Method and device for sharing secret keys in network including satellites

Abstract

To provide a method and a device for sharing secret keys in a network that includes satellites.SOLUTION: A method for distributing secure secret keys is intended to secure communication between a first communication station (S1) and a second communication station (S2) by using first and second satellites (STA and STB) that fly over a common station while each of second and third secret keys is exchanged by using QKD, and fly over the first and second communication stations, respectively, while first and fourth secret keys are exchanged by using QKD.SELECTED DRAWING: Figure 3A

Description

[Technical field]

[0001] The present invention relates to the field of information technology security, and more particularly to the field of sharing satellite encryption keys over a quantum channel. [Background technology]

[0002] There has always been a need to exchange sensitive information that must remain private over public communication channels. Suppose two individuals (Alice and Bob) want to exchange sensitive information over an authenticated public communication channel (e.g. the Internet) away from prying eyes: to achieve this goal, they must encrypt their messages. Let "Eve" refer to the individual who wants to intercept these messages. Using a public (i.e. non-secret) cryptographic function and a private key, Alice must encrypt her message before sending it to Bob over the public channel. Upon receiving the encrypted message, Bob must decrypt it by using the same cryptographic function and the same private key as Alice to access the information as plain text.

[0003] The question now arises regarding the private key shared between Alice and Bob: how the private key may be generated and exchanged securely, away from prying eyes.

[0004] During secure Internet exchanges, a shared secret key is established by using techniques based on public key cryptography (not described here). A new solution was devised at the end of the 20th century. The new solution proposes to use a quantum channel that allows Alice and Bob to establish a common secret key by exchanging quantum particles (e.g. photons) without Eve being able to trace the key by eavesdropping on these particles. However, this technique of sharing keys over a quantum channel (called QKD (Quantum Key Distribution)) suffers from a number of limitations that make it difficult to apply to a large number of users over very long distances. This may ultimately make the technology unattractive and therefore not profitable enough to generate industrial opportunities.

[0005] Figure 1 represents diagrammatically the architecture of a quantum channel that allows two users: Alice (sender of particles) and Bob (receiver of particles) to establish a common secret key. This key is a sequence of random bits and can therefore assume the value 0 or 1. For example, within the context of the BB84 protocol, Alice proposes a key bit that uses two photon polarization modes prepared by her (the choice of polarization mode is random). For each bit (and therefore photon), Alice writes down the randomly selected polarization mode. By using a polarization filter and a photon detector that can be alternately oriented according to the two randomly selected polarization modes, Bob writes down the selected orientation of the filter as well as the detection result (photons leaving the filter via a path "parallel" or "perpendicular" to its axis).

[0006] In BB84, once the entire sequence has been transmitted, Alice transmits the polarization mode used for each detection to Bob over an authenticated plaintext channel. Bob can then infer the values ​​of the bits for which the polarization orientation was the same. Bob then knows with confidence that for the N bits transmitted by Alice, there are on average N / 2 bits (this sequence is called the reconciled key). This process may involve error correction methods using control values ​​and LDPC or cascaded algorithms (using control parameters exchanged over the authenticated channel).

[0007] Finally, Alice and Bob agree on a subset of the arbitration key bits. Then, Alice and Bob compare whether they obtained the same bits in this subset. Any differences are by default attributable to an eavesdropper (although measurement errors could also cause them). In fact, the non-cloning theorem guarantees that if Eve is eavesdropping, she will force the photons into a polarization mode (not necessarily Alice's). Thus, if Eve correctly guesses Alice's mode with a 50% probability, then 25% of the bits in the arbitration key will not match between Alice and Bob. Thus, at this stage, mathematical proofs of security can be used to assess the amount of information thus potentially compromised based on the number of different bits, and to define a one-way function (hash function) that, when applied to the arbitration key, produces a smaller key that no longer contains any compromised information. This is the confidentiality augmentation process that provides a secure key.

[0008] From the perspective of terrestrial networks, key exchange (or, more specifically: key establishment) over a quantum channel is straightforward over optical communication channels based on optical fibers. However, exchanging single-polarized photons (or very low intensity light pulses) over a fiber does not allow Alice and Bob to exchange these photons over tens of kilometers (representing the combined asymptote of the probability of 100% absorption of photons due to the material forming the optical fiber and the shortening of the key due to increased confidentiality).

[0009] As a result, the architecture of quantum channel networks is significantly affected by this physical constraint. Two solutions have been devised to overcome this distance limitation in optical fibers.

[0010] A first contemplated solution involves propagating the keys through nodes called "trusted nodes" placed at regular intervals (e.g. every 50 kilometers). Figure 2A shows a schematic of a key generation device using trusted nodes known from the prior art. The trusted node handles the keys established by Alice (key A) and Bob (key B) in plain text. This trusted node is positioned approximately equidistant between Alice and Bob, and doubles the distance for establishing the keys over the quantum channel. The trusted node then uses a one-time pad technique to transmit this key A to Bob. To this end, the trusted node implements an "XOR" function (a bitwise exclusive OR logic gate) between Alice's key and Bob's key. "XOR" is here and throughout the rest of this document represented by the symbol

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[0011] This first solution is not only based on the following properties of the XOR function:

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[0012] However, this first solution requires that the trusted nodes (which handle the keys in plaintext) be secured with a very high level of security (implying significant additional costs and operational constraints). Secondly, this type of solution is difficult, if not impossible, to implement for the interconnection of two users separated by an ocean (it is difficult and expensive to implement a trusted node every 100 km in the ocean).

[0013] The second solution involves exchanging photons in free space and from space to cover long distances. In this case, the 100 km limit of optical fiber is no longer applicable, and it is possible to distribute cryptographic keys from space to two users located anywhere on Earth by a QKD channel. This configuration is shown in Figure 2B. In this case, the trusted nodes are the ones who establish key A for Alice (e.g. a ground station) and key B for Bob (e.g. another ground station) while the satellite flies over Bob's station, and then exchange the keys.

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[0014] In this second solution, the "pair management" and "pair station overflight" aspects of the ground station are likely to significantly constrain the planning of quantum optical contact between the station and the satellite in a generally considered manner. In fact, the satellite orbit is very strict, and the optical contact period between the satellite and the ground station, while predictable, is not flexible. In addition, cloud cover is likely to interfere with the quantum channel linking the satellite and the ground station, and thus with the establishment of the basic satellite / ground station link. In this case, the planning exchange must therefore be able to be updated.

[0015] These constraints make planning contacts between ground stations and satellites very complex and inefficient. Indeed, the solution shown in FIG. 2B requires a precise sequence for the various contacts due to the fact that it does not manage point-to-point connections, but in fact manages pairs of connections for sharing keys between pairs of ground stations. If this sequence is disturbed by poorly predicted meteorological disturbances, the availability of the service may be jeopardized since the visibility of the connected ground station may not reappear until several days later. As a result, a strategy for optimizing the contacts between satellites and ground stations is very complex and requires the solution of several problems, the optimal solution of which is difficult to determine.

[0016] The present invention aims to overcome some of the aforementioned problems of the prior art. To this end, one object of the present invention is a method and a device for sharing a secure secret key intended to secure communications between a first and a second communication station. The present invention is implemented in a network including at least two satellites flying over a common ground station so as to be optically visible to said common ground station. In the method and process of the present invention, the secure secret keys are distributed via the common station in a step separated in time from the step in which these keys are generated by using QKD. Moreover, the distribution of the secure keys to the first and second stations via the common station is done on a channel that does not necessarily require optical visibility between the satellite and the common station and can occur much later depending on the user requirements. The solution of the present invention avoids the need to implement an orbit of two satellites that successfully fly over the first and second stations to be connected: that is, this task can be shared between several satellites by station packets (and not by paired station packets), as is usually contemplated in the prior art. Summary of the Invention [Means for solving the problem]

[0017] To this end, the object of the invention is a method for sharing a secure secret key intended to secure communications between a first communication station and a second communication station, said method comprising the following steps: A. exchanging a first secret key over a quantum channel linking a first satellite to a first communications station, and exchanging a second secret key over a quantum channel linking the first satellite to a third communications station, called a common station; B. exchanging a third secret key over a quantum channel linking the second satellite to the common station, and exchanging a fourth secret key over a quantum channel linking the second satellite to the second communications station; C. receiving at the first and second satellites a request for communication between the first and second communications stations; D. encrypting the first private key at the first satellite using a common key encryption method using the second private key to generate a first encrypted message, and transmitting the first encrypted message to the common station over the first authentication channel; E. encrypting the fourth secret key at the second satellite using a common key encryption method using the third secret key to generate a second encrypted message, and transmitting the second encrypted message to the common station over a second authentication channel; F. decrypting the first encrypted message at the common station using the second private key to obtain the first private key, and decrypting the second encrypted message using the third private key to obtain a fourth private key; G. Encrypting the first secret key at the common station using a common key encryption method using the fourth secret key to generate a third encrypted message, and transmitting the third encrypted message to the second communication station over a third authenticated channel.

[0018] According to one embodiment, the method of the invention comprises another step H which involves decrypting the third encrypted message in the second communication station by using the fourth private key so as to obtain the first private key forming a secure private key.

[0019] Preferably, the symmetric encryption method is a one-time pad method, in which the secret key and the encrypted message are binary coded, and each encryption step is performed by an XOR logic gate combining the secret key respectively involved in said encryption step, and each decryption step is performed by an XOR logic gate combining the encrypted message with the encrypted secret key respectively involved in said decryption step.

[0020] According to one embodiment, the method of the invention comprises two separate steps I and J which involve: I. encrypting a non-random message at a first communication station by using a first secret key, and then transmitting the encrypted non-random message to a second communication station over a channel called a plaintext channel; J. Decrypting the encrypted non-random message at a second communication station by using a first private key.

[0021] According to one embodiment, steps AH are repeated multiple times to generate multiple secure private keys.

[0022] According to one embodiment, in step A, exchanging a second private key requires that an arbitration sub-step be performed over the first authenticated channel; and / or, in step B, exchanging a third private key requires that an arbitration sub-step be performed over the second authenticated channel.

[0023] According to one embodiment, steps AH are repeated multiple times by a pair of communication stations different from the first and second communication stations.

[0024] Another object of the invention is a communication system comprising first and second communication stations, a third communication station called a common station, and first and second satellites, said first satellite being adapted to exchange a first secret key over a quantum channel linking the first satellite to the first communication station and to exchange a second secret key over a quantum channel linking the first satellite to the common station; said second satellite being adapted to exchange a third secret key over a quantum channel linking the second satellite to the common station and to exchange a fourth secret key over a quantum channel linking the second satellite to the second communication station; said first satellite being further adapted to exchange a plaintext request. the second satellite is further adapted to receive a communication request between the first and second communication stations transmitted over a first authentication channel, and thereafter encrypt the first secret key by using a common key encryption method using the second secret key to generate a first encrypted message, and transmit the first encrypted message to the common station over the first authentication channel; the second satellite is further adapted to receive the communication request, and thereafter encrypt a fourth secret key by using a common key encryption method using the third secret key to generate a second encrypted message, and transmit the second encrypted message to the common station over the second authentication channel; the common station is further adapted to: - decrypting the first encrypted message by using the second private key to obtain the first private key, and decrypting the second encrypted message at the common station by using the third private key to obtain a fourth private key; - Encrypting the first secret key by using a common key encryption method using a fourth secret key to generate a third encrypted message, and transmitting the third encrypted message to the second communication station over a third authenticated channel.

[0025] According to one embodiment, the second communication station is adapted to decrypt the third encrypted message by using the fourth private key to obtain the first private key forming the secure private key.

[0026] According to one embodiment, the first authenticated channel and / or the second authenticated channel are each a bidirectional telemetry / remote control link with the first satellite and the second satellite, respectively.

[0027] In one embodiment, the first authentication channel is repeated by the additional satellite via an optical or RF inter-satellite link linking the additional satellite and the first satellite, and / or the second authentication channel is repeated by the additional satellite via an optical or RF inter-satellite link linking the additional satellite and the second satellite.

[0028] According to one embodiment, the first authenticated channel and the second authenticated channel are each an RF or optical anchor link of a telecommunications service supported by a first satellite and a second satellite, respectively.

[0029] According to one embodiment, the first authentication channel is different from the quantum channel linking a first satellite to said common station (S3), and the second authentication channel is different from the quantum channel linking a second satellite to said common station.

[0030] In one embodiment, the plaintext request channel is relayed by a local station capable of communicating with the first and second satellites via authenticated local links.

[0031] Another object of the invention is a communication station, called a common station, said communication station comprising: - to receive a second secret key exchanged over a quantum channel linking the first satellite to a common station; - to receive a third secret key exchanged over a quantum channel linking the second satellite to the common station; - to receive, via a first authenticated channel, a first encrypted message transmitted by a first satellite, the first encrypted message being generated by a symmetric encryption method encrypting a first secret key exchanged over a quantum channel linking the first satellite to a first communication station with a second secret key; - to receive a second encrypted message transmitted by a second satellite over a second authenticated channel, the second encrypted message being generated by a symmetric key encryption method encrypting a fourth secret key exchanged over a quantum channel linking the second satellite to a second communications station with a third secret key; - adapted for decrypting the first encrypted message by using the second private key to obtain the first private key, and for decrypting the second encrypted message by using the third private key to obtain the fourth private key.

[0032] Further features, details and advantages of the invention will emerge clearly from reading the description given with reference to the attached drawings, given by way of example, in which: [Brief description of the drawings]

[0033] [Figure 1] FIG. 1 is a schematic diagram of a prior art QKD device. [Figure 2A] FIG. 1 is a schematic diagram of a prior art device for generating keys by using QKD via satellite. [Figure 2B] FIG. 1 is a schematic diagram of a prior art device for generating keys by using trusted nodes. [Figure 3A] FIG. 2 is a schematic diagram of a method for distributing secure private keys according to the present invention; [Figure 3B] 1 is a schematic diagram of a system for distributing private keys according to the present invention; [Figure 3C] 1 is a schematic diagram of a system for distributing private keys according to an embodiment of the present invention; [Figure 3D]1 is a schematic diagram of a system for distributing private keys according to an embodiment of the present invention; [Figure 4] FIG. 2 is a schematic diagram of a method for generating a secure private key according to a preferred embodiment of the method of the present invention. [Figure 5A] FIG. 2 is a schematic diagram of a method for generating a secure private key according to an embodiment of the method of the present invention. [Figure 5B] 1 is a schematic diagram of a system for distributing private keys according to an embodiment of the present invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] In the accompanying drawings, elements are not drawn to scale unless otherwise indicated.

[0035] Fig. 3A shows diagrammatically a method according to the invention for distributing a secure secret key intended to secure communications between a first communication station S1 and a second communication station S2. Fig. 3B shows diagrammatically a communication system 1 according to the invention adapted to implement the method shown in Fig. 3A. In addition to the first and second stations S1 and S2, the communication system 1 includes a third communication station S3, called a common station, and first and second satellites STA, STB.

[0036] In a first step A of the method of the invention, a first satellite STA transmits a quantum channel CQ linking the first satellite STA to a first communication station S1. A1 This exchange in step A occurs when the satellite STA is at position A while flying over station S1, and requires that the satellite STA be optically visible to station S1 in order to transmit and detect quantum bits.

[0037] Furthermore, when the satellite STA is at a position B that is optically visible to the common station S3, the satellite transmits a quantum channel CQ A3 The second private key K2 is exchanged via

[0038] For clarity, in the present invention, when describing the process of exchanging keys over a quantum channel linking a station and a satellite, quantum bits may be transmitted from the satellite to the station or vice versa.

[0039] In step B, the second satellite STB transmits a quantum channel CQ linking the second satellite STB to the common station S3. B3 The satellite STB exchanges a third secret key K3 via the quantum channel CQ3. This exchange takes place when the satellite STB is in position C while flying over the common station S3 and requires that the satellite STB is optically visible to station S3. In addition, when the satellite STB is in position D which is optically visible to station S2, the satellite STB exchanges a third secret key K3 via the quantum channel CQ3. B2 The fourth private key K4 is exchanged via

[0040] The objective of the method of the present invention is to transmit a first key K1 to a second station S2 in a completely secure manner, such that stations S1 and S2 can communicate securely with this key K1 without the need for the satellite STA to fly over station S2, whilst significantly facilitating planning of the respective operations of the satellite STA and STB.

[0041] In the present invention, the step of exchanging secret keys over a quantum channel can be performed by any QKD protocol known to those skilled in the art. As a note, cloud cover above a ground-based communication station blocks any free-space optical link in the visible and near-infrared range. In addition, the sun generates a significant amount of parasitic photons due to diffraction in the atmosphere (indirect radiation), which can significantly degrade the detection of quantum bits and even lead to the destruction of the link. Therefore, the current prior art of satellite QKD allows quantum bits to be transmitted through the atmosphere under clear skies over a near-infrared optical link. Therefore, the availability of the quantum channel between the satellite and the ground-based communication station depends on meteorological events and hazards above the ground-based communication station.

[0042] It is understood that in the method of the present invention, the relative order in which steps A and B are performed is not important. Furthermore, the two steps A and B can be performed simultaneously or one after the other (i.e., step A then step B or step B then step A).

[0043] After steps A and B, the method of the invention includes a step C, which involves the satellites STA and STB receiving a communication request between stations S1 and S2, transmitted by a station called a transmitting station and transmitted over a request channel CR. According to the embodiment shown in Fig. 3B, the communication request is transmitted by station S1 (which is therefore a transmitting station). Alternatively, according to another embodiment, the communication request is transmitted by station S2, or even by station S3, or even by another station (not shown in Fig. 3B) other than stations S1-S3 (e.g. a mission control station).

[0044] This request channel CR is an authentication channel that does not necessarily require optical visibility between the satellite STA, STB and the transmitting station. This request channel CR can be, for example, an RF channel or even an optical channel. Furthermore, step C is temporarily disassociated from steps A and B and can therefore be performed at the moment when the satellite STA and STB are not optically visible to the transmitting station (for example because the satellite STA and STB are too far from this station or even because the atmospheric disturbance between the satellite STA, STB and the transmitting station is too great). A communication request can be transmitted only when a secure communication between stations S1 and S2 is required.

[0045] After receiving this request, in step D, the satellite STA transmits the first encrypted message KC 12 The method is adapted to encrypt the first secret key K1 by using a common key encryption method using the second secret key K2 to generate: Further, step D involves transmitting the first encrypted message over the first authentication channel CA1 to the common station S3.

[0046] Similarly, after receiving the request, at step E, the satellite STB transmits the second encrypted message KC 43 The satellite STB is adapted to encrypt the fourth secret key K4 by using a common key encryption method using the third secret key K3 to generate the second encrypted message K4. The satellite STB then transmits the second encrypted message over the second authentication channel CA2 to the common station S3.

[0047] As for the request channel CR, the authenticated channels CA1, CA2 are not channels that necessarily require optical visibility between the satellites STA, STB and station S3 since steps D and E are temporarily disassociated from steps A and B. Thus, the authenticated channels CA1, CA2 can be any conventional authenticated opportunity channel in the RF or optical domain (see below).

[0048] It is further understood that the relative order in which steps D and E are performed is not critical to the method of the present invention. Moreover, the two steps D and E may be performed simultaneously or one after the other.

[0049] After these steps D and E, the method of the invention comprises two steps F, G, which are carried out in the common station S3. Step F consists in encrypting the first encrypted message KC by using the second secret key K2 to obtain the first secret key K1. 12 and obtain a fourth secret key K4 by using the third secret key K3. 43 and deciphering the

[0050] After step F, the common station S3 receives the third encrypted message KC 14 The third encrypted message KC is generated by performing a step G (which involves encrypting the first secret key K1 by using a common key encryption method using a fourth secret key K4) so ​​as to generate a third encrypted message KC. 14 on a third authentication channel CA3 to the second communication station S2.

[0051] Upon completion of the method illustrated in FIG. 3A, station S2 receives the third encrypted message KC 14 can be decrypted by the fourth secret key K4 previously exchanged with the satellite STB by using QKD. Thus, the first secret key K1 can be obtained, which then forms a secure secret key (see the method shown in FIG. 4). Thus, the first and second communication stations S1, S2 can each possess the secure secret key K1. Since the steps implemented in the method of the present invention (QKD and symmetric encryption) are quantum threat-resistant, the method of the present invention allows a secure secret key to be provided for encrypting the communication between stations S1 and S2.

[0052] As mentioned before, in the present invention, steps A and B of exchanging a secret key by using QKD are used to encrypt messages KC in steps D and E, respectively. 12 , K.C. 43 Therefore, the encrypted message KC 12 , K.C. 43 The distribution of the encrypted message KC does not necessarily require optical visibility between the satellite STA, STB and the common station S3. In practical terms, this means that: in step D, the satellite STA transmits the encrypted message KC 12 In step E, the satellite STB may be located farther away from station S3 than it would be at location B for exchanging key K2 with station S3 if it were distributing the encrypted message KC. 43When distributing the secret keys K1, K2, K2, K3 with station S3, the satellite may be significantly further away from station S3 than when it is at position C for exchanging key K3 with station S3. This time separation between exchanging secret keys K1, K2 by using QKD and distributing the encrypted message is not found in the prior art methods. Indeed, in these prior art methods, the distribution of the secure secret key is performed by the satellite during a nearly simultaneous flight over the station after establishing the encryption key by using QKD to encrypt the secure secret key transmission between the satellite and the station (see FIG. 2B).

[0053] The inventive solution allows the operational planning to be made significantly more flexible than the prior art method shown in Fig. 2B. In addition, the inventive solution allows the distribution of encrypted messages to be delayed, for example according to the requirements of the users of the stations S1 and S2, and thus avoids prolonged storage of the (potentially compromised) secret keys K1, K4 and of the encrypted messages in the common station S3. Furthermore, the inventive solution avoids the need to implement orbits of two satellites STA, STB that fly successfully over the first and second stations S1, S2 connected to it: this task can be shared between the satellites STA and STB by packets of stations (S1, S3) and (S3, S2) rather than by packets of a pair of stations (S1, S2) as is usually envisaged in the prior art.

[0054] It is understood that making steps A and B temporarily unrelated to the rest of the steps of the invention implies that the satellite STA and STB may securely store keys K1, K2 and K3, K4, respectively, after they have been exchanged using QKD.

[0055] According to the present invention, the common key encryption method of steps D, E and G can be any block or stream method known to those skilled in the art, such as the One-Time Pad method, the AES (Advanced Encryption Standard) method, or even the Triple DES method.

[0056] In the present invention, step G may alternatively include encrypting a third encrypted message KC by using a common key encryption method using the first secret key K1. 41 and transmitting a third encrypted message KC via a third authentication channel CA3. 14 to communication station S1. These two approaches are equivalent, the only difference being that in the first alternative key K1 is a secure private key for encrypting communications between stations S1 and S2, while in the second alternative key K4 is a secure private key.

[0057] As mentioned above, channels CA1 and CA2 can be any conventional authentication opportunity channels. According to a preferred embodiment of the present invention, denoted MR, channels CA1 and CA2 are quantum channels CQ A3 and CQ B3 Each is different.

[0058] According to one embodiment of the method of the invention, denoted MP, in step A, the arbitration substep required to exchange the private key K2 is performed over channel CA1. Similarly, in step B, the arbitration substep required to exchange the private key K3 is performed over channel CA2. Indeed, the encrypted message KC 12 and K.C. 43 These arbitration sub-steps, such as the step for distributing the STA and STB, do not require the satellite STA and STB to be optically visible to the common station S3. It is therefore possible to simplify the network architecture of the system 1 by pooling some of the links required for its operation. This embodiment MP is compatible with the embodiment MR.

[0059] According to another embodiment compatible with the embodiment MR and the embodiment MP, the first authentication channel and / or the second authentication channel are, respectively, bidirectional telemetry / remote control links of the first and second satellites, respectively. The term "bidirectional telemetry / remote control link of a satellite" is used in this specification to refer to a link that allows, on the one hand, the mission of the satellite to be monitored, configured and programmed, and, on the other hand, the configuration and status of the satellite to be measured. It is therefore possible to simplify the network architecture of the system 1 by pooling some of the links required for its operation.

[0060] According to another embodiment of the system of the present invention, the first authentication channel and the second authentication channel are RF or optical anchor links of telecommunication services supported by the first satellite and the second satellite, respectively. This embodiment also simplifies the network architecture of the system 1.

[0061] The system according to the invention is a system in which the first and second authenticated channels are RF or optical anchor links of a telecommunications service supported by a first satellite and a second satellite, respectively.

[0062] Fig. 3C shows a schematic representation of an embodiment of the system 1 of Fig. 3B, in which the authentication channel CA2 is relayed by an additional satellite SR via an inter-satellite link LI linking the satellite STB and the additional satellite SR. This link LI is for example an optical or RF link. This embodiment improves the availability of the system 1, for example in cases where the satellite STB is too far from the station S3 to be able to communicate directly with the additional satellite SR without the intermediary of the additional satellite SR. More generally, according to an embodiment of the system 1 of the invention, the first and / or second authentication channels CA1, CA2 are respectively relayed by the additional satellite SR via an inter-satellite link LI linking the additional satellite SR and the satellites STA, STB.

[0063] Advantageously, according to an embodiment of the system 1 of Fig. 3C, the additional satellite SR is adapted to operate at an altitude, for example in the stratosphere or above clouds and atmospheric turbulence in orbit. In addition, a part of the channel CA2 between the station S3 and the additional satellite SR uses wavelengths in the RF, visible or LWIR regions so as not to be affected by any disturbances linked to meteorological conditions (for example clouds). Finally, a part of the channel CA2 between the satellite STB and the additional satellite SR uses a length that maximizes the transmission capacity, for example a wavelength in the SWIR (Short Wavelength InfraRed) region (typically 1 μm to 2.7 μm). This embodiment optimizes the operation of the system 1 by making it more tolerant to meteorological hazards.

[0064] Fig. 3D shows a schematic representation of an embodiment of the system 1 of the invention, in which a communication request transmitted by a transmitting station (e.g. station S1 shown in Fig. 3D) is relayed to the satellites STA and STB by local stations SL that can communicate with these satellites via authenticated local links LL. This embodiment is advantageous to exploit the possibility of the satellites STA and STB flying above the stations SL when they are too far from the transmitting station that can transmit a communication request directly to the satellites STA and STB flying above the stations SL.

[0065] Fig. 4 shows a preferred embodiment of the method of the present invention, which is particularly adapted to be implemented by the system 1 of Fig. 3B. The method shown in Fig. 4 encrypts a third encrypted message KC by using a fourth secret key K4. 14 in the second communication station S2. Station S2 thus obtains the first private key K1.

[0066] This step H is necessary so that both stations S1 and S2 have a secure secret key K1 that can be used later to encrypt the non-random messages they want to exchange (see Figures 5A and 5B). Indeed, if the encryption of the secure secret key K1 in step G is done by a common key encryption method, then the first encrypted message KC 14 Decryption of is performed with the same key as that used for encryption (i.e., the fourth secret key K4). The method shown in Fig. 4 thus facilitates distributing the secure secret key K1 in a secure and tamper-proof manner (even against quantum attacks) for encrypting communications between stations S1 and S2 by relaxing key distribution constraints compared to other methods of the prior art.

[0067] According to one embodiment of the method shown in FIG. 4, steps AH are repeated multiple times to form multiple secure private keys, each intended to secure communications between station S1 and station S2.

[0068] In a preferred embodiment of the present invention, the common key encryption method is a one-time pad method. In this embodiment, the secret keys K1, K2, K3, K4 and the encrypted message KC 12 , K.C. 43 , K.C. 41 are binary coded. Each encryption step is therefore performed by an XOR logic gate by combining respectively the secret keys involved in said encryption steps. For example, in step D, the encrypted message is obtained by combining the first and second secret keys K1, K2 by using the following logical operation:

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[0069] To ensure the security of the One-Time Pad technology in the present invention, every step of implementing the One-Time Pad technology combines two secret keys or one secret key and one key and one encrypted message (of the same length (key size)).

[0070] Figure 5A shows a preferred embodiment of the method shown in Figure 4 which allows a non-random message M to be securely exchanged between stations S1 and S2. Figure 5B shows a schematic of an embodiment of a system of the invention adapted to implement the method shown in Figure 5A. In addition to the steps previously described, the method shown in Figure 5A comprises the following steps: I- encrypting a non-random message M in a first communication station S1 by using a first secret key K1 and transmitting the encrypted non-random message MC to a second communication station S2; J- Decrypting the encrypted non-random message by using the first secret key K1 in step F. Thus, station S2 obtains the non-random message M in a completely secure manner. The method includes two separate steps I and J, which involve:

[0071] According to an alternative embodiment to that shown in FIG. 5B, message M is exchanged from station S2 to station S1.

[0072] According to a preferred embodiment of the method shown in Figure 5A, steps I and J are repeated multiple (n) times to encrypt multiple communications between stations S1 and S2. The number n of iterations of steps I and J is smaller than a number (called the critical number) calculated from the cryptoperiod of the key used to encrypt message M and to decrypt message MC. This feature prevents the exchanged messages from being corrupted and is important when encoding non-random messages M (e.g. human languages). "Cryptoperiod" is used herein to refer to the number of uses of a key in a cryptographic algorithm that must not be exceeded to guarantee the security of messages encrypted with this key.

[0073] Finally, another object of the invention is a common station S3 of the above-mentioned system 1. The station S3 is therefore: -Quantum Channel CQ A3 receiving a second private key K2 exchanged via; -Quantum Channel CQ B3 receiving a third private key K3 exchanged via - a first encrypted message KC sent by a first satellite STA 12 is received via channel CA1; - a second encrypted message KC sent by a second satellite STB 43 is received via channel CA2; - Encrypting a first encrypted message KC by using a second secret key K2 to obtain a first secret key K1. 12 and decrypt the second encrypted message KC by using the third private key K3 to obtain the fourth private key K4. 43 Decipher; -Third encrypted message KC 14 and sending the third encrypted message to the second communication station S2 over the third authenticated channel CA3. It has been adapted for

[0074] Thus, common station S3 of system 1 has been adapted to implement methods in which some steps are "localized" within the same geographic area (i.e., station S3) instead of being distributed across various stations and satellites spaced apart by hundreds or thousands of kilometers. [Explanation of symbols]

[0075] A,B,C,D,E,F,G,H,I,J process CA1 1st Authentication Channel CA2 Second Authentication Channel CA3 3rd authentication channel CR Plaintext Request Channel CQ A1 Quantum Channel CQ A3 Quantum Channel CQ B2 Quantum Channel CQ B3 Quantum Channel K1 First private key K2 Second secret key K3 Third secret key K4 Fourth secret key KC 12 First encrypted message KC 14 The third encrypted message KC 41 The third encrypted message KC 43 Second Encrypted Message LI Inter-Satellite Link LL Authentication Local Link M Message MC Non-random messages S1 First communication station S2 Second communication station S3 Common Station SL local station STA 1st Satellite STB 2nd Satellite

Claims

1. A method for sharing a secure secret key intended to secure communications between a first communication station (S1) and a second communication station (S2), comprising the following steps: A. A quantum channel (CQ) linking a first satellite (STA) to said first communication station (S1) Ａ１ ) via the first private key (K １ ), and a quantum channel (CQ) that links the first satellite (STA) to a third communication station called a common station (S3). Ａ３ ) via the second private key (K ２ ) exchanging; B. A quantum channel (CQ Ｂ３ ) via the third private key (K ３ ), and a quantum channel (CQ Ｂ２ ) via the fourth secret key (K ４ ) exchanging; C. receiving at the first and second satellites (STA, STB) a request for communication between the first and second communication stations; D. First encrypted message (KC １２ ) to generate the second private key (K ２ ) by using a symmetric encryption method using the first secret key (K １ ) at the first satellite, and then transmitting the first encrypted message through a first authentication channel (CA １ ) to said common station (S3); E. The second encrypted message (KC ４３ ) to generate the third private key (K ３ ) to generate the fourth secret key (K ４ ) at the second satellite, and transmits the second encrypted message through a second authentication channel (CA ２ ) to said common station (S3); F. The first private key (K １ ) to obtain the second private key (K ２ ) to generate the first encrypted message (KC １２ ) at the common station (S3) and the fourth private key (K ４ ) to obtain the third private key (K ３ ) to generate the second encrypted message (KC ４３ ) decoding the sequence; G. The third encrypted message (KC １４ ) to generate the fourth secret key (K ４ ) by using a symmetric encryption method using the first secret key (K １ ) at the common station (S3), and transmits the third encrypted message to a third authentication channel (CA ３ ) to said second communication station S2. The method includes:

2. The first private key (K １ ) to obtain the fourth secret key (K ４ ) to generate the third encrypted message (KC １４ 2. The method of claim 1, further comprising a step H of decrypting at said second communication station a first secret key, said first secret key being a part of said secure private key.

3. 3. The method according to claim 1, wherein the common key encryption method is a one-time pad method, The private key (K １ , K ２ , K ３ , K ４ ) and the encrypted message (KC １２ , K.C. ４３ , K.C. ４１ ) is binary coded, each of said encryption steps is performed by an XOR logic gate combining the private keys respectively involved in said encryption steps; wherein each of said decryption steps is performed by said XOR logic gate combining said encrypted message with said encrypted private key respectively involved in said decryption step.

4. The following: I. encrypting a non-random message (M) at the first communication station by using the first secret key, and then transmitting the encrypted non-random message (MC) to the second communication station over a channel called a plaintext channel; J. Decrypting the encrypted non-random message at the second communication station using the first private key.

4. The method according to claim 2, comprising two separate steps I and J involving:

5. The method of any one of claims 2 to 4, wherein steps AH are repeated multiple times to generate multiple secure private keys.

6. In step A, exchanging the second private key requires that an arbitration substep be performed over the first authenticated channel; and / or 6. The method according to claim 1, wherein in step B, exchanging the third secret key requires that an arbitration substep be performed over the second authenticated channel.

7. The method according to any one of claims 2 to 6, wherein steps AH are repeated a number of times with pairs of communication stations different from the first and second communication stations.

8. A communication system including first and second communication stations (S1, S2), a third communication station called a common station (S3), and first and second satellites (STA, STB), The first satellite (STA) has a quantum channel (CQ) that links the first satellite (STA) to the first communication station (STA). Ａ１ ) via the first private key (K １ ), and a quantum channel (CQ) linking said first satellite (STA) to said common station (S3). Ａ３ ) via the second private key (K ２ ) adapted to replace; The second satellite (STB) has a quantum channel (CQ) which links the second satellite (STB) to the common station (S3). Ｂ３ ) via the third private key (K ３ ), and a quantum channel (CQ Ｂ２ ) via the fourth secret key (K ４ ) is adapted to replace; The first satellite is further adapted to receive a request for communication between the first and second communication stations transmitted over a plaintext request channel (CR) and thereafter to receive a first encrypted message (KC １２ ) to generate the second private key (K ２ ) by using a common key encryption method using the first secret key (K １ ), and to transmit the first encrypted message to a first authentication channel (CA １ ) adapted for transmitting to said common station (S3); The second satellite is further adapted to receive the communication request and thereafter to transmit a second encrypted message (KC ４３ ) to generate the third private key (K ３ ) by using a common key encryption method using the fourth secret key (K ４ ), and transmitting the second encrypted message to a second authentication channel (CA ２ ) adapted for transmitting to said common station (S3); The common station comprises: The first private key (K １ ) to obtain the second private key (K ２ ) to generate the first encrypted message (KC １２ ), and the fourth private key (K ４ ) to obtain the third private key (K ３ ) to generate the second encrypted message (KC ４３ ) in said common station (S3); The third encrypted message (KC １４ ) to generate the fourth secret key (K ４ ) by using a common key encryption method using the first secret key (K １ ), and transmitting the third encrypted message to a third authentication channel (CA ３ ) to said second communication station S2.

9. The second communication station transmits the first private key (K １ ) to obtain the fourth secret key (K ４ ) to obtain the third encrypted message (KC １４ 9. The system of claim 8, wherein the first private key forms the secure private key.

10. 10. The system of claim 8, wherein the first authentication channel and / or the second authentication channel are / is a bidirectional telemetry / remote control link of the first satellite and the second satellite, respectively.

11. 11. The system according to claim 8, wherein the first authentication channel is relayed by an additional satellite (SR) via an optical or RF inter-satellite link (L1) linking the additional satellite and the first satellite, and / or the second authentication channel is relayed by the additional satellite (SR) via an optical or RF inter-satellite link (L1) linking the additional satellite and the second satellite.

12. 12. The system according to claim 8, wherein the first authentication channel and the second authentication channel are RF or optical anchor links of telecommunications services supported by the first satellite and the second satellite, respectively.

13. 12. The system of claim 8, wherein the first authentication channel is different from the quantum channel linking the first satellite to the common station (S3), and the second authentication channel is different from the quantum channel linking the second satellite to the common station (S3).

14. 13. A system according to any one of claims 8 to 12, wherein the plaintext request channel is relayed by a local station (SL) capable of communicating with the first and second satellites via an authenticated local link (LL).

15. A communication station called a common station (S3), said communication station comprising: - a quantum channel (CQ Ａ３ ) exchanged through the second secret key (K ２ ) to receive; - a quantum channel (CQ Ｂ３ ) exchanged via ３ ) to receive; - a first encrypted message (KC １２ ) linking said first satellite (STA) to a first communication station (S1) via a quantum channel (CQ Ａ１ A first encrypted message (KC) generated by a common key encryption method in which the first secret key exchanged through the second secret key is encrypted by the second secret key. １２ ) via a first authentication channel; - a second encrypted message (KC ４３ ), which includes a quantum channel (CQ Ｂ２ A second encrypted message (KC) generated by a common key encryption method in which the fourth secret key exchanged through the third secret key is encrypted by the third secret key. ４３ ) via a second authenticated channel; - the first private key (K １ ) to obtain the second private key (K ２ ) to generate the first encrypted message (KC １２ ), and the fourth private key (K ４ ) to obtain the third private key (K ３ ) to generate the second encrypted message (KC ４３ ) to decipher; - the third encrypted message (KC １４ ) to generate the fourth secret key (K ４ ) by using a common key encryption method using the first secret key (K １ ) and for transmitting said third encrypted message to said second communication station (S2) over a third authentication channel.

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