WO2008008125A2

WO2008008125A2 - Method and device for secret radio communications

Info

Publication number: WO2008008125A2
Application number: PCT/US2007/011973
Authority: WO
Inventors: Frederic Broyde; Evelyne Clavelier
Original assignee: Zxtalk Assets Llc.
Priority date: 2006-05-18
Filing date: 2007-05-18
Publication date: 2008-01-17
Also published as: WO2008008125A3

Abstract

The invention relates to a method and a device for secret radio communications, using one or more radio links and one or more free-space optical links A device of the invention comprises a key generator (12), a transmitting set for transmission via power distribution lines (13), dedicated electrical conductors (21), a differential circuit-breaker (22), a power-line filter (23), several sources of modulated light (31), a first mobile radio system (40) comprising a radio device B (41) and a 'legitimate receiver for optical transmission' (42), and a second mobile radio system (50) comprising a radio device C (51) and a 'legitimate receiver for optical transmission' (52). Thanks to the free-space optical transmission, secret keys are distributed and changed automatically every 10 seconds. The first mobile radio system (40) and the second mobile radio system (50) can therefore establish a radio link encrypted using these secret keys.

Description

METHOD AND DEVICE FOR SECRET RADIO COMMUNICATIONS

FIELD OF THE INVENTION

The invention relates to a method and a device for secret radio communications, using one or more radio links and one or more free-space optical links.

BACKGROUND ART

The persons skilled in the art know many methods for ciphering or deciphering secret communications. Some of these methods are implemented to cipher (i.e. encrypt) or decipher (i.e. decrypt) radio communications using one or more encryption keys. For instance, the GSM radio communication system uses a key contained in the SIM card of each mobile phone.

However, the persons skilled in the art know that public-key cryptographic systems are not perfectly secure, and that symmetric cryptographic systems face a problem of key distribution.

Specialists know key distribution methods via free-space optical links using lasers. Such methods may for instance be applied to intersatellite links. Some of these methods use the principle of quantum key distribution (QKD). These methods are not appropriate for the implementation of secret radio communications with mobiles moving inside a room or a vehicle.

DESCRIPTION OF THE INVENTION

The purpose of the method of the invention is secret communications through space, without the limitations of known techniques.

The invention is about a method for secret radio communications, comprising the following steps:

- using several sources of modulated light for transmitting ciphering keys, each source of modulated light being an apparatus, the modulated light that any one of said sources of modulated light would produce at a large distance having, in no solid angle less than 0,005 steradian, a radiant flux greater than 50% of the total radiant flux of the modulated light produced by said source of modulated light;

- connecting said sources of modulated light to dedicated electrical conductors, the dedicated electrical conductors being used to provide "electrical control signals" to the sources of modulated light, the "electrical control signals" being processed by each of said sources of modulated light to obtain the modulation of the light produced by said sources of modulated light;

- receiving the modulated light produced by said sources of modulated light using one or more "legitimate receivers for optical transmission", each "legitimate receiver for optical transmission" being capable of providing at its output said ciphering keys when the "legitimate receiver for optical transmission" receives the modulated light produced by said sources of modulated light with a large enough radiant flux;

- arranging said "legitimate receivers for optical transmission" in an "illuminated area" such that, in the "illuminated area", each "legitimate receiver for optical transmission" can receive the modulated light produced by said sources of modulated light with a radiant flux such that this radiant flux is large enough to obtain that the "legitimate receiver for optical transmission" provides at its output said ciphering keys;

- using said ciphering keys appearing at the output of each "legitimate receiver for optical transmission" to cipher and/or decipher information transmitted by radio transmission;

- arranging said sources of modulated light in such a way that the propagation through space of the light produced by said sources of modulated light is limited by the directivity of each of said sources of modulated light and/or by obstacles, this limitation defining firstly said "illuminated area" and secondly a "potentially illuminated area", the "potentially illuminated area" being such that a receiver for optical transmission of known characteristics which is positioned outside the "potentially illuminated area" cannot receive the modulated light produced by said sources of modulated light with a radiant flux such that this radiant flux is large enough to obtain that said receiver for optical transmission of known characteristics provides at its output said ciphering keys; - delivering said "electrical control signals" to said dedicated electrical conductors, using one or more generators producing signals, these signals being such that said ciphering keys may be obtained at the output of the "legitimate receivers for optical transmission" arranged in said "illuminated area". According to the invention, each source of modulated light is an apparatus. The persons skilled in the art understand well the difference between an apparatus which can be used by a final user, for instance a luminaire which can be installed and then used for lighting in a building, and an electronic component, for instance a light- emitting diode intended to be soldered on a printed circuit board, this printed circuit board being a part of an apparatus. In particular, apparatuses have an enclosure or a barrier which can protect them against certain external influences, and which allows to comply with safety rules.

Specialists understand that, at a distance of a source of modulated light which is large enough compared to the largest dimension of the source of modulated light, it is possible to measure, in each direction, the radiant intensity of the modulated light produced by the source of modulated light. The meaning of the wording "the modulated light that any one of said sources of modulated light would produce at a large distance having, in no solid angle less than 0,005 steradian, a radiant flux greater than 50% of the total radiant flux of the modulated light produced by said source of modulated light" is therefore clear. According to the invention, this total radiant flux cannot be focused in a very narrow beam of light, and the corresponding modulated light is therefore radiated with a low directivity by the source of modulated light. For instance, with a laser, more than 50% of the total radiant flux is usually contained in a beam of light which, at a large enough distance, corresponds to a cone, the vertex angle of which is less than 2 milliradians, that is to say a solid angle less than about steradian. According to the invention, the modulated light produced by the source of modulated light must therefore be emitted in a solid angle much larger than the solid angle of the lasers typically used in commercially available devices for free-space optical transmission between buildings. This characteristic renders useless a precise alignment between the sources of modulated light and a "legitimate receiver for optical transmission", which can therefore be mobile with respect to the sources of modulated light, without the need for a tracking mount. However, the use of a relatively large solid angle of emission (that is to say a low directivity) leads to a relatively shorter transmission range. According to the invention, it is possible that no source of modulated light comprises a laser.

If one makes sure that an adversary (specialists also say "an eavesdropper" or "an enemy" or "Eve") equipped with a given receiver for optical transmission cannot enter the "potentially illuminated area", then said receiver for optical transmission cannot give him access to the knowledge of the keys which' are transmitted via optical means, according to the invention. One will generally assume that a "receiver for optical transmission of known characteristics" which an adversary could use is at least as sensitive as the "legitimate receivers for optical transmission" of the legitimate users, such receivers being able to provide the keys used for ciphering and/or deciphering said information transmitted via a radio link. Therefore, said "illuminated area", inside which the legitimate users may obtain, via the reception of the optical signals, exploitable keys for ciphering and/or deciphering, is typically included in the "potentially illuminated area", outside which the reception of exploitable optical signals for ciphering and/or deciphering is not possible by an adversary.

According to the invention, it is possible that the limitation of the propagation through space of the light produced by said sources of modulated light is such that the irradiance of the modulated light produced by said sources of modulated light exceeds, in specified conditions, a given level in the "illuminated area". It is indeed clear for the person skilled in the art that there is a relation between a given minimum irradiance level produced by said sources of modulated light in the "illuminated area" and the fact that a "legitimate receiver for optical transmission" positioned in the "illuminated area" can receive the modulated light produced by said sources of modulated light with a radiant flux which is large enough to obtain that the signal-to-noise ratio is such that the messages corresponding to said keys may be delivered at the output of the "legitimate receiver for optical transmission".

According to the invention, it is possible that the limitation of the propagation through space of the light produced by said sources of modulated light is such that the irradiance of the modulated light produced by said sources of modulated light cannot exceed, in specified conditions, a given level outside the "potentially illuminated area". It is indeed clear for the person skilled in the art that there is a relation between a given maximum irradiance level produced by said sources of modulated light outside the "potentially illuminated area" and the fact that a receiver for optical transmission of known characteristics positioned outside the "potentially illuminated area" cannot receive the modulated light produced by said sources of modulated light with a radiant flux which is large enough to obtain that the signal-to-noise ratio is such that messages may be delivered at the output of said receiver for optical transmission with a low enough bit-error rate to obtain that one of these messages may correspond to one of said keys with a sufficient probability. We see that the method of the invention takes advantage of the possibility of confining a free space optical link, for instance by using opaque natural obstacles such as the walls of a building's room, or the bodywork or hull sides of a vehicle. We note that this characteristic is different from what is observed for a radio signal, because at the frequencies used for radio transmissions, an efficient confinement requires a Faraday cage. According to the invention, said "illuminated area" may for instance be mainly contained inside a building. According to the invention, said "illuminated area" may for instance be mainly contained inside a vehicle.

On the other hand, if r is the distance between one of said sources of modulated light and one receiver for optical transmission, as soon as r is large enough compared to the dimensions of the source of modulated light, the illuminance decreases as l/r2, in the case of a line-of-sight path visibility. Consequently, the signal received by a light sensor of a receiver for optical transmission, this sensor being for instance a photodiode, decreases as 1 /r2, or faster, as soon as r is large enough. A large enough distance of said sources of modulated light may therefore easily contribute to a low maximum irradiance produced by said sources of modulated light outside the

"potentially illuminated area". We note that this characteristic is different from what is observed for a radio signal, because the signal which is received by an antenna only decreases as 1/r, as soon as r is large enough.

It should be noted that the presence of non-modulated light increases the level of noise in a receiver for optical transmission intended to receive the light produced by said sources of modulated light, because of shot noise. Likewise, the presence of light which is modulated in a different way from the light produced by said sources of modulated light increases the level of noise and may also produce a jamming in a receiver for optical transmission intended to receive the light produced by said sources of modulated light. According to the invention, it is therefore possible to use one or more sources of light other than said sources of modulated light, to increase the level of noise and/or to produce a jamming in a receiver for optical transmission positioned outside the "potentially illuminated area".

For instance, let us consider the case where said "illuminated area" is contained in a building's room, this room having windows. If these windows are not opaque at the wavelengths of the modulated light, it is possible that some light produced by said sources of modulated light it is possible that some light produced by said sources of modulated light passes through these windows. Let us assume that the "potentially illuminated area" takes into account the fact that an adversary outside the building can use a telescope having an objective diameter of 0,5 m for watching these windows. In a first case, the decrease in illuminance with the distance may be sufficient to obtain that the "potentially illuminated area" does not spread over places where an adversary could use such a telescope to receive the light produced by said sources of modulated light. In this first case, the arrangement is satisfactory. In a second case, the "potentially illuminated area" spreads up to places where an adversary could use a receiver for optical transmission comprising a telescope. In this second case, the arrangement is not satisfactory and should be modified to obtain that the "potentially illuminated area" does not spread over places where an adversary could use such a telescope. The arrangement could for instance be modified by using, in front of each window, a net curtain illuminated by a light, such light being appropriate to increase sufficiently the noise level and/or the jamming in the receiver for optical transmission used by an adversary watching the windows with a telescope.

According to the invention, it is possible that said dedicated electrical conductors are also used to provide the supply power consumed by each of said sources of modulated light. This option allows to simplify the wiring. In this case, the invention may be such that one delivers said "electrical control signals" to said dedicated electrical conductors, using one or more transmitting sets for transmission via power distribution lines, the output of each transmitting set for transmission via power distribution lines being connected to said dedicated electrical conductors, and the input of each transmitting set for transmission via power distribution lines being connected to one of said generators producing signals, these signals being such that said ciphering keys may be obtained. In this case, we note that, according to the invention, signals relative to said ciphering keys are successively: a) produced by one of said generators, b) converted by a transmitting set for transmission via power distribution lines, c) propagated as electrical signals on said dedicated conductors, d) converted in at least one source of modulated light, e) propagated through space as optical signals, f) converted in one or more "legitimate receivers for optical transmission", g) used for ciphering and/or deciphering information transmitted via a radio link. According to the invention, it is possible that the modulated light produced by sources of modulated light is also used to transmit data other than said ciphering keys. In this case, it is clear that it will generally be desirable that the "legitimate receivers for optical transmission" can also provide at its output said data other than said keys.

According to the invention, said keys are used for ciphering and/or deciphering data transmitted via a radio link. It is possible that the radio transmission does not use any ciphering or deciphering other than the ciphering and the deciphering using said keys. It is also possible that the radio transmission also uses one or more ciphering or deciphering other than the ciphering and the deciphering using said keys. For instance, it is possible that an usual method for ciphering/deciphering using public keys might be combined with a symmetrical ciphering/deciphering using the method of the invention.

Let us now describe a device for the implementation of the method of the invention. This device for secret radio communications comprises: -several sources of modulated light, each source of modulated light being an apparatus, the modulated light that any of said sources of modulated light would produce at a large distance having, in no solid angle less than 0,005 steradian, a radiant flux greater than 50 % of the total radiant flux of the modulated light produced by said source of modulated light, the sources of modulated light being arranged in such a way that the propagation through space of the light produced by said sources of modulated light is limited by the directivity of each of said sources of modulated light and/or by obstacles, this limitation defining firstly an "illuminated area", the irradiance of the modulated light produced by said sources of modulated light exceeding, in specified conditions, a given minimum level in the "illuminated area", and secondly a "potentially illuminated area", the irradiance of the modulated light produced by said sources of modulated light remaining, in specified conditions, less than or equal to a given maximum level outside the "potentially illuminated area";

- dedicated electrical conductors connected to said sources of modulated light, the dedicated electrical conductors being used to provide the supply power consumed by each of said sources of modulated light and to provide "electrical control signals", the "electrical control signals" being processed by each of said sources of modulated light to obtain the modulation of the light produced by said sources of modulated light; -one or more "legitimate receivers for optical transmission" arranged in said "illuminated area", each "legitimate receiver for optical transmission" being able to provide at its output ciphering keys when the "legitimate receiver for optical transmission" receives the modulated light produced by said sources of modulated light with a radiant flux which can be reached with said given minimum level of irradiance applicable to the "illuminated area";

- one or more radio devices, each radio device being able to receive and/or to transmit information via a radio link and to use said ciphering keys appearing at the output of each "legitimate receiver for optical transmission" for ciphering information transmitted via radio by the radio device and/or for deciphering information received via radio by the radio device;

-one or more transmitting sets for transmission via power distribution lines delivering said "electrical control signals" to said dedicated electrical conductors, the output of each transmitting set for transmission via power distribution lines being connected to said dedicated electrical conductors; -one or more generators producing signals at the input of each transmitting set for transmission via power distribution lines, the signals produced by said one or more generators being such that said ciphering keys may be obtained at the output of the "legitimate receivers for optical transmission" arranged in said "illuminated area". A device of the invention may be such that no source of modulated light comprises a laser.

A device of the invention may be such that one of said "legitimate receivers for optical transmission" and one of said radio devices are mechanically connected to a case or enclosure, said case or enclosure being such that it can be moved with respect to the sources of modulated light.

A device of the invention may be such that at least one of the sources of modulated light is also used for lighting, the modulated light produced by this source of modulated light being visible by a human observer. A device of the invention may be such that at least one of the sources of modulated light is also used for lighting, the modulated light produced by this source of modulated light being invisible to a human observer.

A device of the invention may be such that the modulated light produced by at least one source of modulated light is emitted by one or more semiconductor devices other than a solid-state laser, for instance one or more light-emitting diodes.

A device of the invention may be such that the modulated light produced by at least one source of modulated light is emitted by one or more discharge lamps.

In a device of the invention, one may arrange said dedicated electrical conductors, said transmitting sets for transmission via power distribution lines, and all entities where the "electrical control signals" may appear, in such a way that an adversary cannot pick up the "electrical control signals" by using a connection to said dedicated electrical conductors, or by using a connection to other electrical conductors, or by using a connection to an antenna.

BRIEF DESCRIPTION OF THE DRAWINGS Other advantages and characteristics will appear more clearly from the following description of particular embodiments of the invention, given by way of non-limiting examples, with 25 reference to the accompanying drawings, in which:

Figure 1 shows a first embodiment of the invention, - Figure 2 shows a second embodiment of the invention.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

First embodiment (Best mode).

As a first example of a device of the invention given by way of non-limiting example and best mode of carrying out the invention, we have represented in Fig. 1 a device of the invention comprising a key generator (12), a transmitting set for transmission via power distribution lines (13), dedicated electrical conductors (21), a differential circuit breaker (22), a power-line filter (23), several sources of modulated light (31), a first mobile radio system (40), and a second mobile radio system (50). Each source of modulated light (31) uses 2 fluorescent lamps as light source for transmission, as described in the French patent application number 04 09939 of 5 September 2004, entitled "Dispositif d'emission pour Ia transmission optique en espace libre", and in the PCT application number PCT/IB2005/003309 entitled "Transmitting device for free- space optical transmission" of 16 September 2005. The sources of modulated light (31) are arranged inside a large room, in a building, the distance between two sources of modulated light being greater than 2 meters. The room doesn't have any window, and is illuminated with visible light by the sources of modulated light (31), which are therefore used for lighting. The access to this room is only possible through a double- door vestibule which can be reached, outside the room, only through an access control device, the interior of the double-door vestibule being always illuminated by a light source which is not used as a light source for transmission. The doors of the double- door vestibule and the walls of the room being opaque, the propagation through space of the light produced by the sources of modulated light (31) is limited by these obstacles. Taking into account light leakage through the doors of the double-door vestibule, in particular through the gaps, this limitation is such that the irradiance of the modulated light produced by the sources of modulated light (31) cannot, when the doors of the double-door vestibule are closed, exceed a level of 1 gW/m2 outside the room and outside the double-door vestibule, at a distance greater than 1 m from the double-door vestibule. Outside the building, the level of irradiance produced by the sources of modulated light (31) is practically zero. Taking into account the type of modulation which is used, the designers consider that these levels do not allow an adversary to demodulate the light produced by the sources of modulated light (31) and to obtain a key in this manner.

Electrical conductors (24) of a distribution circuit of an energy distribution system at the frequency 50 Hz, running from a service panel, are connected to the input terminals of the power-line filter (23) installed in the room. The output terminals of the power-line filter (23) are connected to the differential circuit breaker (22). When this differential circuit breaker (22) is closed, the electrical energy is delivered to the dedicated electrical conductors (21) and to the sources of modulated light (31), in such a way that the room is illuminated. The key generator (12) produces at the input of the transmitting set for transmission via power distribution lines (13) signals corresponding to ciphering keys. The transmitting set for transmission via power distribution lines (13) delivers "electrical control signals" to the dedicated electrical conductors (21). The electrical control signals are received and processed in each of the sources of modulated light (31) to obtain the modulation of the light produced by said sources of modulated light. The power-line filter (23) is designed in such a way that the signals produced by the transmitting set for transmission via power distribution lines (13) cannot be received by an adversary who would connect a receiving apparatus to the electrical conductors. The room is not designed as a Faraday cage, but the signals produced by the transmitting set for transmission via power distribution lines (13) have a frequency sufficiently low and an amplitude sufficiently small so that an adversary cannot receive them with an antenna and cannot obtain the keys in such a way.

The first mobile radio system (40) comprises a radio device B (41) and a "legitimate receiver for optical transmission" (42). The "legitimate receiver for optical transmission" (42) receives the modulated light produced by the sources of modulated light (31), and outputs ciphering keys. The radio device B (41) can receive and transmit information via a radio link and uses the keys outputed by the "legitimate receiver for optical transmission" (42) for ciphering information transmitted by the radio device B and for deciphering information received by the radio device B.

The second mobile radio system (50) comprises a radio device C (51) and a "legitimate receiver for optical transmission" (52). The "legitimate receiver for optical transmission" (52) receives the modulated light produced by the sources of modulated light (31), and provides (ciphering) keys at the output. The radio device C (51) can receive or transmit information via a radio link and uses the (ciphering) keys at the output of the "legitimate receiver for optical transmission" (52) for ciphering information transmitted by the radio device C and for deciphering information received by the radio device C.

Thanks to the free-space optical transmission, secret keys are distributed and changed automatically every 10 seconds. The first mobile radio system (40) and the second mobile radio system (50) can therefore establish a radio link encrypted using these secret keys. In this first example, this encryption is the only one used. It is clear that other mobile radio systems may be added to the network of the Fig. 1, and that encrypted communications is then possible between all mobile radio systems present in the room.

Second embodiment.

As a second example of a device of the invention given by way of non-limiting example, we have represented in Fig. 2 a device of the invention comprising a fixed radio device A (11), a key generator (12), a transmitting set for transmission- via power distribution lines (13), dedicated electrical conductors (21), a switch (22), a power-line filter (23), several sources of modulated 30 light (31) and a mobile radio system (40). Each source of modulated light (31) uses 12 light - emitting diodes as light source for transmission, as described in the French patent application number 06 01952 of 6 march 2006, entitled "Dispositif d¹ emission electroluminescent pour Ia transmission optique en espace Libre", and in the PCT application number PCT/XXXXX entitled "Electroluminescent transmitting device for free-space optical transmission" of XXX 35 2007. The light-emitting diodes used as light source for transmission produce an infrared radiation at a wavelength near 1,55 pm. Each source of modulated light (31) produces also non- modulated visible light, thanks to white light-emitting diodes. The sources of modulated light (31) are arranged inside a large room, in a building. The room has windows which cannot be opened and which have a reflective coating for the wavelength 1,55 rtm. This room may be illuminated with visible light by the sources of modulated light (31), which provide therefore a lighting function. As in the first embodiment, the access to this room is only possible through a double-door vestibule which can be reached only through an access control device. At the wavelength of 1:55 p.m. the doors of the double-door vestibule are opaque and the windows are essentially reflective: the propagation through space of the light produced by the sources of modulated light (31) is limited by these obstacles. Taking into account the directivity of the emission of the modulated light, this limitation is such that the irradiance produced by the sources of modulated light (31) cannot, when the doors of the double-door vestibule are closed, exceed a level of 10 nW/m2 outside the room and outside the double-door vestibule, at a distance greater than Im from the double-door vestibule. Outside the building, at a distance greater than 40 m, the mean irradiance of the modulated light produced by the sources of modulated light (31) cannot exceed a level of 1 pW/m2. Taking into account the type of modulation which is used, the designers consider that these levels do not allow an adversary to demodulate the light produced by the sources of modulated light (31) and to obtain a key in this manner. Electrical conductors (24) of a distribution circuit of a 48 V d.c. energy distribution system, running from a service panel, are connected to the input terminals of a power-line filter (23) installed in the room. The output terminals of the power-line filter (23) are connected to the switch (22). When the switch (22) is closed, the electrical energy is delivered to the dedicated electrical conductors (21) and to the sources of modulated light (31), in such a way that the room is illuminated.

The key generator (12) produces at the input of the transmitting set for transmission via power distribution lines (13) signals corresponding to ciphering keys. The transmitting set for transmission via power distribution lines (13) delivers "electrical control signals" to the dedicated electrical conductors (21). The electrical control signals are received and processed in each of the sources of modulated light (31) to obtain the modulation of the light produced by said sources of modulated light. The power-line filter (23) is designed in such a way that the signals produced by the transmitting set for transmission via power distribution lines (13) cannot be received by an adversary who would connect a receiving apparatus to the electrical conductors (24). The room is not designed as a Faraday cage, but the dedicated electrical conductors (21) are surrounded by an electromagnetic shield made of two superimposed metallic braids, used for eliminating any significant electromagnetic radiation through the dedicated electrical conductors. Consequently, an adversary cannot receive with an antenna the electrical signals received and processed in each of the sources of modulated light (31), and obtain the keys in this manner. The fixed radio device A (11) can receive and transmit information via a radio link. The fixed radio device (11) is connected to said key generator (12), in such a way that the electrical signals corresponding to keys produced by the key generator (12) are used to cipher information transmitted by the fixed radio device A and/or decipher information received by the fixed radio device A. A mobile radio system (40) comprises a radio device B (41) and a "legitimate receiver for optical transmission" (42). The "legitimate receiver for optical transmission" (42) receives the modulated light produced by the sources of modulated light (31), and outputs ciphering keys. The radio device B (41) can receive and transmit information via a radio link and uses the keys outputed by the "legitimate receiver for optical transmission" (42) for ciphering information transmitted by the radio device B and/or for deciphering info' illation received by the radio device B.

Thanks to the free-space optical transmission, secret keys are distributed and changed automatically every 10 ms. The fixed radio device A (11) and the radio device B (41) can therefore establish a radio link encrypted using these secret keys. In this second example, this ciphering is combined with a public key cryptographic system. It is clear that other mobile radio systems may be added to the network of Fig. 2, and that encrypted communication using the invention is then possible between all mobile radio systems of the network present in the room and the fixed radio system A.

The data rate of the free-space optical link is about 5 megabits per second. The specialists understand that it would be possible to modify the device of the Fig. 2, in such a way that the transmission of information from A to B is made via an optical link and the transmission from B to A is made via a radio link. In this case, the modulated light produced by the sources of modulated light is used to transmit information other than keys, and the "legitimate receiver for optical transmission" (42) outputs information other than keys. This arrangement has advantages for the confidentiality of information.

INDUSTRIAL APPLICABILITY

The invention is particularly appropriate to secret radio communications when a protection of information better than the protection obtained by classical methods based on public keys has to be provided locally. For example, current laptop computers often have a radio communication function. A unit of the size of a USB key may for example be connected to the USB port of a computer which does not have any "legitimate receiver for optical transmission". This unit may become a "legitimate receiver for optical transmission" used in the implementation of the invention. We note that current mobile phones often have a digital camera function and that the image sensor corresponding to this function could, for some appropriate types of light modulation, also be used as a sensor for the function of "legitimate receiver for optical transmission" for the implementation of the invention.

The method and device of the invention are particularly appropriate for establishing secret radio communications with mobiles which can move inside a building or inside a vehicle.

Claims

1. Method for secret radio communications, comprising the following steps:

Using several sources of modulated light for transmitting ciphering keys, each source of modulated light comprising an apparatus producing modulated light at a large distance having, in no solid angle less than 0,005 steradian, a radiant flux greater than 50 percent of the total radiant flux of the modulated light produced by said source of modulated light;

Connecting said sources of modulated light to dedicated electrical conductors, the dedicated electrical conductors providing electrical control signals to the sources of modulated light, the electrical control signals being processed by each of said sources of modulated light to modulate the light produced;

Receiving the modulated light produced by said sources of modulated light using one or more legitimate receivers for optical transmission, each legitimate receiver for optical transmission capable of providing ciphering keys at its output when the legitimate receiver for optical transmission receives the modulated light produced by said sources of modulated light with a large enough radiant flux;

Arranging said legitimate receivers in an illuminated area such that each legitimate receiver can receive the modulated light produced by said sources of modulated light with a radiant flux such that this radiant flux is large enough such that the legitimate receiver for optical transmission may provide said ciphering keys at its output;

Using said ciphering keys to cipher and/or decipher information transmitted by radio transmission; Arranging said sources of modulated light in such a way that the propagation through space of the light produced by said sources of modulated light is limited by the directivity of each of said sources of modulated light and/or by obstacles, this limitation defining an illuminated area and a potentially illuminated area, the potentially illuminated area being such that an optical transmission receiver positioned outside the potentially illuminated area cannot receive enough modulated light produced by said sources of modulated light to provide said ciphering keys; Delivering said electrical control signals to said dedicated electrical conductors, using one or more signal generators, the signals being such that said ciphering keys may be obtained at the output of the legitimate receivers for optical transmission arranged in said illuminated area.

2. The method of claim 1, wherein the propagation of the light produced by said sources of modulated light is limited such that the irradiance of the modulated light cannot exceed a given level outside of the potentially illuminated area..

3. The method of claims 1 to 2, wherein said illuminated area is mainly contained inside a building.

4. The method of claims 1 to 2, wherein said illuminated area is mainly contained inside a vehicle.

5. The method of claims 1 to 3 or 1 to 4, wherein:

Said dedicated electrical conductors provide power to each of the said sources of modulated light;

Said electrical control signals are delivered to said dedicated electrical conductors using one or more transmitting sets for transmission via power distribution lines, the output of each transmitting set being connected to said dedicated electrical conductors, and the input of each transmitting set being connected to one of the said signal generators, the generated signals being such that said ciphering keys may be obtained.

6. The method of claims 1 to 5, wherein modulated light is used to transmit data other than said ciphering keys.

7. A device for secret radio communications comprising: One or more sources of modulated light, each source of modulated light comprising an apparatus producing modulated light at a large distance having, in no solid angle less than 0,005 steradian, a radiant flux greater than 50 percent of the total radiant flux of the modulated light produced, the sources of modulated light being arranged in such a way that the propagation through space of the light produced by said sources of modulated light is limited by the directivity of each of the said sources of modulated light and/or by obstacles, this limitation defining an illuminated area, inside which the irradiance of the modulated light may exceed a given minimum level during certain conditions, and a potentially illuminated area, the irradiance of the modulated light remaining, in specified conditions, less than or equal to a given maximum level outside the potentially illuminated area;

Dedicated electrical conductors connected to said sources of modulated light, the dedicated electrical conductors providing the supply power to each of the said sources of modulated light and providing electrical control signals, the electrical control signals being processed by each of said sources of modulated light to modulate the light produced;

One or more legitimate receivers for optical transmission arranged in said illuminated area, each legitimate receiver for optical transmission capable of providing ciphering keys at its output when receiving the modulated light produced by said sources of modulated light; One or more radio devices, each radio device capable of receiving and/or transmitting information via a radio link and using said ciphering keys from the output of each legitimate receiver to cipher information transmitted via radio by the radio device and/or to decipher information received via radio by the radio device;

One or more transmitting sets for transmission via power distribution lines delivering said electrical control signals to said dedicated electrical conductors, the output of each transmitting set being connected to said dedicated electrical conductors;

One or more signal generators at the input of each transmitting set, the generated signals being such that said ciphering keys may be obtained at the output of the legitimate receivers arranged in said illuminated area.

8. The device of claim 7 wherein at least one of the sources of modulated light is also used for lighting.

9. The device of any of the claims 7 or 8 wherein the modulated light produced by at least one source of modulated light is emitted by one or more light-emitting diodes.

10. The device of any of the claims 7 or 8 wherein the modulated light produced by at least one source of modulated light is emitted by one or more discharge lamps.