Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
  • H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
  • H04L9/0816—Key establishment, i.e. cryptographic processes or cryptographic protocols whereby a shared secret becomes available to two or more parties, for subsequent use
  • H04L9/0852—Quantum cryptography

Definitions

• the invention relates to the field of cryptography.
• a message can only be read by its recipient.
• a key is used to encrypt the message. Only the owner of the key is able to read the message on reception.
• the encryption key must therefore be transmitted by the sender to the recipient of the encrypted message. This transmission is carried out such that only the recipient of the encrypted message receives this encryption key. Interception by a third party of the encryption key is detected by the sender or the recipient. Thus, the encryption key detected as intercepted is not used for encryption of the message.
• An example of the principle of transmitting encryption keys is that of quantum cryptography. It consists of using physical properties to ensure the integrity of a received encryption key.
• the encryption key is made up of a sequence of bits. For example, each bit is associated with a time offset of a pulse on a light flux. Then, the light flux coded in the time domain is attenuated.
• the transmitter can encode the encryption key in two non-orthogonal states.
• the pulses sent by Alice have a time width ⁇ T and an amplitude such that the probability of detecting a photon during the whole duration of the pulse is equal to one (state at one photon) or sufficiently low for the probability to detect two photons or negligible compared to the probability of detecting one (coherent state).
• the detection states are chosen from a base of two states. These two detection states are orthogonal respectively to each of the states of the base used by the transmitter. During transmission, the choice of emission and detection states is made independently of each other.
• the probability of detection is zero.
• the measurement result is certain, there is no ambiguity. If they are not orthogonal, there are two possible measurement results because the probability of detecting the photon is
• Photon detection is a one-time process that can occur at any time during the pulse.
• the spy can, for example, measure all the impulses sent by Alice. It has a quantum efficiency detector equal to one. For each pulse emitted by Alice, it detects the corresponding photon. If it is capable of instantly retransmitting a pulse to a photon to the Bob receiver but with a time width ⁇ T 'shorter than that sent by Alice, it can also read the information without being unmasked as shown in Figure 1.
• the spy can measure the information sent by the transmitter (Alice) and send an equivalent signal back to the receiver (Bob) without it can be detected. Eve then has a copy of the information without it being exposed.
• This type of espionage is difficult to carry out in practice, but its possibility of principle cannot be excluded.
• the present invention proposes to guard against this type of espionage by using a minimal state.
• This minimum state is a state where the product of the uncertainty on the coding parameter and its conjugate parameter is equal to its minimum value.
• the invention provides a method for decoding coded digital data such that two conjugate parameters x and y of the coded particle flow are in a minimal state, knowing that the probability of detecting two particles per period is negligible, characterized in that it comprises minus:
• a filtering step making it possible to separate the particles received verifying the relationship ⁇ xi. ⁇ yi> 1 but where ⁇ xi ⁇ ⁇ x or ⁇ yi ⁇ ⁇ y ( ⁇ x and ⁇ y fixed) of the particles characterized ⁇ x and ⁇ y, and
• the decoding process is implemented by a coded digital data decoder such that two conjugate parameters x and y of the coded particle stream are in a minimal state, knowing that the probability of detecting two particles per period is negligible, characterized in that it includes at least:
• a filter making it possible to separate the particles received verifying the relationship ⁇ xi. ⁇ y-i> 1 but where ⁇ xi ⁇ ⁇ x or ⁇ yi ⁇ ⁇ y ( ⁇ x and ⁇ y fixed) from the particles characterized ⁇ x and ⁇ y, and
• the parameters verifying the minimum state chosen by way of example in the figures and the description are the time width ⁇ T of the pulse carrying the information and its conjugate: the spectral width ⁇ v of this pulse.
• the pulses of temporal width ⁇ T are emitted by Alice. If Eve detects a photon, she can re-emit a pulse of shorter time width ⁇ T so that Bob cannot detect the interception. Indeed, the amplitudes of the pulses emitted by Alice and Eve are such that the probability of detecting a photon is the same for the two types of pulses. Depending on the moment of detection, the pulse sent by Eve carries information or not.
• the pulses are also defined in the frequency space.
• the impulses emitted by Alice are characterized by a temporal width ⁇ T and a spectral width ⁇ v whose product is always greater than a constant whose value is of the order of one: ⁇ T. ⁇ v> 1.
• This relation is analogous to the relations d Heisenberg uncertainties which connect two conjugate parameters x and y: ⁇ x. ⁇ y> h. When this relation becomes an equality, we obtain a minimal state.
• the uncertainty on one of the conjugate variables is directly the opposite of the uncertainty on the other conjugate variable.
• These variables can, for example, be the position p z and the pulse z.
• Figure 2 shows the block diagram of a transmission system implementing the invention.
• the encoder 1 therefore delivers a stream of pulses in a minimal state carrying the information to be transmitted on the value of the offset of the pulses with respect to the initial instant of the period.
• Such an encoder can include: • [ENCODER A] Or a source with 11 +3 coded pulses (for example, a laser generating a discontinuous laser beam with more or less offset pulses according to the coded data and verifying the minimum state relation, • [B coder] a controllable retarder 13 receiving a stream of particle pulses verifying the minimum state relationship and coming from a pulse source 1 1 +2 ,
• the coder 1 comprises a source of particles with pulse 11 + 2 (laser with mode latching, for example) and a retarder 13 such that the coded pulses verify the minimum state relation.
• the coded particle flow delivered by the encoder 1 is then attenuated by the attenuator 2 before being transmitted on the channel.
• This channel is said to be quantum because the probability that two particles are emitted on the channel per period is negligible or the probability that a single particle is emitted on the channel per period is equal to 1.
• the attenuator 2 proposed by FIG. 2 has a half-wave plate 21 followed by a polarizer 22 delivering a "key" beam on the quantum channel.
• the polarizer can also deliver a second, more intense beam.
• This annex "sync" beam can be used as a reference to synchronize the clocks of the transmitter and the receiver of the transmission system by quantum cryptography.
• the receiver comprises at least one decoder 3 receiving the "key” beam. This decoder 3 is possibly synchronized with the transmitter thanks to the "sync" auxiliary beam.
• a first variant of the decoder 3 proposed by FIG. 3 (a) can be used.
• the photons of the received "key” quantum signal are filtered by a filter of spectral width ⁇ v.
• the photons of spectral width ⁇ v are observed by the photon counter 31 'activated on the observation windows on which the value of the transmitted bit is certain (a window for the value bits "0" and a window for the value bits "1").
• the photons reflected by the filter ⁇ v are also counted by a photon counter 31 ".
• the comparator 32 checks whether the number N ⁇ f of reflected photons is greater than much greater than the numbers N ⁇ v of photons observed in the observation windows. If this is the case, the decoder 3 decides that the information transmitted has been intercepted by a third person. Otherwise, depending on whether the photon counter 31 ′ detects a photon in one or the other. Another of the observation windows, the decoder 3 decides that a bit of value "0" or "1" has been emitted.
• the decoder 3 decides that there is non-reception, and cannot determine whether this non-reception is due to poor quality transmission or to interception by a third party.
• Pulses close to the minimum state relation can be produced by, for example, 11 +2 mode latching lasers within an encoder 1.
• the time shifts are produced outside the laser by means of a retarder 13.
• the use of pulses produced by lasers with mode 11 +2 latching has important practical consequences.
• the pulse durations are typically between 10ps and 100fs. These values are much lower than the response times of existing photon counters (31 ') (typically 1 ns). The distinction between a shifted pulse and an unshifted pulse is therefore not possible.
• This function can be performed by an electrically controlled door (not shown) behind which the photon counter (31 ') is located. The possibility of making such a door very much depends on the response times obtained with the technology used, for example: 10 GHz with an electro-optical modulator.
• the frequency filter can be achieved using, for example, an interference filter or a Fabry-Perot with adjustable spacing to choose the spectral bandwidth. If the pulses used are too small compared to the switching time of the door, an interferometer can be used between the transmitter and the receiver as shown in Figure 4.
• An 11 +2 pulse source delivers the flow of particles in the form of a train of pulses of temporal width ⁇ T and of periodicity Tb.
• the retarder 13 then comprises the separating element of the interferometer.
• the attenuator can, for example, use the auxiliary stream as a synchronization signal "sync" from the transmitter to the receiver.
• the decoder 3 then makes or not pass the pulse of the other arm of the interferometer in a delay line of identical duration ( ⁇ T / 2 for example).
• the self-timer 13 and the decoder 3 have chosen the same delay (0 or ⁇ T / 2), then the probability of detecting a photon is 100% in one of the output channels (channel a) and zero in the other track (track b). If the self-timer 13 and the decoder 3 have chosen different delays, then the probability of detecting a photon is 50% in each channel.
• the fact that the counter 31 'detects a particle in the path b, makes it possible to determine without fail what the delay chosen by the retarder 13.
• the counter of particle 31 ′ is placed downstream of a bandpass filter for particles of spectral width ⁇ v or can, for example, be replaced by the device presented in FIG. 3 (b) if the pulses emitted are in a minimal state.
• the coding / decoding systems and methods using the minimum state relation for quantum cryptography have been described above in the particular case of temporal coding.
• the conjugate parameter is then the spectral width of the pulse carrying the information to be transmitted. It is thus possible to separate by simple filtering on the conjugate parameter the emitted particles not verifying the minimal state relation.
• Filtering makes it possible to separate the particles received verifying the relationship ⁇ xi. ⁇ y-i> 1 but where ⁇ xi ⁇ ⁇ x or ⁇ y-i ⁇ ⁇ y ( ⁇ x and ⁇ y fixed by the coder and known a priori from the decoder) from the particles characterized ⁇ x and ⁇ y .

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• Electromagnetism (AREA)
• Theoretical Computer Science (AREA)
• Computer Security & Cryptography (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Optical Communication System (AREA)

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