Random number generator

Abstract

A random number generator 10 receives input of a plurality of time-series measurement results of scattered light intensity of light propagating through an optical transmission line 100, calculates differences between the plurality of measurement results, and assigns a bit string to the result of calculating the differences to generate a random bit string.

Description

Random Number Generator

[0001] The present disclosure relates to random number generators.

[0002] With the development of information and communication services, encryption of highly confidential information has become essential. Cryptographic keys are generated based on random bit strings of 0 and 1, and the unpredictability and irreproducibility of the bit string strongly affect the robustness of the encryption. Therefore, generating random bit strings is important in cryptographic communication technology.

[0003] To achieve unpredictability and irreproducibility, physical random number generators that utilize random physical phenomena have been developed. Here, the physical phenomenon that exhibits random behavior is called an entropy source. Conventional physical random number generators consist of an entropy source and a measurement device.

[0004] Optical fibers are known to have distributed randomness due to materials and perturbations (Non-Patent Document 1). Distributed characteristics can be measured using Optical Time Domain Reflectometry (OTDR) or Optical Frequency Domain Reflectometry (OFDR). By generating physical random numbers using the optical fiber of an existing transmission line as an entropy source, highly robust encryption can be achieved without introducing a new physical encryption generator into the network system.

[0005] Alexander Smirnov et al., “An Optical-Fiber-Based Key for Remote Authentication of Users and Optical Fiber Lines”Sensors, 23, 6390

[0006] Random number generators that use the distributed randomness of optical fibers as an entropy source have the problem that the measurement signal changes due to loss in the optical fiber, reducing the randomness of the measurement signal.

[0007] Non-Patent Document 1 discloses a method of extracting signal intensity changes caused by optical loss by function fitting and removing them from measurement results. However, function fitting requires all measurement results to be stored as electrical information and function estimation to be performed using the least squares method or the like, which degrades real-time performance.

[0008] The present disclosure has been made in view of the above, and aims to suppress the reduction in randomness in generating a random bit string using the distributed randomness of an optical fiber as an entropy source.

[0009] A random number generator according to one aspect of the present disclosure inputs multiple time series measurement results of the scattered light intensity of light propagating within an optical transmission path, calculates the difference between the multiple measurement results, and assigns a bit string to the calculated difference to generate a random bit string.

[0010] According to the present disclosure, it is possible to suppress a decrease in randomness in generating a random bit string using the distributed randomness of an optical fiber as an entropy source.

[0011] FIG. 1 is a diagram showing an example of a method for assigning a bit string to an OTDR measurement result. FIG. 2 is a diagram showing an example of the configuration of a random number generator. FIG. 3 is a diagram showing an outline of how element numbers are assigned to scattered light intensities and differences are calculated. FIG. 4 is a diagram showing an example of a method for assigning a bit string to difference information. FIG. 5 is a diagram showing an example of a configuration using scattered light intensities of multiple transmission channels. FIG. 6 is a diagram showing an example of a configuration using scattered light intensities of multiple optical transmission paths. FIG. 7 is a diagram showing an example of a configuration for generating multiple scattered light intensity information by applying an electrical delay to measurement results. FIG. 8 is a diagram showing an example of a configuration for generating multiple scattered light intensity information by applying an optical delay to scattered light. FIG. 9 is a diagram showing an example of multiple scattered light intensity information obtained by an OTDR. FIG. 10 is a diagram showing the difference between multiple scattered light intensity information. FIG. 11 is a diagram showing the relationship between the intensity division width and the maximum allowable delay amount. FIG. 12 is a diagram showing an example of the hardware configuration of a random number generator.

[0012] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0013] [Generation of Random Bit String Based on Optical Response] A method for generating a random bit string from the optical response of an optical fiber will be described with reference to FIG.

[0014] OTDR is a measurement technology used to evaluate the distributed optical response of optical fibers. OTDR measurement results include the distributed random response of the optical fiber. Figure 1 shows an example of an OTDR measurement result for an optical fiber. The distributed randomness of the optical fiber is acquired as time-series data of the backscattered light intensity, as shown in Figure 1. In other words, light scattered near the input end of the test light is observed at an earlier time, and light scattered at a position farther from the input end is observed at a later time.

[0015] The received light intensity of backscattered light varies randomly, reflecting the random optical response of the optical fiber. By converting the received light intensity into a bit string, a random bit string based on the random optical response of the optical fiber can be generated. In the example shown in Figure 1, the received light intensity is divided into eight levels in the range of 0.002 to 0.006, with an intensity width of 0.0005 (hereinafter referred to as the intensity division width), and a three-bit bit string is assigned to each range. A random bit string can be generated by outputting the bit string assigned to the range to which the received light intensity belongs for each light reception time. Here, we have shown an example in which the received light intensity of backscattered light is divided into eight using OTDR measurement results, but OFDR measurement results can also be used, or the number of divisions can be increased.

[0016] Due to losses incurred by the test light and backscattered light as they propagate through the optical fiber, the received light intensity gradually decreases over time. This gradual decrease in received light intensity reduces the randomness of the generated random bit string. For example, in the case of Figure 1, the first half of the random bit string is likely to contain bits such as 111, 110, and 101, while the second half is likely to contain bits such as 010, 001, and 000.

[0017] Therefore, in the present disclosure, the influence of this gradual decrease in the intensity of received light is eliminated, and the decrease in randomness in the generation of random bit strings is suppressed.

[0018] [Configuration of Random Number Generator] An example of the configuration of the random number generator 10 of this embodiment will be described with reference to Fig. 2. The random number generator 10 shown in Fig. 2 includes an input unit 11, a labeling unit 12, a difference detection unit 13, a conversion unit 14, an output unit 15, a determination unit 16, and a determination display unit 17.

[0019] The input unit 11 receives multiple pieces of scattered light intensity information A and B. The scattered light intensity information is a time-series measurement result of the scattered light intensity of light propagating through an optical transmission path (optical fiber). It is preferable that the multiple pieces of scattered light intensity information decrease to the same extent over time. In other words, it is sufficient that the stationary components of the multiple pieces of scattered light intensity information are sufficiently similar. The multiple pieces of scattered light intensity information may be, for example, the scattered light intensity for each transmission channel obtained by splitting backscattered light in the optical transmission path into individual transmission channels, the scattered light intensity of each piece of backscattered light in multiple optical transmission paths, or the scattered light intensity and the scattered light intensity obtained by adding a delay to the scattered light intensity.

[0020] The input unit 11 may be provided with a light receiver to receive scattered light and acquire the scattered light intensity, or may input the scattered light intensity measured by an external light receiver.

[0021] The labeling unit 12 samples each of the scattered light intensity information A and B to obtain discrete time-series data of the scattered light intensity. Specifically, as shown in Fig. 3, the labeling unit 12 assigns an element number to each of the scattered light intensity information A and B in accordance with the passage of time (sampling timing). The labeling unit 12 increments the element number in accordance with the passage of time, and assigns the same element number to scattered light intensity information A and B that are input at the same timing.

[0022] The difference detection unit 13 finds the difference between the scattered light intensity information A and B. Specifically, as shown in Fig. 3, the difference detection unit 13 calculates the difference between the linear values ​​of scattered light intensity information with the same element number, assigns the element number to the calculation result, and outputs the difference information in the order of the element numbers.

[0023] The conversion unit 14 converts the difference information into a bit string. Specifically, the conversion unit 14 compares the difference information of each element with a preset intensity division width and an assigned bit string in the order of element numbers, and assigns a bit string to each element.

[0024] In the example of Figure 4, the received light intensity is divided into 8 levels in the range from 0.0005 to 0.0025 with an intensity division width of 0.00025, and a 3-bit bit string is assigned to each range. The dashed line in Figure 4 is the scattered light intensity waveform of the single-mode fiber evaluated with an OTDR, and the solid line is the waveform generated by calculating the difference in linear values ​​of two scattered light intensity waveforms obtained from OTDR measurements using two orthogonal polarized waves. In other words, the solid line corresponds to the difference information determined by the difference detection unit 13.

[0025] When generating a bit string based on the dashed line, there is a concern that the resulting bit pattern will be biased depending on the measurement time. This requires the removal of loss components through methods such as function fitting. On the other hand, the solid line removes the gradual intensity change components caused by optical fiber loss by taking the difference between the two scattered light intensity waveforms. As a result, it is expected that the bias in the bit pattern depending on the measurement time will be reduced. This random number generator 10 does not require processing such as function fitting, improving real-time performance.

[0026] The output unit 15 converts the bit string into an arbitrary data format and outputs it as electronic data.

[0027] The determination unit 16 determines whether the loss component has been appropriately removed from the difference information. The determination unit 16 extracts a gradual change in intensity by, for example, fitting a polynomial function to the difference detection unit 13, and makes a determination based on the magnitude relationship between the maximum change in intensity and the intensity division width. The determination unit 16 may also perform straight-line fitting to the output of the difference detection unit 13 and make a determination based on the slope of the line.

[0028] The determination display unit 17 displays the determination result of the determination unit 16. For example, the determination unit 16 displays the determination result as pass or fail.

[0029] Although the random number generator 10 receives two pieces of scattered light intensity information A and B, it may also receive three or more pieces of scattered light intensity information. In this case, the difference between any two pieces of scattered light intensity information can be used.

[0030] The random number generator 10 does not need to include the determination unit 16 and the determination display unit 17 .

[0031] [Method for Acquiring Scattered Light Intensity Information] Next, a method for acquiring a plurality of scattered light intensity information will be described.

[0032] Fig. 5 shows an example of the configuration of a random number generation system that acquires scattered light intensity information from multiple transmission channels. The random number generation system shown in the figure comprises a random number generator 10, a splitter 120, and two or more light receiving units 130. The random number generator 10 is the same as that shown in Fig. 2, so a duplicated description will be omitted here. The random number generator 10 may comprise the light receiving unit 130, and may also comprise the splitter 120.

[0033] The light source 110 inputs test light into the optical transmission line (optical fiber) 100 .

[0034] The demultiplexer 120 receives backscattered light generated when the test light propagates through the optical transmission line 100 and demultiplexes it into individual transmission channels. The optical transmission line 100 has two or more transmission channels. The transmission channels here may be any of wavelengths, polarizations, cores, and spatial modes.

[0035] Each of the light receiving units 130 receives the backscattered light of each transmission channel demultiplexed by the demultiplexer 120, and inputs the light intensity data of the backscattered light of each transmission channel (scattered light intensity information A, B) to the random number generator 10. The light intensity data is time-series light intensity information of the backscattered light, and includes the distributed random characteristics of the optical fiber and intensity variation components caused by losses in the optical fiber.

[0036] The difference detector 13 calculates the difference between the scattered light intensity information A and B and outputs the difference information. When the loss difference between the transmission channels is sufficiently small, the difference information eliminates the light intensity variation component caused by the loss in the optical fiber. Since the distributed optical response of the optical fiber is considered to differ between the transmission channels, the difference information includes the randomness of each transmission channel.

[0037] The conversion unit 14 generates a random bit string from the difference information.

[0038] Fig. 6 is a diagram showing an example of the configuration of a random number generation system that acquires scattered light intensity information from multiple optical transmission paths. The random number generation system shown in the figure includes a random number generator 10 and two or more light receiving units 130. The random number generator 10 is the same as that shown in Fig. 2, so a duplicated description will be omitted here. The random number generator 10 may also include the light receiving units 130.

[0039] The light source 110 inputs test light into the two optical transmission lines 100 .

[0040] Each of the light receiving units 130 receives backscattered light generated during propagation through each optical transmission line 100, and inputs light intensity data (scattered light intensity information A, B) of the backscattered light in each optical transmission line 100 to the random number generator 10. The light intensity data is time-series light intensity information of the backscattered light, and includes the distributed random characteristics of the optical fiber and intensity change components caused by losses in the optical fiber.

[0041] The difference detector 13 finds the difference between the scattered light intensity information A and B and outputs the difference information. When the optical fiber loss in each optical transmission line 100 is the same or sufficiently small, the difference information removes the light intensity variation component caused by the optical fiber loss. Since the distributed randomness of the optical fiber is thought to differ for each optical transmission line 100, the difference information includes the randomness caused by each optical transmission line 100.

[0042] The conversion unit 14 generates a random bit string from the difference information.

[0043] 5 and 6 may be combined. Specifically, the backscattered light of each optical transmission line 100 is separated into individual transmission channels, and distribution characteristics are measured for each of the separated transmission channels to obtain a plurality of optical intensity data, which are then input to the random number generator 10.

[0044] In the systems of Figures 5 and 6, there is a possibility that the loss varies for each transmission channel and optical transmission path. In the random number generation system of Figure 7, one measurement result is duplicated and one of the measurement results is electrically delayed to generate multiple pieces of scattered light intensity information. The random number generation system shown in the figure comprises a random number generator 10, a light receiving unit 130, a branching unit 140, and a delay unit 150. The random number generator 10 is the same as that shown in Figure 2, so a duplicated description will be omitted here. The random number generator 10 may comprise a branching unit 140 and a delay unit 150, and may also comprise a light receiving unit 130.

[0045] The light source 110 inputs test light into the optical transmission line 100 .

[0046] The light receiving unit 130 receives backscattered light generated when the test light propagates through the optical transmission line 100, and outputs light intensity data of the backscattered light. The light intensity data is time-series light intensity information of the backscattered light, and includes intensity variation components caused by the distributed random characteristics of the optical fiber and losses in the optical fiber.

[0047] The branching unit 140 duplicates the light intensity data, inputs one of the light intensity data (scattered light intensity information A) to the random number generator 10 , and inputs the other light intensity data to the delay unit 150 .

[0048] The delay unit 150 applies an electrical delay to the light intensity data, and then inputs the delayed light intensity data (scattered light intensity information B) to the random number generator 10. For example, the delay unit 150 delays the light intensity data by holding the light intensity data for a certain period of time before outputting it.

[0049] The random number generator 10 receives light intensity data without delay (scattered light intensity information A) and light intensity data with a delay (scattered light intensity information B).

[0050] The difference detector 13 finds the difference between the scattered light intensity information A and B and outputs the difference information.

[0051] The conversion unit 14 generates a random bit string from the difference information.

[0052] 7, by applying an optimally designed delay to the delay unit 150 in response to gradual changes in optical intensity caused by losses in the optical fiber, the difference detection unit 13 can remove the intensity changes caused by losses and extract only the distributed random characteristics of the optical fiber. The calculation of the optimal delay will be described later.

[0053] In the random number generation system of Fig. 8, backscattered light is split into multiple backscattered light beams, and one of the backscattered light beams is optically delayed to generate multiple scattered light intensity information. The random number generation system shown in the figure includes a random number generator 10, an optical splitter 160, an optical delay unit 170, and two or more light receiving units 130. The random number generator 10 is the same as that shown in Fig. 2, so a duplicated description will be omitted here. The random number generator 10 may include the light receiving unit 130, or may further include the optical splitter 160 and the optical delay unit 170.

[0054] The light source 110 inputs test light into the optical transmission line 100 .

[0055] The optical branching unit 160 branches backscattered light generated when the test light propagates through the optical transmission line 100, and directs one branch to the light receiving unit 130, while directing the other branch to the light receiving unit 130 via the optical delay unit 170. The system in Fig. 8 includes the optical delay unit 170 and is designed so that the optical path lengths are different in each branch path.

[0056] Each of the light receiving sections 130 receives backscattered light and inputs light intensity data of each backscattered light (scattered light intensity information A, B) to the random number generator 10 .

[0057] The difference detector 13 finds the difference between the scattered light intensity information A and B and outputs the difference information.

[0058] The conversion unit 14 generates a random bit string from the difference information.

[0059] By designing an optimal delay amount for the gradual change in light intensity caused by loss in the optical fiber, the difference detection unit 13 can remove the intensity change caused by loss and extract only the distributed random characteristics of the optical fiber.

[0060] [Relationship Between Loss and Optimum Delay Amount] Next, the relationship between loss and optimum delay amount will be described.

[0061] Figure 9 shows an example of an optical intensity data string obtained from an OTDR waveform when the optical transmission line loss is 0.2 dB / km, the optical transmission line group delay time is 4.9 μs / km, and the optical intensity sampling interval is 1 μs. The black circle data string has a delay difference of 10 elements (10 μs) compared to the white circle data string. The horizontal axis represents the element number assigned to each element in the data string, and the vertical axis represents the measured optical intensity. The vertical axis is normalized by the optical intensity at element number 0 in the black circle data string. Since this explains intensity changes due to loss, random components of the optical intensity are not included. Due to loss in the optical transmission line, the measured optical intensity decreases as the element number increases (over time).

[0062] Figure 10 shows the intensity difference between the black circle data string and the white circle data string. The horizontal axis is the element number, and the vertical axis is the intensity difference. The decrease in the intensity difference over time is suppressed compared to the data string in Figure 9. The intensity difference decreases as the element number increases, converging to zero.

[0063] Therefore, when the intensity difference at element number 0 (hereinafter referred to as the maximum intensity difference) is equal to or less than the intensity division width, sufficient loss component suppression is achieved.

[0064] Figure 11 shows the conditions for the intensity division width and the maximum allowable delay amount required to sufficiently suppress the maximum intensity difference. The horizontal axis represents the normalized intensity division width, which is the intensity division width normalized by the light intensity at element number 0, and the vertical axis represents the maximum allowable delay amount. The normalized intensity division width is the ratio of the intensity division width to the maximum received light intensity.

[0065] In the configuration of FIG. 7, the maximum intensity difference ΔI and the number of delay elements D e are related by the following formula:

[0066]

[0067] Here, α (dB / km) is the optical loss coefficient, t (μs) is the sampling interval of the optical intensity, and τ (μs / km) is the group delay time of the optical transmission line.

[0068] When ΔI≦R for the normalized intensity division width R, ΔI is considered to be sufficiently suppressed. Therefore, the maximum allowable delay Dm is expressed by the following formula:

[0069]

[0070] The solid line, dashed line, and dashed line in Figure 11 represent the cases where the loss α of the optical transmission line is 0.15 dB / km, 0.20 dB / km, and 0.25 dB / km, respectively. 0.15 dB / km corresponds to the lowest loss of currently widely used optical fibers.

[0071] Therefore, in the configuration of FIG. 7, the number of delay elements D e It is desirable to set R so that it satisfies the following equation using τ and t.

[0072]

[0073] In the case of the optical delay configuration shown in FIG. 8, the optimal optical delay amount D l is D l =D e It can be designed as t.

[0074]

[0075] Conversely, if the delay is too small, not only the intensity change due to loss but also the random component may be reduced. In the received light intensity time series data obtained by OTDR, the correlation between elements separated by half or more of the test light pulse width is considered to be sufficiently small, so it is desirable to add a delay of half or more of the test light pulse width.

[0076] As described above, the random number generator 10 of this embodiment receives multiple time-series measurement results of the scattered light intensity of light propagating through the optical transmission line 100, calculates the difference between the multiple measurement results, and assigns a bit string to the calculated difference to generate a random bit string. This makes it possible to suppress the effect of a decrease in received light intensity caused by losses incurred by the test light and backscattered light as they propagate through the optical fiber, thereby suppressing a decrease in randomness in the generation of the random bit string. The random number generator 10 can suppress a decrease in randomness through simple signal processing and optical processing, improving the real-time performance of random number generation.

[0077] The random number generator 10 described above can be, for example, a general-purpose computer system including a central processing unit (CPU) 901, memory 902, storage 903, communication device 904, input device 905, and output device 906, as shown in Figure 12. In this computer system, the random number generator 10 is realized by the CPU 901 executing a predetermined program loaded onto the memory 902. This program can be recorded on a computer-readable non-transitory recording medium such as a magnetic disk, optical disk, or semiconductor memory, or can be distributed via a network.

[0078] REFERENCE SIGNS LIST 10 Random number generator 11 Input unit 12 Labeling unit 13 Difference detection unit 14 Conversion unit 15 Output unit 16 Determination unit 17 Determination display unit 100 Optical transmission path 110 Light source 120 Demultiplexer 130 Light receiving unit 140 Branching unit 150 Delay unit 160 Optical branching unit 170 Optical delay unit

Claims

1. A random number generator that inputs multiple time-series measurement results of the scattered light intensity of light propagating within an optical transmission line, calculates the difference between the multiple measurement results, and assigns a bit string to the calculation result of the difference to generate a random bit string.

2. A random number generator according to claim 1, wherein the multiple measurement results are measurement results obtained by measuring scattered light intensity in multiple optical transmission paths or multiple transmission channels.

3. A random number generator according to claim 1, wherein the random number generator branches the time series measurement results of scattered light intensity or branches the scattered light and applies a delay to generate one of the plurality of measurement results.

4. A random number generator according to claim 3, wherein the delay is determined based on an intensity division width for allocating the bit string and the maximum intensity difference between the plurality of measurement results.

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