Low frequency secure communication method and system

Abstract

This application discloses a low-frequency secure communication method and system, relating to the field of wireless communication security technology. The method includes: at the transmitting end, generating a true random key sequence equal in length to the plaintext data block; further generating a ciphertext symbol sequence; mapping the ciphertext symbol sequence and the true random key sequence to obtain a first baseband signal and a second baseband signal, and using the first and second baseband signals to drive the rotation of a first mechanical antenna and a second mechanical antenna respectively, generating a first alternating magnetic field signal and a second alternating magnetic field signal; at the receiving end, synchronously receiving a composite signal of the first and second alternating magnetic field signals, and generating a three-axis magnetic field signal sequence; demodulating the three-axis magnetic field signal sequence to obtain a demodulated ciphertext symbol sequence and a demodulated true random key sequence; and further obtaining the plaintext data block. This application can overcome the security defects existing in current mechanical antenna communication technology and improve communication security.

Description

Technical Field

[0001] This application relates to the field of wireless communication security technology, and in particular to a low-frequency secure communication method and system. Background Technology

[0002] Traditional low-frequency transmitting systems rely on electrically small antennas and power amplifiers to generate oscillating currents, thereby radiating electromagnetic waves. However, electrically small antennas are typically large in size, have low radiation efficiency, high power consumption, and are inconvenient to move. To overcome these shortcomings, researchers have focused on miniaturizing and reducing the power consumption of low-frequency transmitting antennas, triggering a paradigm shift in antenna technology.

[0003] Unlike traditional antennas, mechanical antennas utilize mechanical energy to drive the movement of charges and magnetic dipoles, converting a static magnetic field into an alternating magnetic field and radiating electromagnetic waves outwards. Their efficiency in the low-frequency band is far superior to that of traditional antennas. Mechanical antennas emitting low-frequency electromagnetic waves possess advantages such as low power consumption, high stability, strong penetration, and high portability. They can achieve communication in challenging environments where traditional high-frequency signals suffer severe attenuation, such as underwater and submarine communication, underground and mine communication, and cross-medium communication. They also have significant application value in beyond-line-of-sight communication and navigation denial, civilian IoT, and special sensing. However, existing research on mechanical antennas mainly focuses on optimizing mechanical structure design and modulation parameters, with relatively less attention paid to communication security issues such as the detectability and confidentiality of low-frequency signals.

[0004] In nature, atmospheric noise generated by phenomena such as lightning introduces significant background noise into the low-frequency band. Simultaneously, mechanical antennas typically operate at low power, and their emitted signal energy is easily submerged under this high-intensity background noise. From a communication security perspective, this makes mechanical antenna communication methods highly concealed, making them difficult to detect by traditional eavesdropping devices that rely solely on energy detection. However, the low-frequency signals radiated by rotating permanent magnet mechanical antennas typically exhibit high signal-to-noise ratios and clear frequency domain characteristics in the near field, making waveform identification and decoding sufficient through simple spectrum analysis and demodulation techniques. This makes short-range communication information from rotating permanent magnet mechanical antennas easily intercepted, posing a significant challenge to the security of short-range communication based on rotating permanent magnet mechanical antennas. Furthermore, the omnidirectional nature of low-frequency signal transmission by rotating permanent magnet mechanical antennas further reduces the security of information transmission.

[0005] Therefore, based on the above problems, there is an urgent need to provide a low-frequency secure communication method that can overcome the security defects in existing mechanical antenna communication technology and improve communication security. Summary of the Invention

[0006] The purpose of this application is to provide a low-frequency secure communication method that can overcome the security defects existing in the current mechanical antenna communication technology and improve communication security.

[0007] To achieve the above objectives, this application provides the following solution: In a first aspect, this application provides a low-frequency secure communication method, comprising: At the sending end, a truly random key sequence with the same length as the plaintext data block is generated; Generate a ciphertext code sequence based on the plaintext data block and the true random key sequence; The ciphertext symbol sequence and the true random key sequence are mapped respectively to obtain a first baseband signal and a second baseband signal; the first baseband signal and the second baseband signal are modulated using different frequency pairs; The first mechanical antenna is driven to rotate using the first baseband signal to generate a first alternating magnetic field signal carrying a ciphertext code sequence; at the same time, the second mechanical antenna is driven to rotate using the second baseband signal to generate a second alternating magnetic field signal carrying a true random key sequence. At the receiving end, a composite signal of the first alternating magnetic field signal and the second alternating magnetic field signal is received synchronously; and a triaxial magnetic field signal sequence is determined based on the composite signal. The triaxial magnetic field signal sequence is demodulated to obtain the demodulated ciphertext code sequence and the demodulated true random key sequence; based on the demodulated ciphertext code sequence and the demodulated true random key sequence, the plaintext data block is obtained.

[0008] Secondly, this application provides a low-frequency secure communication system, including: a transmitter and a receiver; The transmitting end includes: a transmitting host computer and a magnetic field signal generation module; The transmitting host computer is used to generate a true random key sequence with the same length as the plaintext data block; generate a ciphertext code sequence based on the plaintext data block and the true random key sequence; and map the ciphertext code sequence and the true random key sequence to obtain a first baseband signal and a second baseband signal. The magnetic field signal generation module is used to drive the first mechanical antenna to rotate using the first baseband signal to generate a first alternating magnetic field signal carrying a ciphertext code sequence; at the same time, it uses the second baseband signal to drive the second mechanical antenna to rotate to generate a second alternating magnetic field signal carrying a true random key sequence. The receiving end includes: a signal conversion module and a receiving host computer; The signal conversion module is used to synchronously receive the composite signal of the first alternating magnetic field signal and the second alternating magnetic field signal; and to determine the triaxial magnetic field signal sequence based on the composite signal; The receiving host computer is used to demodulate the triaxial magnetic field signal sequence to obtain the demodulated ciphertext code sequence and the demodulated true random key sequence; based on the demodulated ciphertext code sequence and the demodulated true random key sequence, the plaintext data block is obtained.

[0009] According to the specific embodiments provided in this application, this application has the following technical effects: This application provides a low-frequency secure communication method and system. First, a truly random key sequence of equal length to the plaintext data block is generated at the transmitting end, and then a ciphertext codeword sequence is generated. This method is a one-time cipher. The core encryption and decryption method of Pad (OTP) overcomes the security defects of existing mechanical antenna communication technology. By mapping the ciphertext symbol sequence and the true random key sequence separately, a first baseband signal and a second baseband signal are obtained. The first baseband signal is used to drive the rotation of the first mechanical antenna to generate a first alternating magnetic field signal carrying the ciphertext symbol sequence. At the same time, the second baseband signal is used to drive the rotation of the second mechanical antenna to generate a second alternating magnetic field signal carrying the true random key sequence. The ciphertext symbol sequence and the true random key sequence are transmitted through the two mechanical antennas respectively, further improving the security of data transmission. At the receiving end, the composite signal of the first alternating magnetic field signal and the second alternating magnetic field signal are received synchronously, and a three-axis magnetic field signal sequence is generated. The three-axis magnetic field signal sequence is further demodulated to obtain the demodulated ciphertext symbol sequence and the demodulated true random key sequence, and finally the plaintext data block is obtained. The demodulation process can extract signal energy to the maximum extent in extremely low SNR environments, improve noise immunity, and solve the problem of unreliable communication under low signal-to-noise ratio conditions in the far field. This application combines the high efficiency of low-frequency radiation capability of mechanical antennas with the security of OTP, enabling low-frequency secure communication systems to avoid direct detection and cracking in the near field, while utilizing the low detection rate and high communication reliability under far-field conditions, effectively solving the problem of balancing security and performance faced by traditional low-frequency communication in challenging environments such as cross-medium environments. Attached Figure Description

[0010] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0011] Figure 1 This is a flowchart illustrating a low-frequency secure communication method according to an embodiment of this application; Figure 2 This is a schematic diagram of the magnetic field model and coordinate system of RPMA in one embodiment of this application; Figure 3 This is a schematic diagram of the radiation direction of the RPMA in one embodiment of this application; Figure 4 This is a configuration diagram of a low-frequency secure communication system in one embodiment of this application. Detailed Implementation

[0012] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0013] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0014] In one exemplary embodiment, such as Figure 1 As shown, a low-frequency secure communication method is provided, including the following S1 to S6. Wherein: S1: At the sending end, generate a true random key sequence with the same length as the plaintext data block.

[0015] Specifically, at the sending end, a truly random key sequence is generated that is strictly equal in length to the plaintext data block; the plaintext data block consists of a series of binary plaintext symbols; and the truly random key sequence consists of a series of binary key symbols.

[0016] S2: Generate a ciphertext code sequence based on the plaintext data block and the true random key sequence.

[0017] Perform a bit-by-bit XOR operation on the plaintext data block and the true random key sequence to generate the corresponding binary ciphertext code sequence.

[0018] S3: Map the ciphertext code sequence and the true random key sequence respectively to obtain the first baseband signal and the second baseband signal.

[0019] The first baseband signal and the second baseband signal are modulated using different frequency pairs. S3 specifically includes: S31: Map 0s in the ciphertext code sequence to frequencies. And map the 1 in the ciphertext code sequence to a frequency. The first frequency pair is obtained; and the first frequency pair is used to modulate the first baseband signal.

[0020] The ciphertext code sequence is binary. During the mapping process, "0" and "1" in the binary code are mapped to frequencies respectively. With frequency Thus forming a first frequency pair for modulating the first mechanical antenna. The first frequency pair is used to modulate the first baseband signal.

[0021] S32: Map 0s in the true random key sequence to frequencies. And map the 1 in the true random key sequence to a frequency. The second frequency pair is obtained; and the second frequency pair is used to modulate the second baseband signal.

[0022] A truly random key sequence is binary. During the mapping process, "0" and "1" in the binary sequence are mapped to frequencies respectively. With frequency This constitutes a second frequency pair for modulating the second mechanical antenna. The second frequency pair is used to modulate the second baseband signal.

[0023] In this application, the first frequency pair With the second frequency pair There is no overlap in the spectrum, and the frequency interval is at least 1.5 times the symbol rate.

[0024] S4: The first mechanical antenna is driven to rotate using the first baseband signal to generate a first alternating magnetic field signal carrying the ciphertext code sequence; at the same time, the second mechanical antenna is driven to rotate using the second baseband signal to generate a second alternating magnetic field signal carrying the true random key sequence.

[0025] Specifically, a first baseband signal is input to a first driving circuit, which drives a first mechanical antenna to rotate mechanically. The rotating magnetic dipole generates an alternating magnetic field (physical field conversion) in space, thereby radiating a first alternating magnetic field signal carrying the ciphertext code sequence. Simultaneously, a second baseband signal is input to a second driving circuit, which drives a second mechanical antenna to rotate mechanically, thereby radiating a second alternating magnetic field signal carrying a true random key sequence. The first and second mechanical antennas complete the external radiation and transmission of information.

[0026] S5: At the receiving end, the composite signal of the first alternating magnetic field signal and the second alternating magnetic field signal is received synchronously; and the triaxial magnetic field signal sequence is determined based on the composite signal.

[0027] S5 specifically includes: S51: Uses a magnetic sensor to convert composite signals into continuous analog voltage signals.

[0028] Specifically, this application utilizes a highly sensitive magnetic sensor to synchronously receive a composite signal of a first alternating magnetic field signal and a second alternating magnetic field signal, and further performs magneto-electric conversion on the composite signal, that is, converts the composite signal into a continuous analog voltage signal (a continuous voltage or current in the circuit).

[0029] S52: Use a data acquisition card to convert analog voltage signals into a three-axis magnetic field signal sequence.

[0030] The magnetic sensor transmits the converted analog voltage signal to the data acquisition card, which then converts the analog voltage signal into a triaxial magnetic field signal sequence.

[0031] S6: Demodulate the triaxial magnetic field signal sequence to obtain the demodulated ciphertext code sequence and the demodulated true random key sequence; obtain the plaintext data block based on the demodulated ciphertext code sequence and the demodulated true random key sequence.

[0032] S6 specifically includes: S61: Based on the three-axis magnetic field signal sequence, the signal amplitude of each frequency in the first and second frequency pairs within the symbol period is extracted using a coherent demodulation algorithm.

[0033] This application employs a coherent demodulation algorithm based on a digital lock-in amplifier to accurately extract the signal amplitude of each frequency in the first and second frequency pairs within the symbol period from a triaxial magnetic field signal sequence. S61 specifically includes: S611: Determine the in-phase reference signal and quadrature reference signal for each frequency in the first and second frequency pairs.

[0034] Specifically, each frequency in the first and second frequency pairs Generate in-phase reference signal and quadrature reference signals ,in, For time.

[0035] S612: Multiply the triaxial magnetic field signal sequence with the in-phase reference signal and the quadrature reference signal within the symbol period to obtain the multiplication result.

[0036] S613: Use a digital integrator to accumulate and integrate the dot product result within the symbol period to obtain the corresponding in-phase and quadrature components.

[0037] Specifically, the dot product result is passed through a digital integrator in the symbol period. By accumulating and integrating within the range, the frequency can be obtained. The in-phase component corresponding to the component and orthogonal components .

[0038] S614: Determine the signal amplitude of the corresponding frequency within the symbol period based on the in-phase and quadrature components.

[0039] Specifically, the formula for calculating the signal amplitude is as follows: ; in, For frequency The signal amplitude within the symbol period.

[0040] S62: For the first frequency pair, compare the frequencies within the same symbol period using the first-order time difference method. With frequency The magnitude of the amplitude is determined, and the demodulated ciphertext code sequence is determined based on the comparison results.

[0041] The first-order time difference method and symbol decision process are as follows: For a given frequency, calculate the difference in amplitude between the current symbol period and the previous symbol period. Specifically, let... The amplitude of a specified frequency in the current symbol period. If the amplitude of this frequency in the previous symbol period is given, then the first-order time difference result for this frequency is: Because the first-order time-difference method can effectively filter out slowly changing background DC offset and low-frequency noise, it has stronger robustness to complex channel environments.

[0042] Subsequently, during symbol decision-making, the results obtained using the first-order time difference method are compared with those obtained using different frequencies (frequency) within the same symbol period. With frequency The magnitude of the differential amplitude is determined. Based on the comparison of the two sets of amplitude changes, a codeword decision is made, thereby further determining the demodulated ciphertext codeword sequence.

[0043] S63: For the second frequency pair, compare the frequencies within the same symbol period using the first-order time difference method. With frequency The magnitude of the amplitude is determined, and the demodulated true random key sequence is determined based on the comparison results.

[0044] For the second frequency pair, the results obtained using the first-order time difference method are also compared within the same symbol period. With frequency The magnitude of the differential amplitude is determined. Based on the comparison of the two sets of amplitude changes, a symbol decision is made to further determine the demodulated true random key sequence.

[0045] S64: Obtain the plaintext data block based on the demodulated ciphertext code sequence and the demodulated true random key sequence.

[0046] The original plaintext data block is recovered by performing a bit-by-bit XOR operation on the demodulated ciphertext code sequence and the demodulated true random key sequence.

[0047] Furthermore, this application uses a coherent demodulation algorithm to demodulate the triaxial magnetic field signal sequence under far-field low signal-to-noise ratio conditions; and uses an incoherent energy detection demodulation algorithm to demodulate the triaxial magnetic field signal sequence under near-field high signal-to-noise ratio conditions. Moreover, the coherent demodulation algorithm and the incoherent energy detection demodulation algorithm can automatically switch according to different signal-to-noise ratio environments, enabling this application to automatically adapt to different signal-to-noise ratio environments.

[0048] Based on the same inventive concept, this application also provides a low-frequency secure communication system for implementing the low-frequency secure communication method. The solution provided by this system is similar to the implementation scheme described in the low-frequency secure communication method; therefore, the specific limitations in one or more embodiments of the low-frequency secure communication system provided below can be found in the limitations of the low-frequency secure communication method described above, and will not be repeated here.

[0049] In one exemplary embodiment, a low-frequency secure communication system is provided, including a transmitter and a receiver; The transmitting end includes a host computer and a magnetic field signal generation module.

[0050] The transmitting host computer is used to generate a truly random key sequence with the same length as the plaintext data block; based on the plaintext data block and the truly random key sequence, it generates a ciphertext symbol sequence; and it maps the ciphertext symbol sequence and the truly random key sequence to obtain the first baseband signal and the second baseband signal. The transmitting host computer can dynamically adjust the center frequencies of the first and second frequency pairs to avoid adverse effects such as periodic narrowband interference, spurious signals, and harmonics.

[0051] The magnetic field signal generation module includes an electronic speed controller unit, a motor unit, a first mechanical antenna, and a second mechanical antenna. The module uses a first baseband signal to drive the first mechanical antenna to rotate, generating a first alternating magnetic field signal carrying a ciphertext code sequence; simultaneously, it uses a second baseband signal to drive the second mechanical antenna to rotate, generating a second alternating magnetic field signal carrying a truly random key sequence.

[0052] In one exemplary embodiment, the electronic speed controller unit is connected to a transmitting host computer; and the electronic speed controller unit includes a first electronic speed controller and a second electronic speed controller; the first electronic speed controller is used to generate a first control signal based on a first baseband signal; the second electronic speed controller is used to generate a second control signal based on a second baseband signal. The motor unit includes a first motor and a second motor, the motors being selectable as brushless DC motors. The first brushless DC motor (first motor) is driven by the first electronic speed controller and is used to generate a first drive signal based on the first control signal; the second brushless DC motor (second motor) is driven by the second electronic speed controller and is used to generate a second drive signal based on the second control signal. A first mechanical antenna is used to generate a first alternating magnetic field signal carrying a ciphertext symbol sequence based on the first drive signal; the second mechanical antenna is used to generate a second alternating magnetic field signal carrying a true random key sequence based on the second drive signal. In this application, both the first and second mechanical antennas are rotating permanent magnet mechanical antennas. The core radiating components of both antennas are radially magnetized permanent magnets made of neodymium iron boron material with a remanent magnetization greater than 1.0 Tesla. Furthermore, the permanent magnets are cylindrical with a length-to-diameter ratio greater than 5:1 to optimize their magnetic moment, making the operating point closer to the remanent magnetization point, thereby radiating a stronger external quasi-static magnetic field and a time-varying magnetic field. The first and second mechanical antennas are physically encapsulated within identical device housings and are mechanically vibration-isolated using damping materials.

[0053] The receiving end includes a signal conversion module and a receiving host computer.

[0054] The signal conversion module includes a magnetic sensor and a data acquisition card. It synchronously receives a composite signal from a first alternating magnetic field signal and a second alternating magnetic field signal, and determines a triaxial magnetic field signal sequence based on the composite signal. Specifically, the magnetic sensor synchronously receives the composite signal and converts it into a continuous analog voltage signal. The magnetic sensor is one of a fluxgate magnetometer, a giant magnetoresistance sensor, or a magnetometer. The data acquisition card converts the analog voltage signal into a triaxial magnetic field signal sequence. Furthermore, the signal conversion module includes a power supply to power the magnetic sensor and the data acquisition card.

[0055] The receiving host computer demodulates the triaxial magnetic field signal sequence to obtain the demodulated ciphertext code sequence and the demodulated true random key sequence; and obtains the plaintext data block based on the demodulated ciphertext code sequence and the demodulated true random key sequence.

[0056] In an exemplary embodiment, the magnetic field and radiation pattern of a rotating permanent magnet antenna (RPMA) are modeled and analyzed to determine the main working plane or main working axis of the RPMA. A schematic diagram of the magnetic field model and coordinate system of the RPMA is shown below. Figure 2 As shown in the diagram, the radiation direction of a single RPMA is as follows: Figure 3 As shown. Among them, The magnetic flux density generated radially by the mechanical antenna. The magnetic flux density generated by the mechanical antenna along the radial pole angle direction. This represents the magnetic flux density generated by the mechanical antenna along the azimuth direction.

[0057] The structural and system design of RPMA (Low Frequency Secure Communication System) is performed. The RPMA system includes a transmitter and a receiver. The configuration diagram of the low frequency secure communication system is shown below. Figure 4 As shown. The transmitting end mainly includes a transmitting host computer (computer) and a magnetic field signal generation module. The magnetic field signal generation module includes a speed controller (electronic speed controller unit) and an RPMA. The RPMA mainly consists of a motor, a mechanical antenna, and an organic material mounting shell. The assembly process of the shell avoids the use of metal structural components to minimize their influence on the magnetic field. The receiving end mainly includes a signal conversion module and a receiving host computer. The signal conversion module includes a magnetometer (magnetic sensor), a data acquisition card, and a power supply. Both the transmitting and receiving host computers include corresponding control software. The control software in the transmitting host computer implements functions such as ciphertext code sequence generation, OTP encryption, and mapping. The receiving host computer integrates a digital lock-in amplifier and a decryption module.

[0058] After the system is powered on, a plaintext data block, such as "HELLO," is generated and sent to the host computer. This plaintext data block is then converted into a binary sequence according to rules (such as the American Standard Code for Information Interchange, ASCII table). Simultaneously, a true random key sequence of the same length as the plaintext data block is generated. A bit-by-bit XOR operation is performed between the plaintext data block and the true random key sequence to obtain the ciphertext symbol sequence. Subsequently, bit-frequency mapping is performed on both the ciphertext symbol sequence and the true random key sequence: in the ciphertext symbol sequence, "0" is mapped to 6Hz and "1" to 16Hz, resulting in the first frequency pair; in the true random key sequence, "0" is mapped to 10Hz and "1" to 20Hz, resulting in the second frequency pair. Both the first and second frequency pairs are within the operating bandwidth of the mechanical antenna and are sufficiently separated from each other spectrally. The mapped baseband signals are then sent to two electronic speed controllers to drive their respective motors (brushless DC motors). The motors drive the permanent magnets in the mechanical antenna to rotate, thereby radiating the corresponding alternating magnetic field signals.

[0059] The magnetometer at the receiving end receives the alternating magnetic field signal radiated by the mechanical antenna and converts it into an analog voltage signal. Simultaneously, a data acquisition card performs digital sampling, converting the analog voltage signal into a three-axis magnetic field signal sequence. The demodulation software on the host computer demodulates the three-axis magnetic field signal sequence. This software integrates a signal-to-noise ratio (SNR) estimation module. When the SNR is higher than 10dB, it automatically calls a simpler energy detection method (incoherent energy detection demodulation algorithm) to save computational resources. When the SNR is lower than 10dB, it automatically switches to a more computationally intensive but less SNR-sensitive lock-in amplification demodulation method (coherent demodulation algorithm).

[0060] This application overcomes the security deficiencies of existing mechanical antenna communication technologies by constructing a low-frequency secure communication system including dual mechanical antennas and introducing a one-time cipher. It provides a secure communication method that ensures secure information transmission at various communication distances, especially in the near field, enabling low-error-rate communication in scenarios with requirements for security, concealment, and penetration. Secondly, this application employs a demodulation algorithm for low signal-to-noise ratio (SNR). To balance communication quality in both near-field and far-field communication, this application uses digital lock-in amplifier technology for coherent demodulation at low SNR, ensuring that the receiver can still demodulate with a sufficient SNR under far-field communication conditions. The algorithm generates in-phase and quadrature reference signals for each frequency to be detected, multiplies and integrates the received triaxial magnetic field signal sequence with these signals to obtain the in-phase and quadrature components, and then calculates the signal amplitude. This method effectively extracts weak known frequency signals from strong noise backgrounds, ensuring demodulation reliability under harsh channel conditions. The system can also be designed in an adaptive mode, switching between incoherent and coherent demodulation based on the real-time signal-to-noise ratio to balance performance and complexity. Furthermore, this application also designs a mechanical antenna structure suitable for RPMA in the embodiments. To achieve efficient radiation, this application uses a rotating permanent magnet based on neodymium iron boron material as the core of the mechanical antenna. The permanent magnet is designed as a radially magnetized cylinder with a length-to-diameter ratio greater than 5:1. This structure allows its operating point to be closer to the remanent magnetization point, thereby radiating a stronger magnetic field. In the system design, the transmitting end includes a transmitting host computer, an electronic speed controller, a brushless DC motor, and a mechanical antenna; the receiving end includes a magnetic sensor, a data acquisition card, a power supply, and a receiving host computer. To further improve performance, damping materials are used to isolate the mechanical vibration of the dual antennas and prevent mutual interference. The transmitting host computer can also integrate dynamic frequency agility functionality to actively avoid environmental interference.

[0061] The advantages of this application are: (1) By transmitting the ciphertext symbol sequence and the true random key sequence through two mechanical antennas respectively, and combining them with the OTP encryption mechanism, even if the data of the two channels are intercepted simultaneously in the near field, the plaintext cannot be deciphered without knowing the specific frequency and encryption algorithm. The foundation of this security is mathematical unconditional security, which does not rely on computational complexity and improves the security of information transmission.

[0062] (2) It cleverly combines two security advantages. In the near field, it relies on the encryption strength of OTP to resist eavesdropping; in the far field, the low-power signal is naturally hidden in the environmental noise, providing low probability detection / interception characteristics, forming a dual security guarantee that is not easy to decipher in the near field and not easy to detect in the far field, taking into account the security and concealment of near-field and far-field communication conditions.

[0063] (3) The problem of unreliable communication under low signal-to-noise ratio in the far field is solved by the coherent demodulation algorithm, which enables the application to maintain stable communication under the condition of far-field communication, expands the effective communication distance and application scenarios, and ensures low signal-to-noise ratio communication capability.

[0064] (4) Low-frequency communication based on RPMA naturally has strong penetration. Combined with the architecture of this application, it becomes an ideal solution for cross-media communication scenarios such as underwater, underground, tunnel, and reinforced concrete buildings. That is, this application has built cross-media communication capabilities.

[0065] (5) It provides a complete solution from encryption, sending, receiving to demodulation and decryption, and optimizes the design of key hardware such as permanent magnet materials and structure to ensure the feasibility and engineering application value of the solution, and has high integration and practicality.

[0066] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0067] This document uses specific examples to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the methods and core ideas of this application. Furthermore, those skilled in the art will recognize that, based on the ideas of this application, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A low-frequency secure communication method, characterized in that, The low-frequency secure communication method includes: At the sending end, a truly random key sequence with the same length as the plaintext data block is generated; Generate a ciphertext code sequence based on the plaintext data block and the true random key sequence; The ciphertext symbol sequence and the true random key sequence are mapped respectively to obtain a first baseband signal and a second baseband signal; the first baseband signal and the second baseband signal are modulated using different frequency pairs; The first mechanical antenna is driven to rotate using the first baseband signal to generate a first alternating magnetic field signal carrying a ciphertext code sequence; at the same time, the second mechanical antenna is driven to rotate using the second baseband signal to generate a second alternating magnetic field signal carrying a true random key sequence. At the receiving end, a composite signal of the first alternating magnetic field signal and the second alternating magnetic field signal is received synchronously; and a triaxial magnetic field signal sequence is determined based on the composite signal. The triaxial magnetic field signal sequence is demodulated to obtain the demodulated ciphertext code sequence and the demodulated true random key sequence; based on the demodulated ciphertext code sequence and the demodulated true random key sequence, the plaintext data block is obtained.

2. The low-frequency secure communication method according to claim 1, characterized in that, The step of mapping the ciphertext code sequence and the true random key sequence to obtain the first baseband signal and the second baseband signal specifically includes: Map 0 in the ciphertext code sequence to frequency. And map the 1 in the ciphertext code sequence to a frequency. The first frequency pair is obtained; and the first frequency pair is used to modulate the first baseband signal; Map 0 in the true random key sequence to frequency. And map the 1 in the true random key sequence to a frequency. The second frequency pair is obtained; and the second frequency pair is used to modulate the second baseband signal.

3. The low-frequency secure communication method according to claim 1, characterized in that, The determination of the triaxial magnetic field signal sequence based on the composite signal specifically includes: A magnetic sensor is used to convert a composite signal into a continuous analog voltage signal; The analog voltage signal is converted into a triaxial magnetic field signal sequence using a data acquisition card.

4. The low-frequency secure communication method according to claim 2, characterized in that, The demodulation of the triaxial magnetic field signal sequence to obtain the demodulated ciphertext code sequence and the demodulated true random key sequence specifically includes: Based on the triaxial magnetic field signal sequence, the signal amplitude of each frequency in the first and second frequency pairs within the symbol period is extracted using a coherent demodulation algorithm. For the first frequency pair, the first-order time difference method is used to compare the frequencies within the same symbol period. With frequency The magnitude of the amplitude is determined, and the demodulated ciphertext code sequence is determined based on the comparison results; For the second frequency pair, the first-order time difference method is used to compare the frequencies within the same symbol period. With frequency The magnitude of the amplitude is determined, and the demodulated true random key sequence is determined based on the comparison results.

5. The low-frequency secure communication method according to claim 4, characterized in that, The step of extracting the signal amplitude of each frequency in the first and second frequency pairs within the symbol period based on the triaxial magnetic field signal sequence using a coherent demodulation algorithm specifically includes: Determine the in-phase reference signal and quadrature reference signal for each frequency in the first and second frequency pairs; The triaxial magnetic field signal sequence is multiplied by the in-phase reference signal and the quadrature reference signal within the symbol period to obtain the multiplication result. The dot product result is accumulated and integrated within the symbol period using a digital integrator to obtain the corresponding in-phase and quadrature components. Based on the in-phase and quadrature components, determine the signal amplitude of the corresponding frequency within the symbol period.

6. A low-frequency secure communication system for implementing the low-frequency secure communication method according to any one of claims 1-5, characterized in that, The low-frequency secure communication system includes: a transmitter and a receiver; The transmitting end includes: a transmitting host computer and a magnetic field signal generation module; The transmitting host computer is used to generate a true random key sequence with the same length as the plaintext data block; generate a ciphertext code sequence based on the plaintext data block and the true random key sequence; and map the ciphertext code sequence and the true random key sequence to obtain a first baseband signal and a second baseband signal. The magnetic field signal generation module is used to drive the first mechanical antenna to rotate using the first baseband signal to generate a first alternating magnetic field signal carrying a ciphertext code sequence; at the same time, it uses the second baseband signal to drive the second mechanical antenna to rotate to generate a second alternating magnetic field signal carrying a true random key sequence. The receiving end includes: a signal conversion module and a receiving host computer; The signal conversion module is used to synchronously receive the composite signal of the first alternating magnetic field signal and the second alternating magnetic field signal; and to determine the triaxial magnetic field signal sequence based on the composite signal; The receiving host computer is used to demodulate the triaxial magnetic field signal sequence to obtain the demodulated ciphertext code sequence and the demodulated true random key sequence; based on the demodulated ciphertext code sequence and the demodulated true random key sequence, the plaintext data block is obtained.

7. The low-frequency secure communication system according to claim 6, characterized in that, The magnetic field signal generation module includes: an electronic speed controller unit, a motor unit, a first mechanical antenna, and a second mechanical antenna; The electronic speed controller unit is used to generate a first control signal based on a first baseband signal; and simultaneously generate a second control signal based on a second baseband signal. The motor unit is used to generate a first drive signal according to a first control signal; and simultaneously generate a second drive signal according to a second control signal. The first mechanical antenna is used to generate a first alternating magnetic field signal carrying a ciphertext symbol sequence according to the first driving signal; The second mechanical antenna is used to generate a second alternating magnetic field signal carrying a true random key sequence according to the second driving signal.

8. The low-frequency secure communication system according to claim 6, characterized in that, The signal conversion module includes: a magnetic sensor and a data acquisition card; The magnetic sensor is used to synchronously receive a composite signal of a first alternating magnetic field signal and a second alternating magnetic field signal; and convert the composite signal into a continuous analog voltage signal. The data acquisition card is used to convert analog voltage signals into a three-axis magnetic field signal sequence.

9. The low-frequency secure communication system according to claim 8, characterized in that, The signal conversion module further includes: a power supply; The power supply is used to power the magnetic sensor and the data acquisition card.

10. The low-frequency secure communication system according to claim 8, characterized in that, The magnetic sensor is a fluxgate magnetometer or a magnetometer.

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