photon pulses. For QKD this means it can perform the BB84 [24
] protocol without any requirement for decoy states. For PKD it can even be encoded in wavelength giving a broad scope for multiplexing. In both cases the SPS has the highest rates. It should be noted that compared to the other technologies SPS devices are currently at low ‘technology readiness levels’ [25
] or only work at cryogenic temperatures which means they are not a practical choice for deployment in space.
The PKD protocols have higher rates than the QKD protocols because eavesdroppers have no access to the channel and only ‘collective attacks’ are allowed. That means if there are no multi-photon emissions (i.e., not using the WCP source) no privacy amplification is required. PPM PKD suffers from multi-photon emissions so requires privacy amplification. In general the relaxed security assumptions increase key rates and allow for key generation at larger distances. Comparing between different hardware methods of PKD (or between different hardware methods of QKD) is less conclusive, since the ‘pulse’ rates of different implementations of the hardware will vary.
The approach presented in this paper can be applied to key distribution, not only when using satellites, but also in other types of FSO-based systems. For example, in ad-hoc networks where the transmitter and receiver can visually identify each other, it may be straightforward to assess the absence of a man-in-the-middle and use PKD type protocols to establish a secret key. If the possibility of man-in-the-middle attacks can be excluded or monitored by conventional means, then PKD protocols can allow for secure key distribution using quantum communication methods at increased losses using similar or simplified hardware. In a practical key distribution scenario, it is worthwhile to have a better specification of the threat model and to devote efforts in closing physical side-channels, rather than attempting to defeat a hypothetical quantum adversary. Finally, we remark that using QKD hardware it may be possible to switch to a PKD protocol when the channel is assessed to be free of active attacks, and thus increase key distribution rates.