Method for encrypting a transmitted radio signal comprising a sum of pure-carrier signals

Abstract

The invention relates to a method (100) for encrypting a radio-frequency signal transmitted from a transmitter base to a mobile receiver, the radio-frequency signal comprising a sum of unmodulated pure-carrier signals having distinct frequencies, the method comprising the following steps: - establishing a cryptographic key common to the transmitter base and the mobile receiver; - sharing the common cryptographic key with the transmitter bases and the mobile receiver authorized to use the signals; - generating (102) by the transmitter base, an encrypted radio-frequency signal; - receiving (104), by the mobile receiver, the encrypted radio-frequency signal; - demodulating (105), by the mobile receiver, the encrypted radio-frequency signal; - decrypting (106) the encrypted pure-carrier signals to obtain the amplitudes and / or phases of the pure-carrier signals forming the radio-frequency signal.

Description

A method for encrypting a transmitted radio frequency signal comprising a sum of pure carrier signals technical field

[0001] The invention relates to a method for encrypting radio frequency signals transmitted from a transmitting base to a receiving mobile device. The technical field of the invention is that of radio frequency networks for the transmission of unmodulated pure-carrier radio frequency signals. Another object of the invention is a device for transmitting a radio frequency signal. The invention also relates to a method for positioning a receiving mobile device, by the mobile device itself, using the radio frequency signal encryption method according to one aspect of the invention. Prior art

[0002] The location of receiving mobile devices in outdoor environments experienced a significant technological leap with the deployment of the GPS (Global Positioning System) in the 1980s. However, GPS technology does not function well indoors, typically inside buildings, and performs relatively poorly in urban environments. The two main constraints of indoor or urban environments are: the penetration of signals by buildings and various obstacles (which limits the range of the positioning system); and the existence of multipath propagation affecting the emitted signal, resulting in numerous time-shifted replicas of the original signal with varying amplitudes reaching the receiving mobile device. Such multipath signals are called bounces.

[0003] There is therefore a need for mobile receiver positioning systems that have both a long range (typically greater than 10 km) and that work equally well in outdoor and indoor environments.

[0004] As is well known, a location system for outdoor or indoor environments typically includes, in addition to the mobile device to be positioned, a set of at least three beacons. These beacons are unique in that their position is known. Furthermore, they are usually fixed. In such a system configuration, the signals are modulated and cover a continuous frequency band of 30 MHz or more to achieve meter-level accuracy. Such frequency bands are only available above 1 GHz. However, waves at these frequencies do not penetrate walls.

[0005] The applicant has developed a technical solution enabling a receiving mobile device to locate itself using continuous sinusoidal signals. These signals occupy an infinitesimally small spectral band. They can be emitted sporadically in the VHF band (around 100 MHz) and spread over 30 MHz or more. This solution maintains metric accuracy by using an infinitesimally small spectrum width (in practice, ~10 Hz). Small bands of pure carrier signals are used. In practice, 6.25 kHz bands accommodate all the antennas (each antenna uses a slightly different frequency within this band). Approximately thirty 6.25 kHz bands are required for sufficient discrimination. This technical solution relies on the deployment of a network of fixed antennas emitting radio frequency signals, each radio frequency signal being formed by a sum of unmodulated pure carrier signals.Such a location system is disclosed, for example, in patent FR3121756B1. In the technical solution developed by the applicant, each fixed base station emits pure-carrier signals with frequencies distinct from those of the other transmitting bases. The receiving mobile device can then demodulate the received radio frequency signals and, by analyzing the phases of the signals emitted by the data bases, determine its distance from said data bases. By calculating the distances separating it from several bases and knowing the positions of these bases, the receiving mobile device can then locate itself with very high spatial accuracy.

[0006] The deployment of antenna networks emitting radio frequency signals formed by combs of pure unmodulated carriers is therefore essential to locate receiving mobiles both outdoors ("outdoor" according to English terminology) and indoors ("indoor" according to English terminology).

[0007] More generally, radio frequency signals formed by unmodulated frequency combs are now used in other systems as well, for example in multi-frequency radar systems, in medical imaging or in spectroscopy.

[0008] There is therefore a need to secure networks using unmodulated pure carrier signal combs. For example, it is desirable to prevent an unauthorized receiving mobile device from locating itself within a radio frequency antenna network deployed for internal or external geolocation.

[0009] In other words, it is desirable that service providers managing pure sinusoidal signal comb antenna networks have a tool to allow or prevent access to deployed or being deployed radio frequency networks.

[0010] Encryption prevents malicious actors, having developed their own receiver, from taking advantage of the geolocation radio frequency antenna network infrastructure deployed without the operator's authorization.

[0011] Several methods for encrypting radio frequency signals are known to those skilled in the art, particularly methods for encrypting radio frequency signals at the physical layer. Among these methods are: Frequency-Hopping Spread Spectrum (FHSS); Direct-Sequence Spread Spectrum (DSSS); Physical-Layer Scrambling Using Keys; Chaos-Based (Chaotic) Waveform Encryption; Time-Hoping and Ultra-Wideband Signaling; and Channel-Based Key Generation and its Use for Encryption.

[0012] However, these methods are used for encrypting modulated radio frequency signals. Indeed, all these methods require broadening the spectrum used for the signal. To maintain a frequency comb, it is necessary to encrypt the pure carrier signal while keeping the bandwidth infinitely small, as explained in the paragraph

[0005] ,

[0013] There is currently no technical solution to control user access to a network of base stations transmitting unmodulated pure carrier radio frequency signals or pure sinusoidal signals.

[0014] US 5995533 describes another encryption method, illustrating the general technological background. Summary of the invention

[0015] One aim of the present invention is to at least partially overcome the aforementioned drawbacks.

[0016] To this end, an object of the invention relates to a method of encrypting a radio frequency signal transmitted from at least one transmitting base to a receiving mobile, said receiving mobile being authorized to use the radio frequency signal, the radio frequency signal comprising a sum of unmodulated pure carrier signals of distinct frequencies, each pure carrier signal being in the form of an unmodulated sustained wave, said method comprising the following steps: establishment of a cryptographic key common to the transmitting base and the receiving mobile; sharing of said common cryptographic key with the sending base and the receiving mobile; generation, by the transmitting base and from the radio frequency signal, of an encrypted radio frequency signal, said encrypted radio frequency signal comprising a sum of encrypted pure carrier signals, each encrypted pure carrier signal being in the form of a continuous unmodulated wave, the encrypted pure carrier signals having the same frequencies as the pure carrier signals forming the radio frequency signal, each encrypted pure carrier signal being obtained by encryption of the pure carrier signal of corresponding frequency, said encryption comprising a modification, using the cryptographic key, of the amplitude and / or the phase of the pure carrier signal of corresponding frequency; transmission, by the transmitting base, of the encrypted radio frequency signal; reception, by the receiving mobile, of the encrypted radio frequency signal from the transmitting base; demodulation, by the receiving mobile, of the encrypted radio frequency signal, so as to obtain the amplitudes and / or phases of the encrypted pure carrier signals included in said encrypted radio frequency signal; decryption, by the receiving mobile and using the cryptographic key, of the amplitudes and / or phases of the encrypted pure carrier signals so as to obtain the amplitudes and / or phases of the pure carrier signals included in the radio frequency signal.

[0017] A sum of unmodulated pure carrier signals is defined as a discrete sum of signals. An unmodulated pure carrier signal is defined as a signal in the form of a continuous, unmodulated wave. For example, a pure sinusoidal signal is an example of an unmodulated pure carrier signal. A radio frequency signal formed by a sum of unmodulated pure carrier signals is also called a frequency comb.

[0018] A cryptographic key is defined as a sequence of bits, for example, a sequence of 256 bits. Establishing a shared cryptographic key means that the cryptographic key is known by both the sending base and the receiving mobile device. For example, the cryptographic key can be stored in memory elements in both the sending base and the receiving mobile device.

[0019] An authorized receiving mobile device is defined as a receiving mobile device possessing the cryptographic key.

[0020] Thanks to the encryption method according to one aspect of the invention, the transmission of the radio frequency signal from a transmitting base to a receiving mobile device is carried out securely. In other words, a receiving mobile device that does not possess the cryptographic key cannot decrypt the radio frequency signal from the transmitting base.

[0021] Sharing the cryptographic key between the transmitting base station and the receiving mobile device authorized to use the radio frequency signal can be accomplished via encrypted channels known to the expert. The cryptographic key sharing step may, for example, involve transmitting the cryptographic key from the transmitting base station to the receiving mobile device. Alternatively, the cryptographic key can be pre-stored in memory within both the transmitting base station and the receiving mobile device.

[0022] According to particular embodiments, the process according to one aspect of the invention has one or more of the following characteristics, taken individually or in all technically possible combinations: During the step of obtaining the encrypted radio frequency signal, the transmitting base generates, from the cryptographic key, a pseudo-random bit sequence, the phases and / or amplitudes of the encrypted radio frequency signals being a mathematical function of the bits forming the pseudo-random sequence; The pseudo-random bit sequence is generated using a cryptographic key and a cryptographic hash function or pseudo-random sequence generation function.

[0023] Another object of the invention relates to a device for encrypting a transmitted radio frequency signal, the radio frequency signal comprising a sum of unmodulated pure carrier signals of distinct frequencies, each pure carrier signal being in the form of an unmodulated continuous wave, said device comprising a transmitting base and a receiving mobile, said transmitting base being configured to: Generate, from the radio frequency signal or parameters of the radio frequency signal, an encrypted radio frequency signal, said encrypted radio frequency signal comprising a sum of encrypted pure carrier signals, each encrypted pure carrier signal being in the form of an unmodulated continuous wave, the encrypted pure carrier signals having the same frequencies as the pure carrier signals included in the radio frequency signal, each encrypted pure carrier signal being obtained by encrypting the pure carrier signal of the corresponding frequency.said encryption comprising a modification, using a cryptographic key, of the amplitude and / or phase of the pure carrier signal of corresponding frequency, the cryptographic key being common to the transmitting base and the receiving mobile device; Transmit the encrypted radio frequency signal; said receiving mobile device being configured for: Receive said encrypted radio frequency signal from the transmitting base; Demodulate said encrypted radio frequency signal, so as to obtain the amplitudes and / or phases of the encrypted pure carrier signals included in said encrypted radio frequency signal; Decipher, using the cryptographic key, the amplitudes and / or phases of the encrypted pure carrier signals so as to obtain the amplitudes and / or phases of the pure carrier signals included in the radio frequency signal.

[0024] The encryption device is therefore configured to implement the encryption process for a transmitted radio frequency signal according to a first aspect of the invention. Thanks to the encryption of the transmitted radio frequency signal, only a receiving mobile device possessing the cryptographic key can actually decrypt it, thus securing the transmission.

[0025] It is noted that one theoretical attack on this encryption involves placing a mobile receiver at a known location and determining the phase shifts and amplitude changes applied to a theoretically perfect received signal. In practice, this attack is ineffective due to the random nature of the changes in the signal's amplitude and phase caused by inevitable reflections off the ground.

[0026] The invention also relates to a method of positioning a receiving mobile using the method of encrypting a radio frequency signal transmitted according to a first aspect of the invention.

[0027] Another object of the invention is a method for positioning a receiving mobile relative to at least two fixed transmitting bases, said positioning method comprising the following steps: transmission, by each of the at least two transmitting bases and using the encryption method according to a first aspect of the invention, of a radio frequency signal comprising a sum of unmodulated pure carrier signals of distinct frequencies, each pure carrier signal being in the form of an unmodulated continuous wave, the decryption step comprising deciphering the phases and amplitudes of the encrypted pure carrier signals so as to obtain the phases of the pure carrier signals included in the radio frequency signals transmitted by the transmitting bases; calculation of a transition from the frequency domain to the time domain, such as an inverse Fourier transform, on the phases of the pure carrier signals transmitted by each transmitting base, so as to obtain a pseudo-distance or time of flight between said each transmitting base and the receiving mobile; pseudo-distance is understood to be an indirect measurement of distance based on the transmission and reception times of a signal; determination of the position of the receiving mobile, said determination of the position comprising a calculation of a distance separating the receiving mobile and each transmitting base, said calculation of a distance being carried out from the time of flight or pseudo-distance between each transmitting base and the receiving mobile.

[0028] The positioning method according to one aspect of the invention allows a receiving mobile device to position itself by receiving encrypted radio frequency combs. The positioning method according to one aspect of the invention uses the encryption method for a radio frequency signal transmitted according to the first aspect of the invention so as to secure the positioning method according to a second aspect of the invention.

[0029] According to particular embodiments, the device according to one aspect of the invention has one or more of the following characteristics, taken individually or in all technically possible combinations: The positioning process further includes a step of providing the positions of the transmitting bases to the mobile receiver. for each transmitting base, the frequency difference between the minimum frequency and the maximum frequency of the pure carrier signals transmitted by said base is greater than or equal to 30 MHz; advantageously this allows a spatial resolution of about 1m on the position of the receiving mobile; Each pure carrier signal transmitted by a transmitting base has a frequency located in the very high frequency band, in other words, has a frequency in the range from 30 MHz to 300 MHz; the transmitting bases are configured to transmit the same number of unmodulated pure carrier signals; the transmitting bases are configured so as to transmit their sums of pure carrier signals in a frequency-shifted manner, with a predetermined frequency shift between two successive transmitting bases; advantageously, this allows the receiving mobile to associate each radio frequency signal with a given transmitting base so as to be able to position itself relative to said given transmitting base; the frequency shift is within the range of 100 Hz to 1 kHz; the transmitting bases are antennas, preferably antennas distributed in such a way that the distance between two adjacent antennas is between 50m and 5km. The receiving mobile device is an electronic device, such as a chip or an electronic card, or a mobile communication device such as a mobile phone. Brief description of the drawings

[0030] The invention will be better understood upon reading the following description, given solely by way of example, and with reference to the figures in the appendix in which: Figure 1 shows a flowchart representing an embodiment of a method for encrypting a transmitted radio frequency signal according to the first aspect of the invention; Figure 2 shows a flowchart representing an embodiment of a method for positioning a receiving mobile using the method according to Figure 1; Figure 3a shows an example of sinusoidal signals of distinct frequencies; Figure 3b shows the signal obtained by summing the sinusoidal signals of distinct frequencies illustrated in Figure 3a; this summed signal is effectively sent by a transmitting base; Figure 3c shows an example of sinusoidal signals of distinct frequencies after encryption of their phases and amplitudes using the method according to one aspect of the invention; Figure 3d shows the signal obtained by summing the encrypted sinusoidal signals of distinct frequencies illustrated in Figure 3c; this summed signal is effectively sent by a transmitting base when the signal is encrypted using the encryption method according to one aspect of the invention. Description of the implementation methods

[0031] Figure 1 shows a flowchart representing an embodiment of a 100 encryption method for a transmitted radio frequency signal according to one aspect of the invention.

[0032] As illustrated in Figure 1, the method 100 according to an aspect of the invention comprises a step 101.a of establishing a cryptographic key common to the transmitting base and the receiving mobile device, the receiving mobile device being authorized to use the radio frequency signal transmitted by the transmitting base. As illustrated in Figure 1, the method 100 further comprises a step 101.b of sharing the cryptographic key with the common base and the receiving mobile device. The sharing of the cryptographic key can be carried out using encrypted channels known to those skilled in the art. The radio frequency signal to be transmitted comprises a sum of unmodulated pure carrier signals of distinct frequencies, each pure carrier signal being in the form of an unmodulated continuous wave.

[0033] For example, the cryptographic key can be a 256-bit sequence. The cryptographic key can, for instance, be stored in storage devices on the sending base and the receiving mobile device.

[0034] The process 100 further includes a step 102 of generating, by the transmitting base, an encrypted radio frequency signal from the clear radio frequency signal to be transmitted. The encrypted radio frequency signal also comprises a sum of unmodulated pure carrier signals of distinct frequencies, each pure carrier signal being in the form of an unmodulated continuous wave.

[0035] The encrypted pure carrier signals within the encrypted radio frequency signal have the same frequencies as the pure carrier signal within the radio frequency signal. In other words, encryption is performed frequency by frequency using the cryptographic key.

[0036] The encryption implemented in step 103 includes a modification of the amplitude and / or phase of the pure carrier signal of corresponding frequency.

[0037] In one embodiment, the transmitting base is configured to generate, from the cryptographic key, a pseudo-random bit sequence. The phases and / or amplitudes of the encrypted pure-carrier signals are then mathematical functions of the bits forming the pseudo-random sequence and the phases and / or amplitudes of the unencrypted pure-carrier signals.

[0038] According to one embodiment, the pseudo-random bit sequence is generated from the cryptographic key and a cryptographic hash function.

[0039] For example, let's call a 256-bit encoded cryptographic key K, and a cryptographic hash function HKDF that generates a pseudo-random sequence:

[0040] amp1 = 1

[0041] Kamp2 = K

[0042] amp2 = (1+0.5*Kamp2 / 2 A 128)

[0043] Kamp3 = HKDF(Kamp2)

[0044] amp3 = (1+0.5*Kamp3 / 2 A 128)

[0045] Kamp4 = HKDF(Kamp3)

[0046] amp4 = (1+0.5*Kamp4 / 2 A 128)

[0047] ...

[0048] KampN = HKDF(KampN-l)

[0049] ampN = (1+0.5*KampN / 2 A 128)

[0050] phi1 = 0

[0051] Kphi2 = HKDF(KampN)

[0052] phi2 = 2*pi* Kphi2 / 2 A 128

[0053] Kphi3 = HKDF(Kphi2)

[0054] phi3 = 2*pi* Kphi3 / 2 A 128

[0055] Kphi4 = HKDF(Kphi3)

[0056] phi4 = 2*pi* Kphi4 / 2 A 128

[0057] ...

[0058] Terms of type amp1, amp2, ... are the amplitude multipliers of the encrypted pure-carrier signals, while terms of type phi1, phi2, ... are the phases added to the encrypted pure-carrier signals. K, Kamp2, Kamp3, KampN, Kphi2, Kphi3... correspond to the pseudo-random sequence derived from the key K.

[0059] As illustrated in Figure 1, the method 100 according to the invention further comprises an emission step 103 by the transmitting base of the encrypted radio frequency signal and a reception step by the receiving mobile of the encrypted radio frequency signal.

[0060] The process 100 further includes a demodulation step 105, by the receiving mobile, of the encrypted radio frequency signal so as to obtain the amplitudes and / or phases of the encrypted pure carrier signals included in the encrypted radio frequency signal.

[0061] The receiving mobile device can then decipher, during decryption step 106, the amplitudes and / or phases of the encrypted pure carrier signals in order to obtain the pure carrier signals contained within the radio frequency signal. Because the receiving mobile device possesses the same cryptographic key as the transmitting base, it can decipher the encrypted pure carrier signals by applying a mathematical operation inverse to that applied by the transmitting base during decryption.

[0062] Figure 2 illustrates a flowchart representing one embodiment of a localization method 10 of a receiving mobile according to another aspect of the invention.

[0063] As illustrated in Figure 2, the method 10 comprises a step 16 of transmitting radio frequency signals from at least two transmitting bases to a receiving mobile device. During step 16, each transmitting base implements a method for encrypting a radio frequency signal transmitted to the receiving mobile device according to method 100 in a first aspect of the invention. During the implementation of step 16, the at least two transmitting bases are synchronized so as to transmit encrypted radio frequency signals with fixed phase differences.

[0064] During the implementation of the decryption step 106, the receiving mobile decrypts the phases of the encrypted pure carrier signals so as to obtain the phases of the pure carrier signals forming the radio frequency signals and coming from each transmitting base.

[0065] According to one embodiment, the method 10 further includes a step of providing the positions of the transmitting bases to the receiving mobile.

[0066] As illustrated in Figure 2, the method 10 according to one aspect of the invention further comprises a step 17 for calculating a transition from the frequency domain to the time domain on the phases of the pure carrier signals transmitted by a given transmitting base, so as to obtain a time of flight or pseudo-distance between said given transmitting base and the receiving mobile device. Such a calculation is, for example, an inverse Fourier transform calculated on the phases of the transmitted pure carrier signals. Alternatively, the calculation can be a Lasso-type calculation, a neural network, a regression, or a summation.

[0067] The method 10 further includes a step of determining the position 18 of the receiving mobile, said determination of the position including a calculation of a distance separating the receiving mobile and each transmitting base, said calculation of a distance being carried out from the time of flight or pseudo-distance between each transmitting base and the receiving mobile.

[0068] The system's use of pure carrier signals in the form of unmodulated sustained waves limits spectral occupancy, thus ensuring very high system sensitivity by minimizing the equivalent noise band. Such signals exhibit minimal spectral occupancy. This results in a mobile receiver positioning system with a long range and good penetration, capable of operating in free channels of the VHF band, functioning equally well outdoors and indoors, and allowing the mobile device itself to position itself precisely.Such an approach, based on the emission of sums of signals with a pure, unmodulated carrier, is not used in prior art positioning and radiolocation systems, particularly because of multipath propagation (rebounds of the transmitted signal arriving at the receiving mobile with a time delay), which is difficult to distinguish from the original line-of-sight signal when such a signal is a modulated wave. The system according to the invention uses an inverse Fourier transform applied to the demodulation results, consisting of the various measured amplitudes and phases of the radio frequency signals, to allow the receiving mobile to discriminate the rebounds from the direct path and thus measure flight times or pseudo-distances (between the mobile and the base stations) free from multipath errors.This approach allows the receiving mobile device to distinguish between bounces and the direct path, the latter being the first to reach the receiving mobile device (the bounces arriving subsequently). Furthermore, unlike many prior art systems, the transmitting bases of the positioning system according to the invention do not transmit any modulated data. This reduces spectral occupancy and thus allows coexistence with other users of the frequency spectrum.

[0069] Furthermore, method 10 according to one aspect of the invention relies on the transmission of encrypted radio frequency signals from each of the transmitting bases to the receiving mobile device, through the use of method 100 according to a first aspect of the invention. This secures the positioning method of a receiving mobile device so that only devices possessing the cryptographic key can locate themselves relative to the transmitting bases.

[0070] In one embodiment, for each transmitting base station, the frequency difference between the minimum and maximum frequencies of the pure-carrier signals transmitted by said base station is greater than or equal to 30 MHz. Such a difference allows for a resolution of approximately 1 meter in determining the position of the receiving mobile device. If the frequency difference is greater, it is possible to obtain an even better resolution for determining the position of the receiving mobile device.

[0071] Figure 3a illustrates the amplitude as a function of time of three pure carrier signals with distinct frequencies. Figure 3b illustrates the radio frequency signal obtained by summing the pure carrier signals of Figure 3a.

[0072] Figure 3c illustrates that the encrypted pure-carrier signals have the same frequencies as the original unencrypted signals, but with different phases and amplitudes. Figure 3d illustrates a radio frequency signal obtained by summing the encrypted signals shown in Figure 3c.

Claims

Demands 1) A method (100) for encrypting a radio frequency signal transmitted from at least one transmitting base to a receiving mobile, said receiving mobile being authorized to use the radio frequency signal, the radio frequency signal comprising a sum of unmodulated pure carrier signals of distinct frequencies, each pure carrier signal being in the form of an unmodulated continuous wave, said method comprising the following steps: establishment (101. a) of a cryptographic key common to the sending base and the receiving mobile; sharing (101. b) of said common cryptographic key with the sending base and the receiving mobile; generation (102), by the transmitting base and from the radio frequency signal, of an encrypted radio frequency signal, said encrypted radio frequency signal comprising a sum of encrypted pure carrier signals, each encrypted pure carrier signal being in the form of an unmodulated continuous wave, the encrypted pure carrier signals having the same frequencies as the pure carrier signals included in the radio frequency signal, each encrypted pure carrier signal being obtained by encryption of the pure carrier signal of corresponding frequency, said encryption comprising a modification, using the cryptographic key, of the amplitude and / or the phase of the pure carrier signal of corresponding frequency; transmission (103), by the transmitting base, of the encrypted radio frequency signal; reception (104), by the receiving mobile, of the encrypted radio frequency signal from the transmitting base; demodulation (105), by the receiving mobile, of the encrypted radio frequency signal, so as to obtain the amplitudes and / or phases of the encrypted pure carrier signals included in said encrypted radio frequency signal; decryption (106), by the receiving mobile and using the cryptographic key, of the amplitudes and / or phases of the encrypted pure carrier signals so as to obtain the amplitudes and / or phases of the pure carrier signals included in the radio frequency signal.2) Encryption method (100) according to the preceding claim, in which, during the generation step (102) of the encrypted radio frequency signal, the transmitting base generates, from the cryptographic key, a pseudo-random bit sequence, the phases and / or amplitudes of the encrypted pure carrier signals being a mathematical function of the bits forming the pseudo-random sequence. 3) A method (100) of encryption according to the preceding claim, wherein the pseudo-random bit sequence is generated using a cryptographic key and a cryptographic hash function. 4) A radio frequency signal encryption device, the radio frequency signal comprising a sum of unmodulated pure carrier signals of distinct frequencies, each pure carrier signal being in the form of an unmodulated continuous wave, said device comprising a transmitting base and a receiving mobile, said receiving mobile being authorized to use the radio frequency signal, said transmitting base being configured to: Generate, from the radio frequency signal, an encrypted radio frequency signal, said encrypted radio frequency signal comprising a sum of encrypted pure carrier signals, each encrypted pure carrier signal being in the form of a continuous unmodulated wave, the encrypted pure carrier signals having the same frequencies as the pure carrier signals included in the radio frequency signal, each encrypted pure carrier signal being obtained by encryption of the pure carrier signal of corresponding frequency, said encryption comprising a modification, using a cryptographic key, of the amplitude and / or the phase of the pure carrier signal of corresponding frequency, the cryptographic key being common to the transmitting base and the receiving mobile; Transmit the encrypted radio frequency signal; The receiving mobile device being configured for: Receive said encrypted radio frequency signal from the transmitting base; Demodulate said encrypted radio frequency signal, so as to obtain the amplitudes and / or phases of the encrypted pure carrier signals included in said encrypted radio frequency signal; Decipher, using the cryptographic key, the amplitudes and / or phases of the encrypted pure carrier signals in order to obtain the amplitudes and / or phases of the pure carrier signals contained within the radio frequency signal.16 5) A method for positioning (10) a receiving mobile relative to at least two fixed transmitting bases, said positioning method comprising the following steps: Transmission (16), by each of the at least two transmitting bases and using the encryption method (100) according to any one of claims 1 to 3, of a radio frequency signal comprising a sum of unmodulated pure carrier signals of distinct frequencies, each pure carrier signal being in the form of an unmodulated continuous wave, the decryption step comprising decrypting the phases and amplitudes of the encrypted pure carrier signals so as to obtain the phases of the pure carrier signals included in the radio frequency signals transmitted by the transmitting bases; calculation of a transition from the frequency domain to the time domain (17), such as an inverse Fourier transform, on the phases of the pure carrier signals transmitted by each transmitting base, so as to obtain a time of flight between each transmitting base and the receiving mobile. determination of the position (18) of the receiving mobile, said determination of the position comprising a calculation of a distance separating the receiving mobile and each transmitting base, said calculation of a distance being carried out from the time of flight or pseudo-distance between each transmitting base and the receiving mobile. 6) Method (10) of positioning according to the preceding claim, further comprising a step of supplying the positions of the transmitting bases to the receiving mobile. 7) Positioning method (10) according to any one of claims 5 or 6, wherein, for each transmitting base, the frequency difference between the minimum frequency and the maximum frequency of the pure carrier signals transmitted by said each base is greater than or equal to 30 MHz. 8) Positioning method (10) according to any one of claims 5 to 7, wherein each pure carrier signal transmitted by a transmitting base has a frequency in the very high frequency band, in other words has a frequency in the range from 30 MHz to 300 MHz. 9) Positioning method (10) according to any one of claims 5 to 8, wherein the transmitting bases are configured to transmit the same number of unmodulated pure carrier signals. 10) Positioning method (10) according to any one of claims 5 to 9, wherein the transmitting bases are configured to transmit their sums of pure carrier signals in a frequency-shifted manner, with a predetermined frequency shift between two successive transmitting bases. 11) Positioning method (10) according to any one of claims 5 to 10, wherein the frequency shift is in the range from 100 Hz to 1 kHz. 12) Positioning method (10) according to any one of claims 5 to 11, wherein the transmitting bases are antennas, preferably antennas distributed such that the distance between two adjacent antennas is between 50m and 5km. 13) Positioning method (10) according to any one of claims 5 to 12, wherein the receiving mobile is an electronic device. 14) 1 Positioning method (10) according to any one of claims 5 to 13, wherein each transmitting base is configured to synchronize with each other the unmodulated pure carrier signals transmitted by said each transmitting base.

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