Centralized satellite-based quantum cryptographic key pairing process

Abstract

The invention relates to a centralized method for satellite-based quantum cryptographic key pairing for ground stations (A, B), comprising: a quantum key distribution step (1), in which, for each station (A, B), a cryptographic key (KA, KB) is shared by quantum communication between the station (A, B) and a satellite (SAT A, SAT B); a centralized key storage step (3), in which each key (KA, KB) is first transmitted in an encrypted manner from the satellite (SAT A, SAT B) to a terrestrial central key management unit (UC), and then stored by this unit (UC); and a pairing step (6) between a first (A) and a second (B) of said stations, in which, upon receipt of a communication request (5), the unit (UC) transmits in an encrypted manner the key (KB) linked to the second station (B) to the first station (A). Figure 2

Description

Title of the invention: Centralized method for pairing quantum cryptographic keys by satellite

[0001] The invention relates to a centralized method for satellite-based quantum cryptographic key pairing for a plurality of ground stations. The invention further relates to a centralized system for satellite-based quantum cryptographic key pairing for a plurality of ground stations, using a method according to the invention.

[0002] There is an interest in enabling the exchange of sensitive information via an open communication channel, such as for example a wireless communication channel, while preserving its secret nature. By open communication channel or open link is meant a communication channel or a link whose content can be intercepted by a third party without the possibility for the transmitter and the receiver to detect it. Such a problem is typically represented by a situation in which two people, Alice and Bob, wish to exchange sensitive information away from prying eyes, through an authenticated open communication channel (for example the internet). For this, there are cryptographic techniques by which Alice and Bob encrypt their messages.We know of symmetric cryptographic techniques using only a shared private key (also called a "shared secret") and asymmetric cryptographic techniques based on a combination of public and private keys. The principle of a symmetric cryptographic technique is briefly detailed below. Let us call Eve, the person who wants to intercept these messages. Alice, using a cryptographic function and a private key shared with Bob, encrypts her messages before sending them to Bob through an open channel. When Bob receives the encrypted messages, he decrypts them using the same cryptographic function and the shared private key, to be able to access the information in clear text. Eve, not having the key, will not be able to decrypt the message.

[0003] In the implementation of symmetric cryptographic techniques, a crucial question concerns the preservation of secrecy, in particular the ways of sharing the secret key securely between Alice and Bob, away from prying eyes.

[0004] A new solution was devised at the end of the 20th century, proposing to use quantum communications to securely share a common key between Alice and Bob. Quantum communication consists of establishing a quantum link between a transmitter and a receiver for an exchange of information. The quantum link notably includes an optical link transmitting a series of photons. A qubit (or qbit) is the quantum information carried by a photon. This information is, for example, encoded in the polarization state of the photon.

[0005] Quantum Key Distribution or QKD (for the English "Quantum Key Distribution") refers to a cryptographic protocol aiming to establish a secret key shared between two participants, Alice and Bob, using quantum communication. Thanks to quantum properties, in particular those due to the quantum no-cloning theorem, any attempt to attack the communication, such as an attempt at eavesdropping by a third party Eve, can be detected with certainty by Alice and Bob, which allows them to quickly invalidate the key. As a result, the QKD method allows a secure establishment of a symmetric cryptographic key.

[0006] The Prepare & Measure (P&M) protocol is an example of a known QKD method. In this P&M protocol, the transmitter Alice encodes a conventional bit (0 or 1) into an individual optical signal, notably a photon, before transmitting it as a qubit to the receiver Bob. Bob performs a set of specific measurements on each of the individual optical signals, such as photons, emitted by Alice in order to recover the encoded bits. An example of a P&M protocol is the BB84 protocol in which the polarization of an individual photon makes it possible to encode a bit. The BB84 protocol is notably described in the publication "Quantum cryptography: Public key distribution and coin tossing", Charles H. Bennett and Gilles Brassard, Theoretical Computer Science, vol. 560, 1984, p. 7-11 (D01:10.1016 / j.tcs.2014.05.025).

[0007] When Alice and Bob are ground users, the quantum link can be implemented via an optical communication channel carried by optical fiber. However, in optical fiber, optical signals are progressively attenuated, which only allows a quantum link below a limiting distance, typically around 100 km. For quantum key distribution over a longer distance, it is known to propagate the key via one or more so-called "trusted nodes" placed at regular intervals (typically every 100 kilometers). Such a trusted node, positioned between Alice and Bob, makes it possible to extend the range of key establishment by quantum communication. For this purpose, the trusted node manipulates in clear a KA key shared by quantum communication with Alice and a KB key shared by quantum communication with Bob.Therefore, the trusted node must be secured with a very high level of security. Then, the trusted node relies in particular on the disposable mask encryption technique (or OTP for "One Time Pad cryptography" in English) to transmit to Bob the key KA previously shared with Alice. To do this, the trusted node applies an XOR function, that is to say an "exclusive OR" function. between the KA key shared with Alice and the KB key shared with Bob, to obtain a parity sequence KA®KB. The XOR function between two elements is represented by the symbol #. The parity sequence ka@KB is transmitted to Bob through an authenticated open channel. Bob, who holds the KB key, will again be able to perform an XOR function between the sequence received from the trusted node and his KB key to recover Alice's Ka key. This is based on the property of the XOR function according to which KASKBSKB = KA® {KBSKB) = -RAbut also on the disposable mask encryption technique which guarantees that it is not possible to trace one of the keys KA, KB by knowledge of the KA® sequence alone <B- Cependant, une connexion via fibre optique entre deux utilisateurs séparés par un océan peut être désavantageuse en termes de garantie de sécurité car nécessitant un grand nombre de nœuds de confiance.

[0008] It is known to carry out the exchange of optical signals in free field, that is to say in particular from space, via a satellite and a ground station equipped with a telescope. The quantum link is then implemented by a laser source, in particular in the infrared or near infrared range. By crossing the vacuum of space and the Earth's atmosphere, the laser signal implementing the quantum communication undergoes less attenuation, at least over long distances, compared to propagation in an optical fiber. The publication "Progress in satellite Quantum Key Distribution", Robert Bedington, Juan Miguel Arrazola, Alexander Ling, npj Quantum Information 3, Article number: 30 (2017) (DOI:10.1038 / s41534-017-0031-5) describes QKD processes by satellite.

[0009] Figure 1 illustrates a classic example of quantum key distribution by satellite. In a first step, the satellite allows the establishment of a shared key KA with the ground station Alice by quantum communication. After moving on its orbit, in a second step, the satellite allows the establishment of a second shared key KB with the ground station Bob by quantum communication. Then, in a third step, the satellite transmits to Bob the parity sequence KA®KB resulting from an XOR function applied to the keys KA, KB. The transmission of the parity sequence KA®KB is carried out on an open communication channel, in particular non-quantum, for example by optical link or RF radiofrequency. In a fourth step, Bob can then extract Alice's key KA by performing an XOR function between his own key Kb and the parity sequence KA®KB that the satellite transmitted to him. Thus, the key KA is finally shared between Alice and Bob.They can then use this KA key to encrypt communications between them over an open communication channel. By moving relative to Alice and relative to Bob, the satellite acts as a mobile space-based trusted node. Key sharing between Alice and Bob can therefore be achieved. regardless of the distance between them, provided that the same satellite flies over both stations.

[0010] Since the quantum signals used are by nature very weak in intensity, the use of a low-orbit satellite is preferred to that of a geostationary satellite, which has a higher orbit. As a result, temporal visibility is more reduced for a given station. In addition, the laser flux, which supports quantum communication, remains very sensitive to the weather and environmental conditions between the ground station and the satellite. For example, clouds or atmospheric turbulence can disrupt, or even block, the optical signal. For each ground station, the satellite must therefore wait until the conditions are favorable for key establishment by quantum communication.Thus, the time taken to implement a pairing request between Alice and Bob can be significant, because the satellite must move from one station to another and the weather conditions must be favorable for each of them at the time of the satellite's passage.

[0011] We know the use of a constellation of satellites by which the key is propagated from Alice to Bob. However, such a solution is relatively complex and expensive. In addition, depending on the geometry of the constellation, it is not always possible to establish an inter-satellite link (or ISL for "Inter Satellite Link" in English) between any two satellites in the constellation. A key could be stored for a long time on board a satellite, waiting for it to have a visibility slot with a destination satellite, before it can be erased. However, the storage capacities of a satellite are limited.

[0012] A solution is therefore sought to reduce the delay between a pairing request between two ground stations and its execution, in a method of quantum distribution of cryptographic key by satellite.

[0013] To this end, the invention relates to a centralized method for pairing quantum cryptographic keys by satellite for a plurality of ground stations, the method comprising in this order: • a quantum key distribution step, in which, for each ground station, at least one respective cryptographic key, called “ground station-related key”, is shared by quantum communication between the ground station and a satellite, called “key establishment satellite”; • a centralized key storage step, in which each key linked to a ground station is first transmitted in encrypted form from the key establishment satellite to a central terrestrial key management unit, and then stored by this central terrestrial key management unit; and • a pairing step between a first and a second of said ground stations, in which, upon receipt of a communication request between the first ground station and the second ground station, the central terrestrial key management unit transmits encrypted the key linked to the second ground station to the first ground station.

[0014] Thus, a secure stock of cryptographic keys is first constituted for the ground stations, then these keys are distributed upon request for the establishment of encrypted communication between ground stations. Thanks to these steps of distribution and centralized storage of keys, the keys are available during a subsequent request for connection between ground stations. Once the stock of cryptographic keys has been constituted, the process of pairing the cryptographic keys shared between two ground stations is therefore no longer dependent on the constraints of the satellite system (visibility, mission, availability of the atmospheric channel, etc.). The pairing time between two ground stations is then improved compared to the prior art and the distribution of the key pairs is adaptable according to the needs of the moment.

[0015] According to one embodiment, in the pairing step between the first ground station and the second ground station, the terrestrial central key management unit further transmits in encrypted form the key linked to the first ground station to the second ground station.

[0016] According to one embodiment, in the pairing step, the encrypted transmission or transmissions comprise the transmission of a parity sequence, the result of an exclusive OR XOR function between the key linked to the first ground station and the key linked to the second ground station.

[0017] According to one embodiment, in the centralized key storage step, each key linked to a ground station is transmitted in encrypted manner using a key shared between the key establishment satellite and the terrestrial central key management unit.

[0018] According to a variant, in the centralized key storage step, the encrypted transmission of the key linked to the ground station between the key establishment satellite and the terrestrial central key management unit comprises the transmission of a parity sequence, the result of an exclusive OR XOR function between the key linked to the ground station and the key shared between the key establishment satellite and the terrestrial central key management unit.

[0019] According to a variant, the method comprises, before the centralized key storage step, a first preliminary step of quantum distribution of satellite keys, in which the key shared between the key establishment satellite and the terrestrial central key management unit is shared by quantum communication between the key establishment satellite and the terrestrial central key management unit.

[0020] According to one embodiment, the method further comprises, the key establishment satellite belonging to a constellation of satellites, a selection of a satellite among the satellite constellation as a key establishment satellite from communication conditions with the ground station.

[0021] According to one embodiment, the terrestrial central key management unit is at least partly included in a mission control center of the key establishment satellite or the constellation of satellites.

[0022] According to one embodiment, for at least one ground station, several keys linked to the ground station shared by quantum communication are stored in the terrestrial central key management unit.

[0023] The invention further relates to a centralized satellite-based quantum cryptographic key pairing system for a plurality of ground stations, the system comprising at least one satellite, called a "key establishment satellite", and at least one terrestrial central key management unit, configured to implement a method according to the invention.

[0024] Other characteristics and advantages of the present invention will appear more clearly on reading the description which follows in relation to the following appended figures:

[0025] [Fig. 1] [Fig. 1], already described, representing a method of quantum distribution of cryptographic key by satellite according to the prior art;

[0026] [Fig.2] [Fig.2], representing an example of a centralized key pairing method satellite quantum cryptography according to the invention.

[0027] The steps of an example method according to the invention will be described with reference to [Fig.2]. The method allows pairing by quantum key between two ground stations A, B. The method can be applied to more than two ground stations. The ground stations A, B include in particular means of communication to one or more satellites.

[0028] In a quantum key distribution step 1, for each ground station A, B, at least one cryptographic key KA, KB, called a “key linked to the ground station”, is shared by quantum communication between the ground station A, B and a satellite SAT A, SAT B, called a “key establishment satellite”. Thus, a quantum communication is established between a first ground station A and a first key establishment satellite SAT A. A key KA linked to the first ground station A is shared between the first ground station A and the first key establishment satellite SAT A at the end of this quantum communication. Still in this quantum key distribution step 1, in a similar manner, a quantum communication is established between a second ground station B and a second key establishment satellite SAT B.A KB key linked to the second ground station B is shared between the second ground station B and the second key establishment satellite SAT B by this quantum communication.

[0029] At the end 2 of the quantum key distribution step, the key KA linked to the first ground station A is shared between the first ground station A and the first key establishment satellite SAT A; the key KB linked to the second ground station B is shared between the second ground station B and the second key establishment satellite SAT B. In other words, each key KA, KB linked to a ground station A, B is stored in the respective ground station A, B and in the respective key establishment satellite SAT A, SAT B.

[0030] Then, in a centralized key storage step 3, each key KA, KB, linked to a ground station A, B is transmitted in encrypted form from the key establishment satellite SAT A, SAT B to a central key management unit UC. The central key management unit UC stores, in particular in a secure manner, the keys KA, KB linked to the ground stations A, B. In other words, the first key establishment satellite SAT A sends in encrypted form the key KA linked to the first ground station A to the central key management unit UC; and the second key establishment satellite SAT B sends in encrypted form the key KB linked to the second ground station B to the central key management unit UC. Thus, the central key management unit UC centralizes all the keys KA, KB linked to the ground stations A, B, previously shared between the ground stations A, B and the key establishment satellites SAT A, SAT B during the key distribution step 1.The central UC key management unit is terrestrial, i.e. located on the ground, which makes it less complex to implement and more easily accessible compared to a satellite.

[0031] At the end 4 of the centralized key storage step 3, the keys KA, KB are therefore all recorded and available in the central key management unit UC, for pairing between two ground stations A, B. The key establishment satellite SAT A, SAT B can delete the key KA, KB linked to the ground station A, B in particular to free up on-board memory space.

[0032] A service request 5 to the central unit UC initiates a pairing step 6 between the first ground station A and the second ground station B. Such a service request 5 requests the pairing of the first ground station A with the second ground station B, namely the provision of one of the keys KA, KB linked to the ground stations which must communicate with each other.

[0033] In the pairing step 6, the central key management unit UC then transmits in encrypted form the key KB linked to the second ground station B to the first ground station A.

[0034] At the end 7 of the pairing step 6, the KB key linked to the second ground station B is available in the first ground station A.

[0035] Then, in a communication step 8, the first ground station A can then send an encrypted message to the second ground station B using the key KB linked to the second ground station B. However, the first ground station A and the second ground station B may not be the users of the keys KA, KB. They then provide their key KA, KB to their respective user, in particular by a terrestrial link. The users can connect to the ground stations A, B by a communication network, local or remote. Preferably, the link between the ground station A, B and the user is encrypted by a key shared by quantum communication. However, the key could be shared by other means. For example, the key could be defined during the design of the ground station A, B and the user. For example, the users can be satellites.

[0036] Thanks to the steps of quantum distribution of keys and centralized storage of these in the central key management unit UC, during a subsequent pairing request, the keys are immediately available, in particular within the limit of the performance of terrestrial links through which the pairing request and the key Kb linked to the second ground station B pass. Thus, an urgent pairing request can be satisfied, which was not the case in the prior art. Its satisfaction is no longer likely to be delayed due to unfavorable weather and / or overflight conditions at the ground stations. Indeed, the method transfers these uncertainties to the prior key distribution step. Advantageously, the requested pairing between ground stations can be carried out at the latest, in particular when the need for communication between the ground stations A, B is felt.There is no need to plan a pairing long in advance with the risk that it will be useless. The advantages of the method make it possible to reduce the differences between the operation of terrestrial QKD infrastructures and that of satellite QKD infrastructures, thus facilitating the integration between terrestrial and satellite systems.

[0037] In particular, the key distribution steps 1 and centralized key storage steps 3 are carried out for all ground stations A, B, before any pairing step 6 between ground stations A, B.

[0038] In particular, in the centralized key storage step 3, for the transmission of the key KA, KB linked to the ground station A, B to the central unit UC, the key establishment satellite S AT A, S AT B uses a respective key KMA, KMB shared between the key establishment satellite SAT A, SAT B and the central key management unit UC.

[0039] In particular, the key establishment satellite SAT A, SAT B performs an exclusive OR function between the key KA, KB linked to the ground station A, B and the key KMA, KMB shared between the key establishment satellite SAT A, SAT B and the central management unit of UC keys. The obtained parity sequence KA®KMA, KBSKMB is then transmitted to the central key management unit UC. For this purpose, the central key management unit UC can implement the disposable mask encryption technique (or OTP for "One Time Pad cryptography" in English). Thus, the link between the key establishment satellite S AT A, S AT B and the central key management unit UC can be implemented by an open link, in particular non-quantum, for example by optical link or RF radio frequency.

[0040] In particular, at the end 4 of the centralized key storage step 3, the central key management unit UC extracts the key KA, KB linked to the ground station A, B from the parity sequence KA@KMA, KBSKMB transmitted by the key establishment satellite SAT A, SAT B. In particular, if the security level is thereby increased, this decryption of the key KA, KB linked to the ground station A, B can be carried out upon receipt of the communication request 5 between the first ground station A and the second ground station B.

[0041] In particular, the key KMA, KMB shared between the key establishment satellite SAT A, SAT B and the central key management unit UC is shared in a quantum manner in a first preliminary step of quantum distribution of satellite keys. The first preliminary step of quantum distribution of satellite keys is implemented before the centralized key storage step 3, to allow a secure encrypted transmission of the keys KA, KB linked to the ground stations A, B to the central key management unit UC. Thus, a first key KMA is shared by quantum communication between the first key establishment satellite SAT A and the central key management unit UC; and a second key KMB is shared by quantum communication between the second key establishment satellite SAT B and the central key management unit UC.Preferably, the first preliminary step of quantum key distribution is implemented before the step of quantum distribution 1 of the keys linked to the ground stations A, B, to allow a transmission of the keys KA, KB linked to the ground stations A, B immediately after their sharing between the ground station A, B and the key establishment satellite SAT A, SAT B.

[0042] However, the key KMA, KMB shared between the key establishment satellite SAT A, SAT B and the central key management unit UC could be obtained otherwise, if the security level allows it. For example, it could be defined during the design of the key establishment satellite SAT A, SAT B and embarked before its launch.

[0043] In particular, the first key establishment satellite SAT A and the second key establishment satellite SAT B may correspond to the same satellite traveling a distance for quantum-sharing of the key KA linked to the first ground station A, then the KB key linked to the second ground station B, or vice versa. Alternatively, the first key establishment satellite SAT A and the second key establishment satellite SAT B may correspond to two different satellites, in particular belonging to a constellation of satellites.

[0044] In particular, when a constellation of satellites is available, the step of quantum distribution of keys KA, KB linked to the ground stations A, B, comprises for each ground station A, B, a selection of a satellite of the constellation as a key establishment satellite SAT A, SAT B, according to the conditions of communication with the ground station A, B. The key establishment satellite SAT A, SAT B is in particular that of the constellation of satellites having the most favorable conditions of communication with the ground station A, B, for example that closest to the ground station A, B and / or benefiting from meteorological conditions characterized by the lowest cloud cover and / or that benefiting from a night transmission rather than a day transmission.The use of a constellation of satellites makes it possible, among other things, to multiply the opportunities for establishing quantum communications with ground stations A, B when conditions are favorable.

[0045] In the centralized key storage step 3, the encrypted transmission of the keys KA, Kb linked to the ground stations A, B uses in particular the infrastructure of the satellite constellation. For example, the key establishment satellite SAT A, SAT B transmits the key Ka, Kb in an encrypted manner via an open link to a telemetry, telecommand and control station or TT&C (for "Telemetry, Telecommand & Control" in English), although another path is possible. The TT&C station is in particular a relay antenna for the satellite constellation known per se. The TT&C station then relays the encrypted transmission of the key KA, KB linked to the ground station A, B to a mission control center or MCC (for "Mission Control Center" in English). In a manner known per se, the function of the MCC is the management of the satellite constellation.In particular, the encrypted transmission of the key KA, KB linked to the ground station A, B is carried out via a TM / TC link, i.e. a telemetry-remote control link, in particular in the form of a radio frequency link.

[0046] The encrypted transmission of the keys KA, KB linked to the ground stations A, B can also use inter-satellite links. One or more satellites of the constellation can serve as a relay between the key establishment satellite SAT A, SAT B and the TT&C station. The transmission can then be encrypted end-to-end and / or be encrypted step by step. The use of one or more keys shared by quantum communication is preferred.

[0047] In particular, the central key management unit UC is included in the mission control center. This is particularly advantageous, because by its function As a satellite constellation manager, the mission control center is highly secure, both digitally to prevent any computer infiltration, and physically through guarding, differentiated access authorizations, safes, for example. The security level of the mission control center is particularly compatible with a trusted node function. The central key management unit UC can then benefit from these security means. The central key management unit UC is preferably completely included in the mission control center. Alternatively, the central key management unit UC is partially included in the mission control center or completely outside the mission control center.Communication between the mission control center and the central key management unit UC is then preferably highly secure, for example by a key shared by quantum communication.

[0048] The central key management unit UC may be at a single site.

[0049] Alternatively, the central key management unit UC may be distributed over several sites. Where appropriate, in the centralized key storage step 3, each key Ka, Kb linked to a ground station A, B is transmitted to a site of the central key management unit UC. In particular, a synchronization mechanism is implemented between the sites of the central key management unit UC, in order to manage the distribution of the keys linked to the ground stations A, B.

[0050] Preferably, the method then comprises a second preliminary step of quantum distribution of site keys, in which keys are shared by quantum communication between the sites of the central key management unit UC. The second preliminary step of quantum distribution of site keys is implemented before the key storage step 3, to allow a secure encrypted transmission of the key Ka, Kb linked to the ground station A, B to the respective site of the central unit UC. Preferably, the second preliminary step of quantum distribution of keys is implemented before the step of quantum distribution 1 of the keys linked to the ground stations A, B, to allow a transmission of the keys KA, KB linked to the ground stations A, B immediately after their sharing between the ground station A, B and the key establishment satellite SAT A, SAT B.

[0051] In particular, the first preliminary step of quantum distribution of satellite keys and / or the second preliminary step of quantum distribution of site keys are implemented during an initial configuration of the system implementing the method, in particular before any step of quantum distribution 1 of keys linked to the ground stations A, B. Such an initial configuration can take place during a first commissioning of the system or after an update of the system.

[0052] In particular, in the pairing step 6, the encrypted transmission of the key KB linked to the second ground station B to the first ground station A comprises the transmission of a parity sequence KA@KBresult of the exclusive OR function between the key Ka linked to the first ground station A and the key KB linked to the second ground station B. For this purpose, the central key management unit UC can implement the disposable mask encryption technique (or OTP for "One Time Pad cryptography" in English). It is therefore not necessary to define a shared key between each ground station A, B and the central key management unit UC.

[0053] In particular, at the end 7 of the pairing step 6, the first ground station A extracts the key Kb linked to the second ground station B from the parity sequence KAë KB transmitted by the central key management unit UC.

[0054] Then, in communication step 8, the first ground station A can then send a parity sequence KÀ^KB resulting from the exclusive OR function between the key Ka linked to the first ground station A and the key KB linked to the second ground station B, so that the second ground station B also has the key KA of the first ground station A.

[0055] The key KA of the first ground station A can also be transmitted in encrypted form to the second ground station B during the pairing step 6. For this purpose, the central key management unit UC also sends the parity sequence KÀSKB to the second ground station B, so that the latter extracts the key KA linked to the first ground station A, as described in relation to the first ground station A.

[0056] In particular, the links between the ground stations A, B and the central key management unit UC are terrestrial, or even solely terrestrial, i.e. implemented by ground infrastructures, such as for example communication cables, optical fibers or mobile telecommunications systems.

[0057] The key establishment satellite(s) S AT A, S AT B and the central key management unit UC are therefore part of a centralized quantum cryptographic key pairing system for a plurality of ground stations A, B.

[0058] The key establishment satellite(s) S AT A, S AT B and / or the central key management unit UC, as trusted nodes in the key exchange between the ground stations A, B, are preferably highly secured against digital and physical attacks by appropriate protection means, according to the rules of the art for critical infrastructures. Thus, the central key management unit UC securely stores the keys KA, KB linked to the ground stations A, B. The key establishment satellite(s) S AT A, S AT B can transiently securely store the keys KA, KB linked to the ground stations A, B.

[0059] To carry out step 1 of quantum key distribution linked to the ground stations A, B, the key establishment satellite(s) S AT A, S AT B and the ground stations A, B are preferably each equipped with quantum optical terminals. For example, the quantum optical terminal of the key establishment satellite SAT A, SAT B comprises a photonic source; and the quantum optical terminal of the ground station A, B comprises, among other things, a qubit analyzer and single photon detectors. The central key management unit UC may also be provided with a quantum optical terminal, to be able to implement the first preliminary step of quantum satellite key distribution.

[0060] A ground station A, B can be fixed or mobile, such as a ship or a submarine.

[0061] For at least one of the ground stations A, B, or even all of them, several keys linked to the ground station can be shared by quantum communication between the ground station and the key establishment satellite, and stored in parallel in the central key management unit UC. These keys are in particular shared between the ground station and the key establishment satellite, in the same key distribution step 1 or in key distribution steps 1 separated in time. Thus, several keys linked to the ground station A, B can be available in advance, which improves the responsiveness of the method when receiving a pairing request. This is particularly advantageous for a ground station which is in high demand.

[0062] It is preferable that for each ground station A, B at least a minimum quantity of key Ka, Kb is available in the central unit UC at any time, so that a pairing request concerning any of the ground stations A, B can be satisfied immediately.

[0063] For example, the quantum communication between the ground station A, B and the key establishment satellite SAT A, SAT B or that between the key establishment satellite SAT A, SAT B and the central key management unit UC, or any other quantum communication described in relation to the invention, implement a QKD method known per se, for example with a Prepare and Measure protocol, such as the BB84 protocol.

[0064] By sharing a key in a quantum manner or by quantum communication, we mean in particular the establishment by quantum communication between two entities of a cryptographic key established by exploiting effects of quantum physics, the quantum cryptographic key being shared between the two entities at the end of this quantum communication. In particular, the cryptographic key is a random sequence of logical bits 0 and 1; and it is only called "quantum" because it is established thanks to quantum communication.

Claims

Claims

1. A centralized method for pairing quantum cryptographic keys by satellite for a plurality of ground stations (A, B), the method comprising in this order: a quantum key distribution step (1), in which, for each ground station (A, B), at least one respective cryptographic key (KA, KB), called "key linked to the ground station", is shared by quantum communication between the ground station (A, B) and a satellite (SAT A, SAT B), called "key establishment satellite"; - a centralized key storage step (3), in which, each key (Ka, Kb) linked to a ground station (A, B) is first transmitted in an encrypted manner from the key establishment satellite (SAT A, SAT B) to a terrestrial central key management unit (UC), and then stored by this terrestrial central key management unit (UC);and - a pairing step (6) between a first (A) and a second (B) of said ground stations, in which, upon receipt of a communication request (5) between the first ground station (A) and the second ground station (B), the terrestrial central key management unit (UC) transmits in encrypted form the key (KB) linked to the second ground station (B) to the first ground station (A).;

2. Method according to the preceding claim, wherein: in the pairing step (6) between the first ground station (A) and the second ground station (B), the terrestrial central key management unit (UC) further transmits in encrypted form the key (KA) linked to the first ground station (A) to the second ground station (B).

3. Method according to claim 1 or 2, wherein, in the pairing step (6), the encrypted transmission or transmissions comprise the transmission of a parity sequence KA^KB resulting from an exclusive OR XOR function between the key (KA) linked to the first ground station (A) and the key (KB) linked to the second ground station (B).

4. Method according to one of the preceding claims, wherein, in the centralized key storage step (3), each key (KA, KB) linked to a ground station (A, B) is transmitted in encrypted manner by means of a key (KMA, KMB) shared between the key establishment satellite (SAT A, SAT B) and the terrestrial central key management unit (UC).

5. Method according to the preceding claim, wherein, in the centralized key storage step (3), the encrypted transmission of the key (KA, KB) linked to the ground station (A, B) between the key establishment satellite (SAT A, SAT B) and the terrestrial central key management unit (UC) comprises the transmission of a parity sequence (KA®KMÀ, Result of an exclusive OR XOR function between the key (KA, KB) linked to the ground station (A, B) and the shared key (KMA, KMB) between the key establishment satellite (SAT A, SAT B) and the terrestrial central key management unit (UC).

6. Method according to claim 4 or 5, comprising, before the centralized key storage step (3), a first preliminary step of quantum distribution of satellite keys, in which the key (KMA, Kmb) shared between the key establishment satellite (SAT A, SAT B) and the terrestrial central key management unit (UC) is shared by quantum communication between the key establishment satellite (SAT A, SAT B) and the terrestrial central key management unit (UC).

7. Method according to one of the preceding claims further comprising, the key establishment satellite (SAT A, SAT B) belonging to a constellation of satellites, a selection of a satellite from the constellation of satellites as a key establishment satellite (SAT A, SAT B) from communication conditions with the ground station (A, B).

8. Method according to one of the preceding claims, in which the terrestrial central key management unit (UC) is at least partly included in a mission control center (MCC) of the key establishment satellite (SAT A, SAT B) or of the satellite constellation.

9. Method according to one of the preceding claims, in which, for at least one ground station (A, B), several keys (KA, KB) linked to the ground station (A, B) shared by quantum communication are stored in the terrestrial central key management unit (UC).

10. Centralized satellite-based quantum cryptographic key pairing system for a plurality of ground stations (A, B), system comprising at least one satellite (SAT A, SAT B), called "key establishment satellite", and at least one terrestrial central key management unit (UC), configured to implement a method according to one of the preceding claims.

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