Apparatus and method for secure spatial communication

Abstract

A method for free-space quantum key-free private communication using a communication protocol in which information is exchanged between a transmitting device (100) and a receiving device (200) through a primary quantum classical channel, and in which the eavesdropping device eavesdrops on the primary channel through the eavesdropping channel based on an eavesdropping channel model, where the overall degradation of the eavesdropping channel is greater than the degradation of the primary channel, includes the steps of: in the transmitting device (100), preparing a message M composed of classical bits; encoding the message M to convert it into a coded message X; actually modulating the amplitude and / or phase of optical pulses of the coded classical bits; transmitting the coded message to the receiving device (200) through a classical quantum channel (500) such that only partial information about the state is provided to an eavesdropping device (300) eavesdropping on the channel; and detecting and decoding the received message through quantum security analysis.

Description

Technical Field

[0001] The present invention relates to secure space communication, and more particularly, to an apparatus and method for providing secure communication between a satellite and a ground station.

Background Art

[0002] Free space optical communication (FSO) is an optical communication technology that uses light propagating through free space to wirelessly transmit data for long-distance communication or computer networking. "Free space" means air, outer space, vacuum, or the like where light propagates linearly. This is in contrast to guided optics such as optical fibers or more generally optical waveguides, where light is guided and induced by a waveguide. Free space technology is useful when a physical connection is not practical due to high cost or other considerations.

[0003] Similar to any other type of communication, free space optical communication requires security to prevent eavesdropping. Examining different security means for free space optical communication reveals that several solutions have been investigated to provide a solution that enables a transmitting device and a receiving device to share secret information through FSO.

[0004] Currently, there are several concepts for deploying networks through space links. In particular, there is increasing unprecedented interest in deep space communication networks.

[0005] Over the past few decades, solutions for overcoming eavesdropping have been developed according to different scenarios. Usually, in FSO communication, two eavesdropping scenarios can be considered. Both are shown in Figure 1a.

[0006] In the first scenario, eavesdropping device Eve1 300 is located on the optical path between transmitting device 100 and receiving device 200. Thus, Eve1 300 can intercept the optical signal and retransmit the potentially tampered optical signal to receiving device 200. This is called an active scenario, which can be addressed, for example, by the QKD communication protocol. In the second scenario, Eve2 305 is not located on the optical path between transmitting device 100 and receiving device 200. Thus, it is limited to the ability to extract a portion of the optical signal transmitted from transmitting device 100 to receiving device 200. In this scenario, the eavesdropping device (Eve2 305) cannot retransmit any optical signal to receiving device 200. This is called a passive scenario, which will be described later.

[0007] The first scenario can be solved by the application of QKD in free space communication. QKD is a protocol that enables the exchange of secret keys in an active scenario when an eavesdropping device is located on the optical path between the transmitting device and the receiving device. In the QKD protocol, the communication channel between two users is known as a quantum channel. A quantum channel is a communication channel that transmits quantum particles, typically photons, in a way that preserves their quantum properties. There are two sets of parameters used for quantum encoding. One is the polarization of photons, and the second is the phase that requires the use of an interferometer. Both have their advantages and disadvantages depending on the physical layer of the quantum channel and the type of QKD protocol.

[0008] The basic idea behind QKD is that an eavesdropping device can intercept the signal and process it in any way compatible with quantum mechanics. Nevertheless, legitimate users known as the transmitting and receiving devices can still exchange a secure key.

[0009] There are several protocols for QKD, such as the BB84 protocol, E91, B92, and COW, but these are conventional and well-known in the art and will not be repeated here.

[0010] All of these protocols are based on the transmission of a single photon through a quantum channel and are known as discrete-variable QKD or DV-QKD. These require the use of a single-photon detector on the receiving end. To mitigate this need, another type of QKD, called continuous-variable QKD or CV-QKD, has been proposed and demonstrated. CV-QKD is typically used with a phase parameter.

[0011] Commercial systems for terrestrial QKD distributed over optical fibers have been developed. In all practical implementations of terrestrial QKD, the parameters used for quantum coding are phase, or the relevant timing parameters for the COW protocol. This is because polarization methods require complex and expensive components, as polarization is not preserved within the optical fiber. Interferometric methods, on the other hand, are easier to implement in single-mode optical fibers, making them the preferred medium for terrestrial QKD.

[0012] One of the most severe limitations of ground-based QKDs is the distance limitation. Due to unavoidable losses in optical waveguides and the fact that optical amplifiers cannot be used within the quantum channel, the distance between the transmitter and receiver is limited to approximately 100 kilometers in commercial settings and up to 300 kilometers in academic experiments. Therefore, to extend the distance range, FSO QKDs have been proposed, in which the quantum channel is free space and does not have the same loss limitations.

[0013] In recent years, FSO QKD has been studied for secure key exchange between transmitters and receivers in free space, typically between satellites or flying drones and ground stations.

[0014] European Patent No. 3337063 describes a conventional free-space key distribution method that includes the step of exchanging information between a transmitter and a receiver based on a physical layer eavesdropping channel model. This system is a QKD communication system consisting of exchanging keys between a transmitter and a receiver, which follows a QKD protocol-based communication model: a quantum part and an iterative classical (post-processing) part, where information is modulated as qubits, and the system operation uses a measured QBER as input to the security-providing process. However, the drawbacks of this invention are that, although it allows for stronger signals, the signal strength is still weak, requiring post-processing, sorting, and distillation of the key, and requiring a bidirectional communication protocol.

[0015] However, QKD with satellites is extremely difficult due to the small signal strength of about one photon per pulse, high channel loss, and sensitivity to background noise, which does not allow for daytime key exchange. On the other hand, undetected eavesdropping also appears to be very complex, even in free-space scenarios.

[0016] Furthermore, QKD requires a bidirectional protocol between the transmitter and receiver, resulting in lower reliability than a unidirectional protocol. In fact, bidirectional protocols over satellites require a long time to distill a useful key due to propagation times that can range from tens to hundreds of milliseconds per hop. This delay accumulates during each iteration of the QKD protocol.

[0017] Regarding the second scenario, the conventional communication channel is known as the eavesdropping channel, first introduced by Wyner. However, the concept of the eavesdropping channel was later expanded to a more abstract level by Czisar and Korner. In these cases, the eavesdropping channel is an abstract model that includes any three-way channel (between the transmitter, receiver, and eavesdropping device) with no restrictions on the eavesdropping device. In this abstract model, the eavesdropping channel has two separate channels, one between the transmitter and receiver and the other between the transmitter and receiver; see Figure 1b.

[0018] However, this technique requires assumptions regarding the noise level and signal extraction capability of the Eve detector. For this reason, the noise on the Eve detector must be below bound, and this limit must be known. This is a significant problem because we have no idea what quality of detector will be provided for the Eve detector. In fact, generally speaking, to consider the best eavesdropping capability and therefore the best security, the Eve is considered a passive eavesdropping device, limited by the laws of quantum electrodynamics, given that it is not located on the optical path between the transmitter 100 and the receiver 200, and does not require physical measurements, but is limited only by its own spatial location, and therefore limited only by its own position.

[0019] Therefore, there is an urgent need for a system and method that provides secure FSO communication under the assumption that Eve is not located on the optical path between the transmitter 100 and the receiver, and is restricted only by the laws of quantum physics.

[0020] Therefore, the object of the present invention is to provide a system and method for providing secure FSO communication that overcomes the aforementioned drawbacks and provides safe and easy FSO communication. [Overview of the project]

[0021] This objective is achieved by combining the physical layer eavesdropping channel hypothesis (that eavesdropping devices are limited to listening) with a simple quantum channel that limits the amount of information available to Eve, through the principles of quantum mechanics and quantum electrodynamics. For example, a free-space quantum keyless private communication method by a communication protocol comprising the exchange of information between a transmitter (100) and a receiver (200) through a principal classical quantum channel, wherein, based on an eavesdropping channel model, an eavesdropping device eavesdrops on the principal classical quantum channel through an eavesdropping channel, the free-space quantum keyless private communication method includes the steps of: preparing a message M composed of classical bits in the transmitter (100); encoding the message M to be converted into a coded message X; converting the classical bits of the coded message X into a signal to be transmitted to the receiver (200) by modulating the amplitude and / or phase of the coherent state of the signal; transmitting the signal including the coded message X to the receiver (200) through the principal classical quantum channel; and detecting and decoding the coded message X in the receiver (200). The aforementioned free-space quantum keyless private communication method further, Exclusion radius r surrounding the receiving device (200) E To define the exclusion radius r E Based on this, calculate a degradation parameter γ that allows the overall degradation of the eavesdropping channel to be greater than the degradation of the principal classical quantum channel. Includes The calculated degradation parameter γ The exclusion radius r satisfies the predetermined conditions E is defined ru.

[0022] A first aspect of the present invention is a free-space quantum-key-free private communication method by a communication protocol that exchanges information between a transmitting device (100) and a receiving device (200) through a main classical quantum channel and in which an eavesdropping device eavesdrops on the main channel through an eavesdropping channel based on an eavesdropping channel model, wherein an overall degradation of the eavesdropping channel is greater than a degradation of the main channel. In the transmitting device (100), a step of preparing a message M composed of classical bits, a step of encoding the message M so as to convert it into an encoded message X, and a step of converting classical bits of the encoded message into a signal transmitted to Bob by modulating an amplitude and / or a phase of a coherent state, and a step of transmitting a signal including the encoded message to the receiving device (200) through a quantum classical channel (500) so that only partial information regarding the state is provided to an eavesdropping device (300) eavesdropping on the channel, and a step of detecting and decoding the received message.

[0023] Preferably, the conversion step is a probabilistic encoding step.

[0024] Advantageously, the communication protocol is a one-way communication protocol.

[0025] In a preferred method, the classical bits modulate a coherent state modeled by quantum electrodynamics.

[0026] According to a preferred embodiment, the method further includes a step of calculating a degradation parameter γ according to parameters of the receiving device as follows:

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[0027] According to a preferred embodiment, the free-space key distribution method further includes the step of defining exclusions surrounding the receiving device (200) based on a degradation parameter γ.

[0028] Advantageously, the exclusion surrounding the receiver (200) is defined such that the degradation parameter γ is lower than a given value less than 1.

[0029] Preferably, the exclusion surrounding the receiving device (200) is defined such that the degradation parameter γ is lower than 0.1.

[0030] Advantageously, the signal is an optical signal. [Brief explanation of the drawing]

[0031] Preferred embodiments of the present invention are described below with reference to drawings intended to illustrate preferred embodiments of the present invention, not for the purpose of limiting these preferred embodiments. In the drawings, [Figure 1a] This diagram illustrates different eavesdropping scenarios involving eavesdropping channels and physical layer eavesdropping channels. [Figure 1b] This diagram illustrates the principle of eavesdropping channels using conventional technology. [Figure 1c] This is a diagram representing a passive scenario. [Figure 2] This figure illustrates the general principles of the present invention. [Figure 3] This is a diagram representing Bob and Eve's probabilistic detection model. [Figure 4] This graph shows the numerical values ​​of Cp(γ), the sensitivity to OOK and noise, assuming pdark≈0 and ηo=1. [Modes for carrying out the invention]

[0032] Figure 1 illustrates various eavesdropping scenarios as described and explained above. Transmitter 100 and receiver 200 exchange data through an FSO communication channel defined by a dashed line. According to the eavesdropping channel scenario, eavesdropping device eve 1 300 may be located between 100 and 200 and intercepts and retransmits the beam. According to the physical layer eavesdropping channel scenario, eavesdropping device eve 2 305 may be located on the ground and can collect a portion of the beam without retransmitting it. Both attempt to obtain information about the exchanged data. Its description is not repeated in detail herein.

[0033] Figure 1b illustrates the general principle of eavesdropping channels. This was introduced by Wyner (relaxing Shannon's perfect secrecy condition) and generalized by Csiszar and Korner. It models a common communication system architecture having a transmitter that encodes a message M into a codeword X of n symbols for transmission to a receiver, in the presence of an eavesdropping device that acquires a noisy observation Z of X. The literature has demonstrated the existence of coding schemes that simultaneously guarantee reliable transmission and secrecy for discrete memoryless channels, which have been extended to Gaussian and radio channels. As a further development of the eavesdropping channel concept, it has been recognized that secrecy is closely related to the concept of channel decomposability from the information spectrum method developed by Han and Verdu. This approach, pioneered by M. Hayashi, offers a completely different way of analyzing systems and constructing codes to provide strong security. For stronger eves, different architectures have been considered, typically involving public channels aimed at distilling secret keys. However, this requires two-way communication.

[0034] Figure 2 shows the general principle of the present invention, namely the main channel and the eavesdropping channel, at the top, and the information-theoretic representation of the classical quantum degraded eavesdropping channel at the bottom. This represents the classical input random variable X and the output quantum systems B and E.

[0035] More specifically, it represents a general direct communication protocol as a one-way eavesdropping protocol, where the secret bit is channel-encoded and transmitted over n uses of the optical channel. This protocol includes the following steps, with the codeword received by Bob being, i.e., this is a noisy version of the transmitted codeword Xn.

[0036] 1) The transmitter transmits a stream of secret information bits, such as video, audio, etc., to an encoder, preferably a probabilistic eavesdropping encoder. In practice, the transmitter generates a message and then encodes this message. This encoding step involves the transmitter using probability q. x Codeword X to send secret message M n This includes selecting the codeword Y, which was received by Bob. n Send this to Receive Bob, i.e., this is the transmitted codeword X n A noisier version of the same, but the channel also has Z n Information is also leaked to the environment represented by the receiving eavesdropping device (EVE).

[0037] Message confidentiality is given by a rate R = k / n (where k is the number of secret bits), and the error probability after decryption is ∈ n , and security measurement δ n This depends on the structure of this encoder, which is characterized by [specific characteristics].

[0038] 2) For each channel used, and therefore after coding and before transmission, the transmitter prepares a coherent state modulation by the random variable X ∈ X = {0, 1}, where X = 0 at probability q and X = 1 at probability 1-q. The OOK state transmitted by Alice is the vacuum state |α0>=|0>,

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[0039] 3) After n transmissions across the quantized propagation field, the receiving device B n Upon receiving the signal, the eavesdropping device E n The receiving device receives the light, and after measuring the incoming light, it estimates its own received coherent state, Y n It obtains. The receiving device processes the state-of-the-art detector. Eve is Z n To obtain it, the best quantum detection strategy is applied. The receiver (Bob) receives the coded message and decodes it after measuring the incoming light.

[0040] According to the eavesdropping theory of the present invention, even when the eavesdropping device is computationally infinite, the eavesdropping code of the present invention is ε if R is an achievable rate. n and δ n This ensures that both tend to be zero for large n, where ε n δ is the error probability, n This is a security measure,

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[0041] In other words, the private capacity (or concealed capacity) of the classical quantum eavesdropping channel of the present invention is the performance criterion for this protocol.

[0042] More specifically, strong security means that, given a uniform distribution of messages transmitted through a channel between a transmitting and receiving device, an eavesdropping device cannot obtain any information about it. This criterion is the most common security criterion in classical quantum information theory.

[0043] The metrics for strong security are δ n = This is the amount of mutual information leaked to Eve, using something that can be expressed as I(X;Z).

[0044] When the communication channel between the transmitting device and the receiving device is degradable, channel N = (N B ,N E We can assume the detection and decoding of each symbol of ). The probabilistic description of the degradation potential of classical channels is that X, Y, and Z form a Markov chain XYZ.

[0045] Figure 2 shows a physical description of the protocol implemented in a coherent state across degradable channels according to the present invention.

[0046] The main channel between Alice (transmitter) and Bob (receiver) and the eavesdropping channel between Alice (transmitter) and Eve (eavesdropping device) are preferably discrete memoryless channels. In this case, the private capacity of the quantum eavesdropping protocol (having quantum channels and information) is as follows:

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[0047] Here, we describe a degradable channel for a practical (energy-constrained) protocol across spatial links, which is used to derive private capacity.

[0048] First, we consider an alphabet consisting of two purely coherent states modulated by the random variable X ∈ X = {0, 1}, where X = 1 for probability q and X = 0 for probability 1-q.

[0049] This model assumes on-off modulation (OOK), but it can also be applied to two-phase shift modulation (BPSK). The OOK state transmitted by Alice is the vacuum state |α0)=|0),

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[0050] The coefficient γ∈(0,1) characterizes the channel power degradation, and therefore the channel transmittance of Eve is γη.

[0051] The received state is either simply a vacuum, or Bob and Eve respectively

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[0052] Therefore, assuming Alice sent x, the conditional probability that Bob detects y is ε0 = (1-p dark )e -ηoΔ and ε1=(1-p dark )e -(η|α1|2+ηoΔ) This is shown in Figure 3.

[0053] On the other hand, Eve is constrained only by her spatial position, so she instead performs optimal quantum detection. In a single observation, this is the optimal error probability.

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[0054]

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[0055] Here

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[0056] In binary source scenarios, this limitation is achieved by the Holevo-Helstrom projector, and several practical implementations have been proposed for this purpose, such as the Kennedy receiver, Dolinar receiver, or Sasaki-Hirota receiver.

[0057] The optimal error probability for Eve, obtained from the above formula, is:

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[0058]

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[0059] Here, I(X,Y) is Alice's Shannon mutual information using a state-of-the-art photon coefficient detector, and I(X;Z│γ) is the maximum Shannon mutual information that Eve can physically detect.

[0060] When the optimized capacity is uniform, i.e., q = 1 / 2, we obtain the following:

[0061]

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[0062] Here, [...] + The positive part is represented by the notation h(), which is the binary Shannon entropy.

[0063] Figure 4 shows the numerical results of the above equation, and it was observed that the concealment rate was high at 0.68 with γ=0.1, indicating almost no sensitivity to noise. Furthermore, even under unfavorable channel conditions (γ close to 1), a positive concealment rate (for example, Δ=10) was achieved with reasonable noise. -4 There is a maximum value of γ = 0.9, but in the following example, it can be seen that aiming for γ < 0.1 is actually the best approach.

[0064] In fact, the method of the present invention also preferably includes a step of calculating a degradation parameter γ depending on the parameters of the receiving device. In fact, in order to provide a communication channel between Alice and Bob that is not as degraded as the channel between Alice and Eve, this degradation must be suppressed and fixed.

[0065] The percentage of light collected by Bob (receiver), free-space loss η B This can be roughly calculated as the ratio of the telescope area to the occupied area.

[0066]

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[0067] Here, d B D is the distance between the transmitter and the receiver. R D is the diameter of Bob's telescope. F is the diameter of the area occupied around Bob.

[0068] Therefore, the number of photons detected by Bob can be calculated as follows:

[0069]

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[0070] Here, η b This represents additional losses depending on the experimental conditions.

[0071] Regarding Eve (eavesdropping device), as mentioned above, η E The (percentage of light collected by Eve) is calculated, but a coefficient is added that takes into account the light intensity outside the excluded angle, assuming a Gaussian angular distribution of the beam, as follows.

[0072]

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[0073] Subsequently, assuming that Eve has no additional losses, the number of photons detected by Eve is simply η E N t =γηN t Therefore, for a fixed antenna size, γ can be easily calculated as follows.

[0074]

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[0075] Therefore, γ can be easily defined and adjusted depending on the parameters of the device being used. [Examples]

[0076] As an example, we consider a realistic physical scenario, using a recent QKD experiment using China's LEO satellite Micius as a reference.

[0077] Here, the satellite has an orbit approximately 500 km above the Earth's surface and exchanges keys at a distance of up to 1200 km when the satellite is near the horizon. The transmitter has a far-field divergence θ of 10 μrad. div (1 / e 2 It is equipped with a 300mm Cassegrain telescope featuring a maximum angle of view (at ). The ground station receiver is located at a 1m diameter D R It has a telescope that has [a certain feature].

[0078] In the Micius experiment, these were found to be atmospheric turbulence of 3-8 dB(η) atm ), direction error (η p ) <3dB, overall light loss from the telescope input lens to the detector (η o ), 7.4dB detector, detector efficiency η d et is 50% (-3dB). Below is the overall η of 20dB (1%). b This can be considered rationally.

[0079] d B =d E =1200km and θ E =r E / d provides information on the Micius system parameters, as well as the receiving antenna D of a very large eavesdropping device at 2m. E and small exclusion radius r E Assuming = 12.5m, we obtain the following:

[0080]

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[0081] Even if other values ​​< 1 are theoretically possible, it is recommended to fix the exclusion radius so that γ < 0.1, which represents a good trade-off. In fact, this value has been shown to be a good choice because it results in a high secret capacity > 0.6 and has little sensitivity to noise and signal fluctuations at a reasonable exclusion radius. This sensitivity is driven by the discriminability of coherent states in Eve's Holevo-Helstrom detector, and the lower γ, the lower the sensitivity of discriminability to signal dynamics.

[0082] Here, we compare the present invention with the conventional QKD protocol.

[0083] To do this, Alice and Bob need to calculate private capacities with different geometric configurations, assuming they have satellites and ground stations equivalent to those of the Micius experiment.

[0084] Here, we consider an OOK with a clock speed of 1 GHz. With a time window of 1 ns, the state-of-the-art single-photon detector is p dark <10 -7 Because of this characteristic, detector noise does not have a significant impact on the secret capacity.

[0085] Considering the average number of noise photons, the achievable values ​​for a clear daytime sky and a clear night with a full moon are 10 for different collection angles, filter bandwidths, and time windows. -4 and 10 -7 It becomes Δ. On a cloudy day, one cloud is 10 -2 We anticipate a Δ of this, but if channel transmission is not significantly reduced, it will still be a positive private rate.

[0086] Table I below shows the private capacities of LEO, MEO, and GEO satellites, as well as different ambient light conditions.

[0087] [Table 1]

[0088] Table I shows that the private capacitance of the eavesdropping channel is superior to QKD in terms of rate and, most importantly, in terms of noise immunity. Note that the laser power required to achieve an optimal signal intensity of approximately 4 photons on average is moderate, e.g., 15 mW and 15 μW in GEO and LEO settings, respectively, and therefore this is not a limitation.

[0089] The above illustrates that in FSO communications, a protected area is required for all types of secure communications, and how this can be achieved. While the above describes downlink communications, uplink communications can similarly be considered, and their channel degradation γ can be estimated for Eve's satellite under reasonable assumptions.

[0090] The protocol of the present invention is sensitive to jam attacks, as is the QKD protocol. However, the protocol of the present invention can also be used in conjunction with security mechanisms for communications above the physical layer to provide availability, integrity, and confidentiality for satellite systems.

[0091] Considering these boundary conditions, the above demonstrates that physical layer encryption can provide information-theoretically secure communication even in the presence of Eve, which is limited only by quantum physics. For eavesdropping code, explicit structures are available that can provide strong security.

[0092] One of the main advantages is that the invention provides a significantly higher achievable private rate than the QKD rates of actual systems. Furthermore, in contrast to QKD, direct secure communication is possible even near illuminated cities and during daylight hours. Also, considering the low rate, the secret key generated by QKD is not actually used in conjunction with a one-time pad, but rather with a symmetric encryption system like AES. This means that a legitimate user must choose to trust either physical security, including the exclusion area around Alice and Bob also required for QKD, or computational security of the encryption algorithm.

Claims

1. A free-space quantum keyless private communication method based on a communication protocol comprising the exchange of information between a transmitting device (100) and a receiving device (200) through a principal classical quantum channel, wherein, based on an eavesdropping channel model, an eavesdropping device eavesdrops on the principal classical quantum channel through an eavesdropping channel, The transmitting device (100) includes the step of preparing a message M composed of classical bits, The steps include: encoding the message M so that it is converted into coded message X; The steps include: converting the classical bits of the coded message X into a signal to be transmitted to the receiving device (200) by modulating the amplitude and / or phase of the coherent state of the signal; The steps include transmitting the signal, including the coded message X, to the receiving device (200) through the principal classical quantum channel, The receiving device (200) detects and decodes the coded message X. Includes, The aforementioned free-space quantum keyless private communication method further includes an exclusion radius r surrounding the receiving device (200). Ｅ To define the exclusion radius r Ｅ This includes calculating a degradation parameter γ that allows the overall degradation of the eavesdropping channel to be greater than the degradation of the principal classical quantum channel, and defining the exclusion radius r E such that the calculated degradation parameter γ satisfies a predetermined condition. Free-space quantum keyless private communication method.

2. The free-space quantum keyless private communication method according to claim 1, characterized in that the coding step is a probabilistic coding step.

3. The free-space quantum keyless private communication method according to claim 2, characterized in that the communication protocol is a one-way communication protocol.

4. The free-space quantum keyless private communication method according to claim 1, characterized in that the coherent state modeled in quantum electrodynamics is modulated by the classical bit.

5. The following is a calculation step for the degradation parameter γ: [Math 1] d Ｂ is the distance between the transmitting device (100) and the receiving device (200), d Ｅ is the distance between the transmitting device (100) and the eavesdropping device, D Ｅ Ｒ is the diameter of the antenna of the eavesdropping device, D Ｂ Ｒ is the diameter of the antenna of the receiving device (200), η Ｂ is the (additional) loss of the receiving device (200) such as atmosphere, directivity, optical circuit, and detector efficiency, θ ｄｉｖ is far-field divergence, θ Ｅ = r Ｅ / d Ｅ where θ Ｅ is the angle of the eavesdropping device with respect to the satellite, and a free-space quantum-key-free private communication method according to any one of claims 1 to 4, characterized by a deterioration parameter γ calculation step

6. The free-space quantum keyless private communication method according to claim 5, characterized in that the exclusion area surrounding the receiving device (200) is defined such that the degradation parameter γ is lower than a given value less than 1.

7. The free-space quantum keyless private communication method according to claim 6, characterized in that the exclusion area surrounding the receiving device (200) is defined such that the degradation parameter γ is less than 0.

1.

8. The free-space quantum keyless private communication method according to any one of claims 1 to 7, characterized in that the signal is an optical signal.

9. A free-space quantum keyless private communication system comprising a transmitting device (100) and a receiving device (200) adapted to exchange information through a principal classical quantum channel, wherein, based on an eavesdropping channel model, an eavesdropping device eavesdrops on the principal classical quantum channel through an eavesdropping channel, wherein the overall degradation of the eavesdropping channel is greater than the degradation of the principal classical quantum channel, and the system is adapted to perform the free-space quantum keyless private communication method according to any one of claims 1 to 8.

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