Soft information codeword reconstruction security key extraction method and system for rail transit

Abstract

A method and system for reconstructing a secure key from soft information codewords for rail transit, relating to the field of digital information transmission, includes the following steps: a transmitter and a legitimate receiver alternately transmit pilot signals to each other via a preset channel to calculate a channel characteristic sequence for the preset channel; the transmitter generates a master key from a preset random source and encodes the master key to obtain a codeword sequence; the transmitter generates an information interaction sequence based on a first characteristic sequence and the codeword sequence, and sends the information interaction sequence to the legitimate receiver; after receiving the information interaction sequence, the legitimate receiver reconstructs a second characteristic sequence based on the information interaction sequence to obtain a codeword structure sequence; the legitimate receiver decodes the codeword structure sequence to obtain a final key. This application is used to improve the consistency rate of keys between legitimate users, thereby improving the efficiency of key generation and extraction, and further enhancing the security of rail transit systems.

Description

Technical Field

[0001] The present application relates to the field of digital information transmission, and in particular to a method and system for extracting a security key for reconstructing soft information codewords for rail transit. Background Art

[0002] With the explosive growth in the number of connected devices in rail transit systems, the increasing complexity of network structures, and the increasing computing power between communication nodes, traditional security mechanisms face severe challenges. Once the key management center is attacked, the pre-set keys are at risk of being stolen, potentially leading to system-level information leakage. Consequently, data transmission in rail transit scenarios is vulnerable to eavesdropping, tampering, and other security threats, potentially causing equipment failure, disrupting the dispatching system, and even threatening driving safety.

[0003] In related technologies, cryptographic mechanisms are often used to ensure secure communications, such as key management and distribution, identity authentication, and digital signatures. These mechanisms, based on cryptographic principles, rely on the intractability of complex mathematical problems to ensure key security, such as large number factorization, discrete logarithms, and elliptic curves. The classic Diffie-Hellman protocol is often used to establish secure channels between the dispatching center and train communication terminals. The security of these mechanisms is effective under the assumption that computational complexity holds, and they can provide a certain degree of resistance to traditional attacks.

[0004] The aforementioned related technologies primarily derive keys by quantizing or partitioning channel characteristics or combinations thereof. However, when the channel samples from the transmitter and the legitimate receiver fall near the quantization boundary, it is easy for the legitimate user's quantization information to differ. When the legitimate user's quantization information differs, repeated negotiations are required to ensure that the legitimate user obtains a completely consistent key. This repeated negotiation process increases the time required for key generation and the vulnerability to attack, thereby reducing the security of the rail transit system. Summary of the Invention

[0005] The present application provides a method and system for reconstructing a secure key for soft information codewords for rail transit, which is used to improve the consistency rate of keys between legitimate users, thereby improving the efficiency of key generation and extraction, and further improving the security of the rail transit system.

[0006] In a first aspect, the present application provides a method for extracting a security key for reconstructing soft information codewords for rail transit, wherein a transmitting end and a legitimate receiving end alternately send pilot signals to each other through a preset channel, so that the transmitting end and the legitimate receiving end calculate a channel characteristic sequence of the preset channel based on the pilot signals, wherein the channel characteristic sequence includes a first characteristic sequence and a second characteristic sequence;

[0007] The sender generates a master key from a preset random source and encodes the master key to obtain a codeword sequence;

[0008] The transmitting end generates an information interaction sequence according to the first feature sequence and the codeword sequence, and sends the information interaction sequence to the legal receiving end;

[0009] After receiving the information interaction sequence, the legal receiving end reconstructs the second feature sequence according to the information interaction sequence to obtain a codeword structure sequence;

[0010] The legal receiving end decodes the codeword structure sequence to obtain the final key.

[0011] By adopting the above technical solution, after the sending end generates a master key and encodes it into a codeword sequence, it generates an information interaction sequence with the first characteristic sequence and sends it to the receiving end. The receiving end uses the received information interaction sequence and the second characteristic sequence obtained by itself to reconstruct the codeword sequence and decode it to obtain the final key. This codeword reconstruction method based on soft information reduces the information loss caused by the traditional hard decision method during the quantization process and improves the efficiency of key generation. At the same time, because the channel characteristic sequence has spatial correlation, it is difficult for an eavesdropper located between the sending end and the receiving end to accurately obtain the channel characteristic information. Even if the information interaction sequence is intercepted, it is difficult to reconstruct the correct codeword sequence, thereby improving the security of key generation. The consistency rate of keys between legitimate users is improved, thereby improving the efficiency of key generation and extraction, and thus improving the security of the rail transit system.

[0012] In conjunction with some embodiments of the first aspect, in some embodiments, the transmitting end generates an information interaction sequence based on the first feature sequence and the codeword sequence, specifically including:

[0013] The transmitter maps the codeword sequence into a real number sequence through a mapping function. The mapping function is: ;

[0014] In the above function, is a real number sequence, is the amplitude, is the codeword sequence, is the independent variable;

[0015] The sending end calculates the distance between the real number sequence and the first characteristic sequence by using a preset first function to obtain an information interaction sequence. The preset first function is: ;

[0016] In the above function, is the information interaction sequence, For distance, is the real or imaginary part of the first characteristic sequence, is a real number sequence, is the independent variable.

[0017] By employing this technical solution, the distance-based soft information transfer method preserves the fine-grained information in the channel characteristic sequence, enabling the receiver to more accurately reconstruct the codeword sequence. Because the information exchange sequence represents the distance relationship between the real number sequence and the channel characteristics, rather than directly transmitting the codeword information, even if an eavesdropper obtains the information exchange sequence, it would be difficult to deduce the original codeword sequence without knowing the receiving end's channel characteristics, thus improving the security of the transmission process.

[0018] In conjunction with some embodiments of the first aspect, in some embodiments, the legitimate receiving end reconstructs the second feature sequence according to the information interaction sequence to obtain the codeword structure sequence, specifically including:

[0019] The legal receiving end inputs the information interaction sequence and the second feature sequence into the preset second function to calculate the codeword structure sequence. The preset second function is: ;

[0020] In the above function, is the codeword structure sequence, is the real or imaginary part of the second characteristic sequence, is the information interaction sequence, is the independent variable.

[0021] By adopting this technical solution and leveraging the principle of channel reciprocity, the receiver can accurately recover the transmitter's codeword information based on its acquired channel signature sequence. Because the entire reconstruction process does not involve setting a hard decision threshold, information loss caused by quantization errors is reduced, improving the accuracy of key generation.

[0022] In conjunction with some embodiments of the first aspect, in some embodiments, after decoding the codeword structure sequence at the legal receiving end to obtain the final key, the method further includes:

[0023] The legitimate receiving end calculates the channel change rate based on the time change of the channel characteristic sequence;

[0024] The legitimate receiving end determines the key dynamic sharding parameters based on the channel change rate. The key dynamic sharding parameters include the number of shards and the validity period of each shard.

[0025] The legal receiving end divides the final key into multiple key shards according to the key dynamic sharding parameters;

[0026] During the communication process, the sender and the legitimate receiver switch to use different key shards in the order of the validity period of each key shard;

[0027] When it is detected that the channel characteristic sequence has changed to a preset degree, the legitimate receiving end sends a switching instruction to the sending end so that the sending end sends the next key fragment according to the switching instruction.

[0028] By adopting the above technical solution, the channel change rate is obtained by calculating the time change of the channel characteristic sequence, and the key sharding parameters are determined based on this rate, thereby realizing dynamic key sharding management. The number of shards and the effective duration are adaptively adjusted according to the channel change rate, so that the key update cycle matches the channel change characteristics. During the communication process, different key shards are used in sequence according to the effective duration of each shard. When a preset degree of change in the channel characteristics is detected, the key shard is switched in time, thereby improving the security of the key in a time-varying channel environment. The key sharding dynamically adjusted based on the channel characteristics enables the system to flexibly adjust the key usage strategy according to the actual channel conditions, thereby improving the adaptability of key management. By dividing a single key into multiple time-limited shards, the security risk caused by key leakage is reduced.

[0029] In conjunction with some embodiments of the first aspect, in some embodiments, the legitimate receiving end determines the key dynamic sharding parameter based on the channel change rate, specifically including:

[0030] The legal receiving end obtains a preset channel change rate threshold set and a corresponding slice parameter mapping table;

[0031] The legitimate receiving end compares the channel change rate with a preset channel change rate threshold set to determine the rate interval to which the channel change rate belongs;

[0032] The legitimate receiving end obtains the corresponding number of fragments and the validity period of each fragment from the fragmentation parameter mapping table according to the rate range as the key dynamic fragmentation parameter.

[0033] By adopting the above technical solution, by obtaining a preset channel change rate threshold set and a corresponding sharding parameter mapping table, the channel change rate is compared with the threshold set to determine the rate range to which it belongs, and then the corresponding shard number and validity period are obtained from the mapping table as the dynamic key sharding parameters, so that the key sharding parameters can be dynamically adjusted according to the actual channel changes. When the channel changes rapidly, the system automatically increases the number of shards and shortens the validity period of each shard to ensure that each shard is replaced in time before the channel characteristics change significantly; when the channel changes slowly, the system correspondingly reduces the number of shards and extends the validity period to avoid unnecessary shard switching operations, thereby improving the flexibility of key management, reducing the risk of key mismatch caused by channel changes, and reducing the system overhead of key updates and distribution.

[0034] In conjunction with some embodiments of the first aspect, in some embodiments, after the legitimate receiving end sends a switching instruction to the sending end so that the sending end sends the next key fragment according to the switching instruction, the method further includes:

[0035] The legitimate receiving end performs a fast Fourier transform on the channel characteristic sequence to obtain the frequency domain distribution of the channel change;

[0036] The legitimate receiving end divides the frequency domain distribution into high-frequency components and low-frequency components based on the preset frequency threshold. The high-frequency components correspond to channel changes caused by multipath propagation, and the low-frequency components correspond to channel changes caused by physical location changes.

[0037] The legitimate receiving end recalculates the channel change rate based on the low-frequency component to obtain the corrected channel change rate;

[0038] The legitimate receiving end re-determines the key dynamic fragmentation parameters based on the corrected channel change rate.

[0039] By adopting the above technical solution, a fast Fourier transform is performed on the channel characteristic sequence to obtain a frequency domain distribution. This is then divided into high- and low-frequency components corresponding to multipath propagation and physical location changes based on a preset frequency threshold. The corrected channel change rate is recalculated based on the low-frequency component, and the dynamic key sharding parameters are then adjusted. By filtering out high-frequency channel fluctuations caused by multipath propagation and retaining low-frequency information reflecting actual physical location changes, the calculated channel change rate more accurately reflects the relative motion state of the communicating parties. This channel change rate correction method based on frequency domain analysis improves the accuracy of channel characteristic assessment, reduces the interference of multipath effects on key sharding parameter selection, and enhances the adaptability of the key sharding mechanism to changes in actual physical scenarios.

[0040] In conjunction with some embodiments of the first aspect, in some embodiments, the legitimate receiving end recalculates the channel change rate based on the low-frequency component, specifically including:

[0041] The legitimate receiving end performs an inverse fast Fourier transform on the low-frequency component to obtain a filtered channel feature sequence;

[0042] The legal receiving end calculates the variation range of the filtered channel characteristic sequence within a preset time window;

[0043] The legitimate receiving end divides the change amplitude by the duration of the preset time window to obtain the corrected channel change rate.

[0044] By adopting the above technical solution, the low-frequency component is subjected to an inverse fast Fourier transform to obtain a filtered channel characteristic sequence. The amplitude of its change within a preset time window is calculated and then divided by the length of the time window to obtain the corrected channel change rate. By setting an appropriate observation time window, the system can accurately capture the gradual trend of channel characteristics and avoid the impact of instantaneous fluctuations. This time window-based rate calculation method improves the stability of channel change rate assessment, reduces the interference of instantaneous fluctuations in channel characteristics on the selection of key sharding parameters, and enables the dynamic key sharding mechanism to operate more smoothly.

[0045] In the second aspect, an embodiment of the present application provides a soft information codeword reconstruction security key extraction system for rail transit, the system comprising: one or more processors and a memory; the memory is coupled to one or more processors, the memory is used to store computer program code, the computer program code includes computer instructions, and one or more processors call the computer instructions to enable the system to execute the method described in the first aspect and any possible implementation method of the first aspect.

[0046] In a third aspect, an embodiment of the present application provides a computer-readable storage medium comprising instructions, which, when executed on a system, enables the system to execute the method described in the first aspect and any possible implementation of the first aspect.

[0047] In a fourth aspect, an embodiment of the present application provides a computer program product, which, when executed on a system, enables the system to execute the method described in any possible implementation manner in the first aspect.

[0048] One or more technical solutions provided in the embodiments of this application have at least the following technical effects or advantages:

[0049] 1. The present application provides a method for reconstructing a secure key and extracting a soft information codeword for rail transit. After the transmitting end generates a master key and encodes it into a codeword sequence, it generates an information interaction sequence with the first characteristic sequence and sends it to the receiving end. The receiving end uses the received information interaction sequence and the second characteristic sequence obtained by itself to reconstruct the codeword sequence and decode it to obtain the final key. This codeword reconstruction method based on soft information reduces the information loss caused by the traditional hard decision method in the quantization process and improves the efficiency of key generation. At the same time, since the channel characteristic sequence has spatial correlation, it is difficult for an eavesdropper between the transmitting end and the receiving end to accurately obtain the channel characteristic information. Even if the information interaction sequence is intercepted, it is difficult to reconstruct the correct codeword sequence, thereby improving the security of key generation. The consistency rate of keys between legitimate users is improved, thereby improving the efficiency of key generation and extraction, and further improving the security of the rail transit system.

[0050] 2. The present application provides a method for extracting secure keys for reconstructing soft information codewords for rail transit. The channel change rate is obtained by calculating the time change of the channel characteristic sequence, and the key sharding parameters are determined based on the rate, thereby realizing dynamic sharding management of the key. The number of shards and the effective duration are adaptively adjusted according to the channel change rate, so that the key update cycle matches the channel change characteristics. During the communication process, different key shards are used in sequence according to the effective duration of each shard. When a preset degree of change in the channel characteristics is detected, the key shard is switched in time, thereby improving the security of the key in a time-varying channel environment. The key sharding based on dynamic adjustment of the channel characteristics enables the system to flexibly adjust the key usage strategy according to the actual channel conditions, thereby improving the adaptability of key management. By dividing a single key into multiple time-limited shards, the security risk caused by key leakage is reduced.

[0051] 3. The present application provides a method for extracting secure keys from soft information codeword reconstruction for rail transit. The frequency domain distribution is obtained by performing a fast Fourier transform on the channel characteristic sequence, and the frequency domain distribution is divided into high- and low-frequency components corresponding to multipath propagation and physical position changes according to a preset frequency threshold. The corrected channel change rate is recalculated based on the low-frequency component, and the dynamic sharding parameters of the key are adjusted. By filtering out the high-frequency channel fluctuations caused by multipath propagation and retaining the low-frequency information reflecting the actual physical position changes, the calculated channel change rate can more accurately reflect the relative motion state of the two communicating parties. This channel change rate correction method based on frequency domain analysis improves the accuracy of channel characteristic evaluation, reduces the interference of multipath effects on the selection of key sharding parameters, and enhances the adaptability of the key sharding mechanism to changes in actual physical scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 This is a flow chart of a method for extracting a soft information codeword reconstruction security key for rail transit in an embodiment of the present application.

[0053] Figure 2 This is another flow chart of a method for extracting a soft information codeword reconstruction security key for rail transit in an embodiment of the present application.

[0054] Figure 3 This is a schematic diagram of the physical device structure of a soft information codeword reconstruction security key extraction system for rail transit provided in an embodiment of the present application. DETAILED DESCRIPTION

[0055] The terms used in the following examples of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application. As used in the specification and appended claims of this application, the singular expressions "a," "an," "said," "above," "the," and "this" are intended to include plural expressions as well, unless the context clearly indicates otherwise. It should also be understood that the term "and / or" used in this application refers to any or all possible combinations comprising one or more of the listed items.

[0056] In the following, the terms "first" and "second" are used for descriptive purposes only and should not be understood to imply or suggest relative importance or implicitly indicate the number of the technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the embodiments of this application, unless otherwise specified, "plurality" means two or more.

[0057] The following uses an embodiment and combines Figure 1 , a method for extracting a security key for reconstructing soft information codewords for rail transit in an embodiment of the present application is described:

[0058] See also Figure 1 , which is a flow chart of a method for extracting a soft information codeword reconstruction security key for rail transit in an embodiment of the present application.

[0059] S101, a transmitting end and a legitimate receiving end alternately send pilot signals to each other through a preset channel, so that the transmitting end and the legitimate receiving end calculate a channel characteristic sequence of the preset channel based on the pilot signals, where the channel characteristic sequence includes a first characteristic sequence and a second characteristic sequence;

[0060] The transmitter and the authorized receiver communicate through a pre-set channel, alternately sending pilot signals to each other during the communication process. The pilot signal is a pre-defined, known signal, typically used for purposes such as channel estimation and synchronization. After receiving the pilot signal sent by the other party, the transmitter and the authorized receiver can estimate the channel characteristics of the current preset channel based on the pilot signal. Channel characteristics typically include factors that affect signal transmission quality, such as channel fading, multipath, and noise. Based on the estimated channel characteristics, the transmitter and the authorized receiver respectively obtain a first characteristic sequence and a second characteristic sequence. The specific form, transmission, and alternation method of the pilot signal are not limited here.

[0061] In one possible implementation, the pilot signal can use a commonly used pilot sequence, such as a pseudorandom noise (PN) sequence or a Zadoff-Chu sequence. The transmitter and legitimate receiver can alternately act as the transmitter and receiver within each communication time slot, sending a pilot signal when acting as the transmitter and estimating channel characteristics based on the received pilot signal when acting as the receiver. For example, in odd-numbered time slots, the transmitter sends a pilot signal and the legitimate receiver estimates channel characteristics, while in even-numbered time slots, the legitimate receiver sends a pilot signal and the transmitter estimates channel characteristics. Alternatively, the pilot signal can be sent alternately every multiple time slots.

[0062] In another possible implementation, the transmitter and authorized receiver can use multiple antennas to simultaneously transmit and receive pilot signals from multiple channels. For example, each transmitting antenna on the transmitter can send a different pilot sequence, and each receiving antenna on the authorized receiver can estimate the channel characteristics of the subchannels between it and each transmitting antenna on the transmitter. These are then combined to obtain an overall characteristic sequence for the multi-antenna channel. This approach can obtain more channel characteristic information.

[0063] S102: The transmitting end generates a master key from a preset random source and encodes the master key to obtain a codeword sequence;

[0064] The sender first generates a master key from a pre-defined random source. The master key is the foundation of the entire key generation scheme and is typically a sufficiently random and unpredictable binary sequence. Master keys can be generated using physical random sources such as thermal noise, quantum random processes, or cryptographically secure pseudorandom number generators. The length of the master key depends on the required key security strength and is typically 128 to 256 bits.

[0065] After generating the master key, the transmitter needs to perform channel coding on it, converting it into a codeword sequence suitable for transmission over the channel. The purpose of this coding is to, first, enhance the robustness of the master key against interference such as channel noise; second, to tightly integrate the master key with the subsequently generated channel signature sequence, ensuring key security. Each element of the codeword sequence can be a binary bit or a multi-base symbol. The coding method can be flexibly selected based on factors such as channel conditions and security requirements. The specific generation method and coding method for the master key are not limited here.

[0066] In one possible implementation, the transmitter can use common channel coding schemes such as convolutional codes, LDPC codes, and Turbo codes. Taking convolutional codes as an example, the transmitter can use a convolutional encoder with a rate of 1 / 2 and a memory length of 3 to encode the master key. The encoder receives a 1-bit master key in each clock cycle and outputs a 2-bit codeword with a code rate of 1 / 2. The length of the encoded codeword sequence is twice the length of the master key. Within the codeword sequence, there is a certain correlation between adjacent codewords, which facilitates error correction and master key recovery.

[0067] In another possible implementation, the transmitter can combine channel coding with security enhancement techniques such as scrambling and scrambling to further enhance the security of the codeword sequence. For example, while encoding the master key, the transmitter can also use a pseudo-random sequence, such as the Gold sequence, to scramble the codeword. This involves performing an exclusive-OR operation on each element of the codeword sequence with the corresponding element of the pseudo-random sequence. This introduces randomness into the codeword sequence, making it more difficult for attackers to decipher it.

[0068] S103: The transmitting end generates an information interaction sequence according to the first feature sequence and the codeword sequence, and sends the information interaction sequence to the legal receiving end;

[0069] The transmitting end generates an information interaction sequence according to the first feature sequence and the codeword sequence. Specifically, the transmitting end maps the codeword sequence into a real number sequence through a mapping function. The mapping function is: ;

[0070] In the above function, is a real number sequence, is the amplitude, is the codeword sequence, is the independent variable;

[0071] The sending end calculates the distance between the real number sequence and the first characteristic sequence by using a preset first function to obtain an information interaction sequence. The preset first function is: ;

[0072] In the above function, is the information interaction sequence, For distance, is the real or imaginary part of the first characteristic sequence, is a real number sequence, is the independent variable.

[0073] After obtaining the information interaction sequence, the sending end sends the information interaction sequence to the legal receiving end.

[0074] The information exchange sequence combines codeword information and channel characteristics and serves as the basis for key negotiation between the transmitter and the authorized receiver. Specifically, the transmitter first converts each codeword element in the codeword sequence into a real number using a mapping function. This mapping function can be a simple positive-negative symmetric mapping, where codeword 1 is mapped to a positive real number and codeword 0 is mapped to a negative real number, with an amplitude of A. A can be set based on the specific application scenario. The resulting real number sequence is the same length as the codeword sequence.

[0075] Next, the transmitter calculates a certain "distance metric" between the real number sequence and the first characteristic sequence using a preset first function, and uses the calculation result as the information exchange sequence. The preset first function can directly express the distance between the two sequences by the difference between the corresponding elements. The first characteristic sequence can take either the real part or the imaginary part for participation in the calculation. Each element of the information exchange sequence actually measures the difference between the codeword sequence and the channel characteristics at a sampling point. On the one hand, this difference is related to the codeword and can reflect the key information; on the other hand, it is affected by the random channel characteristics and is difficult for attackers to accurately obtain. Finally, the transmitter sends the generated information exchange sequence through the channel to the legitimate receiver for key recovery. The specific form of the first function is not limited here.

[0076] In a possible implementation, if the codeword sequence and the first feature sequence are both real-valued sequences, the first function can take the Euclidean distance, that is, the square root of the sum of the squares of the differences between the corresponding elements of the two sequences, to characterize their differences. For example, for a sequence of length N, let the real number sequence after codeword mapping be { , , …, }, the first characteristic sequence is { , , …, }, then the information interaction sequence The larger the Euclidean distance, the greater the difference between the two sequences.

[0077] In another possible implementation, if the codeword sequence and the first feature sequence are complex sequences, the first function can define a distance metric in the complex domain. A common complex domain distance is the Hermitian distance, which is the sum of the real parts of the conjugate product of the corresponding elements of the two complex sequences. For example, for a sequence of length N, let the real sequence after the codeword mapping be the complex sequence { , , …, }, the first characteristic sequence is { , , …, }, then the Hermitian distance is , where h̃ represents the conjugate of the complex number h. The Hermitian distance can be viewed as the inner product of the complex field, which measures the correlation between two complex sequences.

[0078] S104. After receiving the information interaction sequence, the legal receiving end reconstructs the second feature sequence according to the information interaction sequence to obtain a codeword structure sequence;

[0079] After receiving the information interaction sequence, the legitimate receiving end reconstructs the second feature sequence based on the information interaction sequence to obtain a codeword structure sequence. Specifically, the legitimate receiving end inputs the information interaction sequence and the second feature sequence into a preset second function to calculate the codeword structure sequence. The preset second function is: ;

[0080] In the above function, is the codeword structure sequence, is the real or imaginary part of the second characteristic sequence, is the information interaction sequence, is the independent variable.

[0081] Because the legitimate receiver also independently estimates the channel characteristics, it obtains a second characteristic sequence that is highly correlated with the first characteristic sequence from the transmitter. Leveraging this channel reciprocity, the legitimate receiver can remove the influence of the channel characteristics from the information exchange sequence and restore the original codeword sequence.

[0082] Specifically, the legal receiving end inputs the information interaction sequence and its own estimated second characteristic sequence into a preset function (second function), and performs calculations to obtain a new sequence, called a codeword structure sequence. The second function is actually the "inverse transform" of the first function used by the transmitter to generate the information interaction sequence. It uses the information interaction sequence as the independent variable and the second characteristic sequence as the parameter, and calculates the real number sequence after codeword mapping by distance subtraction. Due to the reciprocity of the channel, the second characteristic sequence corresponds to and is close to the first characteristic sequence, so this calculation can offset most of the channel effects introduced by the transmitter, so that the codeword structure sequence is similar to the structural characteristics of the original codeword sequence. The second function is preset here to take the real part or imaginary part of the second characteristic sequence as input, which is consistent with the first function. The specific form of the preset second function is not limited here, but it should be a pair of "mutually inverse" operations with the first function.

[0083] In one possible implementation, if the first function takes the form of a difference , then the second function can be directly expressed in the form of summation as The legitimate receiver subtracts each element of the second signature sequence from the corresponding element of the information interaction sequence, and the result is the corresponding real codeword value. This method has low computational complexity but places high demands on channel reciprocity, requiring the transmitter and legitimate receiver to obtain channel estimates that are as similar as possible.

[0084] In another possible implementation, if the channel reciprocity is not ideal and there is estimation error, some error tolerance mechanism can be introduced into the second function. For example, when calculating the codeword structure sequence, an estimation error compensation amount is applied to the second feature sequence. ,Right now Compensation The design can be based on prior information such as the channel's signal-to-noise ratio and the distance between the transmitter and the legitimate receiver. By introducing an appropriate compensation amount, deviations in channel reciprocity can be tolerated to a certain extent, improving the accuracy of the reconstructed codeword.

[0085] S105: The legal receiving end decodes the codeword structure sequence to obtain the final key.

[0086] In the previous step, the legitimate receiver leveraged channel reciprocity to remove the influence of channel characteristics from the information exchange sequence, resulting in a codeword structure sequence similar to the original codeword sequence. In this step, the legitimate receiver must perform channel decoding on the codeword structure sequence to recover the original master key bit sequence. This decoding process is effectively the inverse of the channel encoding performed by the transmitter. Using a decoding algorithm pre-agreed with the transmitter, the legitimate receiver determines each codeword element in the codeword structure sequence, obtaining a bit sequence as close as possible to the master key as the final key. The specific decoding algorithm is not specified here.

[0087] In one possible implementation, if the transmitter uses a memory-rich code type, such as a convolutional code, the legitimate receiver can use the Viterbi decoding algorithm for decoding. Viterbi decoding leverages the convolutional code's trellis structure and uses dynamic programming to search for the maximum likelihood path within the trellis to obtain the most likely information sequence. The decoding process includes branch metric calculation, state metric update, survivor path recording, and backtracking decision making. The legitimate receiver takes the codeword structure sequence as input, calculates the branch metric and state metric for each time instant and each state, records the survivor path with the highest metric, and backtracks to determine the bit value based on the survivor path after receiving the entire sequence. Viterbi decoding fully utilizes the inter-symbol memory to achieve optimal decoding performance.

[0088] In another possible implementation, if channel errors are a concern, the legitimate receiver can first perform error correction on the codeword structure sequence before sending it to the channel decoder. Common error-correcting codes such as BCH and RS codes can be used for error correction. Taking BCH codes as an example, the legitimate receiver first performs BCH encoding on the codeword structure sequence to obtain the corresponding error-correcting codeword. The BCH code's error-correcting capabilities are then used to detect and correct errors in the codeword structure sequence. The corrected sequence is then sent to the channel decoder for decoding, which can reduce the bit error rate. Error correction and channel decoding can also be designed jointly using concatenated codes or product codes to further improve decoding accuracy.

[0089] In the above embodiment, after the transmitting end generates a master key and encodes it into a codeword sequence, it generates an information interaction sequence with the first characteristic sequence and sends it to the receiving end. The receiving end uses the received information interaction sequence and the second characteristic sequence obtained by itself to reconstruct the codeword sequence and decode it to obtain the final key. This codeword reconstruction method based on soft information reduces the information loss caused by the traditional hard decision method in the quantization process and improves the efficiency of key generation. At the same time, because the channel characteristic sequence has spatial correlation, it is difficult for an eavesdropper located between the transmitting end and the receiving end to accurately obtain the channel characteristic information. Even if the information interaction sequence is intercepted, it is difficult to reconstruct the correct codeword sequence, thereby improving the security of key generation. The consistency rate of keys between legitimate users is improved, thereby improving the efficiency of key generation and extraction, and further improving the security of the rail transit system.

[0090] The above embodiment realizes the secure extraction of keys through soft information codeword reconstruction technology, but in actual application scenarios, especially in rail transit environments, the relative positions of the two communicating parties will continue to change, causing the channel characteristics to change dynamically. If a single static key is used, it may be difficult to adapt to the continuous changes in channel characteristics, affecting the security of communications. Therefore, the embodiment of the present application also proposes another method for extracting secure keys by reconstructing soft information codewords for rail transit, which enables the key system to better adapt to changes in channel characteristics in rail transit scenarios by dynamically sharding and switching the generated keys. Figure 2 , another method for extracting a soft information codeword reconstruction security key for rail transit in an embodiment of the present application is described:

[0091] See also Figure 2 , which is another flow chart of a method for extracting a soft information codeword reconstruction security key for rail transit in an embodiment of the present application.

[0092] S201, the legal receiving end calculates the channel change rate based on the time change of the channel characteristic sequence;

[0093] The legitimate receiving end calculates the speed at which the channel characteristics change over time based on the continuously acquired channel characteristic sequence, and obtains a quantitative indicator that characterizes the channel change rate. The channel characteristic sequence can be the second characteristic sequence estimated by the legitimate receiving end in step S101, or it can be a sequence of channel characteristic values ​​continuously estimated by the legitimate receiving end in subsequent communications. Channel characteristics may include parameters that affect communication quality, such as channel amplitude-frequency response, channel impulse response, channel gain, and multipath delay. The channel change rate reflects the severity of the change of these parameters over time, and can be measured by statistics such as the mean and variance of the channel characteristic change per unit time. The specific calculation method of the channel change rate is not limited here.

[0094] In one possible implementation, a legitimate receiver can calculate the channel change rate based on the first-order difference of the channel characteristic sequence. Assume that the second characteristic sequence is a discrete time series {h(n), n=1, 2, ..., N}, where n is the sampling time index and N is the total length of the sequence. Taking the first-order difference of this sequence yields a series of variations {Δh(n)=h(n+1)-h(n), n=1, 2, ..., N-1}. The mean of this series of variations is then calculated as the channel change rate, i.e., v=(1 / (N-1))∑|Δh(n)|. This method is computationally simple and can reflect the overall trend of channel changes.

[0095] In another possible implementation, the legitimate receiver can also use the short-time Fourier transform (STFT) method to calculate the channel change rate. The second characteristic sequence is windowed, and then the subsequence within each window is Fourier transformed to obtain the time-frequency spectra corresponding to different time windows. By comparing the time-frequency spectra of adjacent time windows, the rate of change of the channel at different frequency components can be obtained. For example, the sum of the squares of the energy differences of the corresponding frequency points of adjacent time-frequency spectra is calculated and then divided by the total number of frequency points to obtain the average change rate of the window. Finally, the change rates of all windows are averaged as the channel change rate of the entire sequence. The STFT method can reflect the changing characteristics of the channel in both time and frequency dimensions, providing richer information.

[0096] S202: The legitimate receiving end determines the key dynamic sharding parameter according to the channel change rate;

[0097] The legal receiving end determines the key dynamic sharding parameters based on the channel change rate. The key dynamic sharding parameters include the number of shards and the effective duration of each shard. Specifically, the legal receiving end obtains a preset channel change rate threshold set and a corresponding sharding parameter mapping table; the legal receiving end compares the channel change rate with the preset channel change rate threshold set to determine the rate range to which the channel change rate belongs; the legal receiving end obtains the corresponding number of shards and the effective duration of each shard from the sharding parameter mapping table according to the rate range as the key dynamic sharding parameters.

[0098] In this step, the legitimate receiver uses the channel change rate calculated in the previous step to design a dynamic sharding strategy to determine how to shard the key. The primary purpose of key sharding is to divide a long key into multiple, independently usable, shorter keys. Dynamically switching between different key shards improves key security. Key sharding parameters include the number of shards and the effective usage time of each shard. The legitimate receiver determines the values ​​of these two parameters based on the channel change rate.

[0099] To establish a correspondence between channel change rates and key sharding parameters, the legitimate receiver pre-acquires a set of channel change rate thresholds, dividing different change rates into several intervals. For example, for the three change rate levels of low, medium, and high, the threshold set can be defined as {v1, v2}, where v1 is the boundary between low and medium speeds, and v2 is the boundary between medium and high speeds. At the same time, the legitimate receiver also pre-defines a mapping table from rate intervals to key sharding parameter values. For example, for the three rate intervals of low, medium, and high, the mapping table provides recommended values ​​for the number of shards N and the effective duration T, which can be {(N_low, T_low), (N_mid, T_mid), (N_high, T_high)}.

[0100] After calculating the channel change rate, the legitimate receiver compares it against the threshold set one by one to determine which rate range it falls within. It then queries the mapping table to obtain the key sharding parameter value (N, T) corresponding to that rate range and determines the number of N shards the key should be divided into for this communication, with each shard valid for T. The specific configuration of the threshold set and mapping table is not restricted here and can be flexibly designed based on factors such as system security requirements and computing resource constraints.

[0101] In one possible implementation, a legitimate receiver can use linear interpolation to obtain key sharding parameters based on the channel change rate. This assumes a linear relationship between the number of shards N, the effective duration T, and the channel change rate v: N = k1·v+b1, T = k2·v+b2. k1, k2, b1, and b2 are linear model parameters. Using parameter estimation methods such as least squares, the model parameters are trained using a set of known (v, N, T) sample values. The calculated channel change rate v is then substituted into the linear model to calculate the corresponding sharding parameter values. This method can provide parameter estimates for any rate value without requiring rate intervals.

[0102] In another possible implementation, the legitimate receiving end can also dynamically adjust the rate threshold set and mapping table. Initially, the sharding parameters are determined using preset thresholds and mapping relationships. In subsequent communications, communication quality assessment indicators such as bit error rate and key negotiation success rate are tracked and recorded, forming a feedback loop. When communication quality deteriorates, the rate threshold is appropriately raised to expand the low-rate range; or the mapping table is adjusted to match more shards and a shorter effective duration for the same rate range. Conversely, when communication quality improves, the threshold is lowered, the number of shards is reduced, and the effective duration is extended. This adaptive adjustment method optimizes sharding parameters based on actual communication performance and improves anti-interference capabilities.

[0103] S203: The legal receiving end divides the final key into multiple key shards according to the key dynamic sharding parameter;

[0104] The legal receiving end uses the key dynamic sharding parameters determined in the previous step to divide the final key generated in step S105 to obtain multiple independently usable key shards. The sharding process needs to follow two parameters: the number of shards N and the effective duration T of the shards. According to the number of shards N, the final key is divided into N sub-keys of equal length, and each sub-key is called a key shard. Two adjacent key shards are connected end to end in time, and the end time of the previous shard is the start time of the next shard. The effective use time of each key shard is equal to the parameter T, that is, within the T time after generation, the communicating parties only use the shard for encrypted communication. After the duration T is exceeded, the shard is automatically discarded and switched to the next shard. The implementation details such as the sequential numbering method and storage method of the shards are not limited here.

[0105] In one possible implementation, a legitimate receiving end can generate key shards using a sequential slicing method. Assume that the binary sequence of the final key is K = {k(1), k(2), ..., k(L)}, where L is the bit length of the key. This sequence is divided equally into N subsequences, each of length M = L / N (not considering rounding errors). The binary sequence corresponding to the i-th key shard is K_i = {k((i-1)*M+1), k((i-1)M+2), ..., k(iM)}, where i = 1, 2, ..., N. The clear correspondence between each shard and the final key facilitates rapid slicing.

[0106] In another possible implementation, the legitimate receiving end can also regard the final key as a ring sequence and use overlapping slicing to generate key shards. For example, for a key sequence of length L, it is still divided into N equal-length subsequences, but the length M of each subsequence can be greater than L / N. Starting from the first element of the key sequence, M elements are sequentially taken to form the first shard; then, starting from the second element, M elements are sequentially taken to form the second shard; and so on, until the end position of the Nth shard returns to the first element. This overlapping slicing method allows for a certain overlap between adjacent shards, blurring the shard boundaries, and all shards are connected end to end to form a ring, avoiding the problem of incomplete slicing at the end of the key sequence. Overlapping slicing has better security.

[0107] S204: During the communication process, the sending end and the legitimate receiving end switch to use different key fragments in the order of the validity period of each key fragment;

[0108] In the previous step, the legitimate receiving end has divided the final key into multiple key shards based on the dynamic key sharding parameters. Each key shard has a corresponding validity period, indicating that the shard is used for communication during that period. The sending end and the legitimate receiving end use the corresponding key shard to encrypt and decrypt communication data in a pre-agreed order within the validity period of each shard. When the validity period of a shard expires, both parties automatically switch to the next key shard. This dynamic switching mechanism allows the key to be periodically updated over time, enhancing communication security. At the same time, the frequency of key updates is determined by the validity period and is adapted to the previously determined channel change rate. The specific implementation details of the switching method and timing are not specified here.

[0109] In one possible implementation, the sender and legitimate receiver can use a synchronized switching method. During the key negotiation phase, both parties agree on the validity period and switch time for each key shard. Each party independently maintains a timer to record the remaining validity period of the currently used key shard. When the timer expires, both parties simultaneously switch to the next key shard. To avoid communication errors caused by asynchronous switching, both parties can temporarily suspend data transmission for a short period before the switch, waiting for the other party to complete the switch. A synchronization flag is appended to the first data packet after the switch, indicating that the sender has switched to the new key shard. Upon receiving this data packet, the receiver also immediately switches to the corresponding key shard, thus achieving synchronization between the two parties.

[0110] In another possible implementation, the sender and authorized receiver can also use asynchronous switching. Initially, both parties communicate using the first key shard, but do not pre-agreed on the switching time. When the sender completes the validity period of a key shard, it automatically switches to the next shard. However, to notify the authorized receiver of the synchronous switch, the sender includes a switch notification in the first data packet after the switch. This notification contains the number or index of the new key shard. Upon receiving the switch notification, the authorized receiver also switches to the corresponding key shard, thus completing an asynchronous key update. Asynchronous switching simplifies timer management, but requires additional switch signaling in the data packet.

[0111] S205: When it is detected that the channel characteristic sequence has changed to a preset degree, the legitimate receiving end sends a switching instruction to the sending end so that the sending end sends the next key fragment according to the switching instruction.

[0112] The key sharding scheme determined based on channel characteristics mainly targets the average statistical characteristics of channel changes, but the actual channel may experience some instantaneous drastic changes. In order to adapt to such sudden channel changes, this step introduces a triggered dynamic switching mechanism. The legal receiving end monitors the channel characteristic sequence in real time during the communication process. When it detects that the amplitude or rate of change of the sequence exceeds a preset threshold, it immediately sends a switching instruction to the sending end, requiring the sending end to switch to the next key shard immediately regardless of whether the currently used key shard has expired. After receiving the switching instruction, the sending end complies with it. This triggered switching can be regarded as a supplement and optimization of the original timed switching scheme, so that the key update mechanism can adapt to changing channel conditions more flexibly. There is no limitation on the selection of specific indicators and thresholds for detecting channel characteristic changes.

[0113] In one possible implementation, a legitimate receiver can determine the severity of channel changes by calculating the mean square error (MSE) or standard deviation (SD) of the channel characteristic sequence over a period of time. When the variance or standard deviation at a given moment exceeds a preset threshold, the channel is considered to have undergone a dramatic change, necessitating a timely key update. For example, assume the channel characteristic sequence during the i-th key shard is h[i], its mean is mu[i], and its MSE is sigma^2[i]. sigma^2[i] = (sum((h[i][j] - mu[i])^2)) / M, where j is the symbol index and M is the total number of symbols in the i-th shard. The legitimate receiver sets a variance threshold, sigma_th^2. When sigma^2[i] > sigma_th^2, a key update is triggered, and a switch instruction is sent to the transmitter. Upon receiving the instruction, the transmitter uses the next key, starting with the i+1th symbol. Setting the sigma_th^2 threshold requires a balance between update sensitivity and frequency.

[0114] In another possible implementation, the legitimate receiver can also detect channel changes through the autocorrelation function of the channel characteristic sequence. The autocorrelation function reflects the correlation of the sequence under different time delays. When the channel remains stable, the channel characteristics of adjacent code elements are strongly correlated. When the channel changes dramatically, the correlation decreases rapidly. Therefore, the non-stationarity of the channel can be judged by the rate of decrease of the peak or mean of the autocorrelation function. The legitimate receiver sets an autocorrelation rate threshold rho_th. During each key sharding period, a channel characteristic subsequence h[i] of length L is intercepted and its autocorrelation function R(tau)=(sum(h[i][j]*h[i][j+tau])) / L is calculated, where tau is the delay and j is the code element index. When the rate of decrease of |R(tau) / R(0)| exceeds rho_th, it is considered that the channel is no longer stable, triggering a key update. The advantage of the autocorrelation method is that it can detect the time-varying characteristics of the channel rather than the instantaneous amplitude.

[0115] In the above embodiment, the channel change rate is obtained by calculating the time change of the channel characteristic sequence, and the key sharding parameters are determined based on the rate, thereby realizing dynamic sharding management of the key. The number of shards and the effective duration are adaptively adjusted according to the channel change rate, so that the key update period matches the channel change characteristics. During the communication process, different key shards are used in sequence according to the effective duration of each shard. When a preset degree of change in the channel characteristics is detected, the key shard is switched in time, thereby improving the security of the key in a time-varying channel environment. The key sharding dynamically adjusted based on the channel characteristics enables the system to flexibly adjust the key usage strategy according to the actual channel conditions, thereby improving the adaptability of key management. By dividing a single key into multiple time-limited shards, the security risk caused by key leakage is reduced.

[0116] Furthermore, in another embodiment, after the legitimate receiving end sends a switching instruction to the transmitting end so that the transmitting end sends the next key fragment according to the switching instruction, the legitimate receiving end performs a fast Fourier transform on the channel characteristic sequence to obtain a frequency domain distribution of the channel change;

[0117] The legitimate receiving end divides the frequency domain distribution into high-frequency components and low-frequency components based on the preset frequency threshold. The high-frequency components correspond to channel changes caused by multipath propagation, and the low-frequency components correspond to channel changes caused by physical location changes.

[0118] The legitimate receiving end recalculates the channel change rate based on the low-frequency component to obtain a corrected channel change rate, specifically comprising: performing an inverse fast Fourier transform on the low-frequency component to obtain a filtered channel characteristic sequence; calculating the change amplitude of the filtered channel characteristic sequence within a preset time window; and dividing the change amplitude by the duration of the preset time window to obtain a corrected channel change rate.

[0119] The legitimate receiving end re-determines the key dynamic fragmentation parameters based on the corrected channel change rate.

[0120] This embodiment proposes a channel change rate correction method based on frequency domain analysis. This method performs a fast Fourier transform (FFT) on a channel signature sequence to obtain its frequency domain distribution. The frequency domain components are then divided into high-frequency and low-frequency components based on a preset frequency threshold. The high-frequency components correspond to rapid channel changes caused by multipath propagation, while the low-frequency components correspond to relatively slow channel changes caused by changes in the physical locations of the communicating parties. The legitimate receiving end then performs an inverse FFT on the low-frequency components to filter out high-frequency noise, obtaining a corrected channel signature sequence. Based on this corrected sequence, the channel change rate within a time window is recalculated as the corrected channel change rate. Finally, the legitimate receiving end uses the corrected channel change rate to adjust dynamic key sharding parameters, such as the number of shards and their validity period. The specific implementation of the FFT, the selection of the frequency threshold, and the setting of the time window length are not limited herein.

[0121] In one possible implementation, after obtaining the channel characteristic sequence, the legitimate receiving end first performs an FFT transform on it. The FFT can use a radix-2 time-division multiplexing FFT algorithm, such as the Cooley-Tukey algorithm. Suppose the channel characteristic sequence is h[n], n=0, 1, ..., N-1, then its frequency domain representation is , k=0, 1, …, N-1. N is the number of FFT points, usually a power of 2. After the transformation, N frequency components H[k] are obtained on the frequency axis. Next, the legal receiving end divides the frequency domain into a high-frequency part (|f|>f_th) and a low-frequency part (|f|<=f_th) according to the preset frequency threshold f_th. The selection of the threshold f_th needs to be comprehensively considered based on parameters such as the maximum relative motion speed of the system and the signal wavelength. Then, the high-frequency component H[k] is set to zero, and the remaining low-frequency components are subjected to inverse FFT. , and obtain the modified time domain channel characteristic sequence h'[n]. At this time, the multipath interference of the high frequency part has been filtered out.

[0122] In another possible implementation, the legitimate receiver can also use a time-frequency analysis method based on a window function to process the channel characteristic sequence in both the time domain and the frequency domain. For example, using the short-time Fourier transform (STFT), the channel characteristic sequence h[n] is windowed with a sliding time window w[n], and then FFT is performed within each window. Let the frequency domain result of the mth window be , k = 0, 1, …, N-1. The window function w[n] can be a rectangular window, a Hanning window, or other options. The length of the time window, M, must balance time resolution and frequency resolution. For each time window, the high-frequency components of H[m, k] are filtered out using the aforementioned method. An inverse FFT transform is then performed into the time domain to obtain the corrected sequence h'[m, n] within that window. The results from each window are then concatenated to obtain the final corrected sequence h'[n]. This joint time-frequency filtering method can more precisely characterize the frequency characteristics of the channel at different time scales.

[0123] In the above-described embodiment, a fast Fourier transform is performed on the channel characteristic sequence to obtain a frequency domain distribution. This distribution is then divided into high- and low-frequency components corresponding to multipath propagation and physical location changes based on a preset frequency threshold. A corrected channel change rate is recalculated based on the low-frequency component, and the dynamic key sharding parameters are then adjusted. By filtering out high-frequency channel fluctuations caused by multipath propagation and retaining low-frequency information reflecting actual physical location changes, the calculated channel change rate more accurately reflects the relative motion state of the communicating parties. This frequency-domain analysis-based channel change rate correction method improves the accuracy of channel characteristic assessment, reduces the interference of multipath effects on key sharding parameter selection, and enhances the key sharding mechanism's adaptability to changes in actual physical scenarios.

[0124] The following describes the system in the embodiment of the present invention from the perspective of hardware processing. Figure 3 , which is a schematic diagram of the physical device structure of a soft information codeword reconstruction security key extraction system for rail transit provided in an embodiment of the present application.

[0125] It should be noted that Figure 3 The structure of the system shown is only an example and should not limit the functions and scope of use of the embodiments of the present invention.

[0126] like Figure 3 As shown, the system includes a central processing unit (CPU) 301, which can perform various appropriate actions and processes, such as the methods described in the above embodiments, based on programs stored in a read-only memory (ROM) 302 or programs loaded from a storage unit 308 into a random access memory (RAM) 303. RAM 303 also stores various programs and data required for system operation. CPU 301, ROM 302, and RAM 303 are interconnected via a bus 304. An input / output (I / O) interface 305 is also connected to bus 304.

[0127] The following components are connected to the I / O interface 305: an input section 306 including a camera, infrared sensor, and the like; an output section 307 including a liquid crystal display (LCD) and speakers; a storage section 308 including a hard disk and the like; and a communication section 309 including a network interface card such as a LAN (Local Area Network) card or a modem. The communication section 309 performs communication processing via a network such as the Internet. A drive 310 is also connected to the I / O interface 305 as needed. Removable media 311, such as a magnetic disk, optical disk, magneto-optical disk, or semiconductor memory, is installed in the drive 310 as needed, so that computer programs read from the media can be installed in the storage section 308 as needed.

[0128] In particular, according to embodiments of the present invention, the processes described above with reference to the flowcharts can be implemented as computer software programs. For example, embodiments of the present invention include a computer program product comprising a computer program carried on a computer-readable medium, the computer program including a computer program for executing the methods illustrated in the flowcharts. In such embodiments, the computer program can be downloaded and installed from a network via the communication section 309 and / or installed from removable media 311. When executed by the central processing unit (CPU) 301, the computer program performs the various functions defined in the present invention.

[0129] It should be noted that the computer-readable medium described in the embodiments of the present invention may be a computer-readable signal medium or a computer-readable storage medium, or any combination thereof. A computer-readable storage medium may be, for example, but not limited to, an electrical, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any combination thereof. More specific examples of computer-readable storage media may include, but are not limited to, an electrical connection having one or more conductors, a portable computer disk, a hard disk, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM), flash memory, optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination thereof. In the present invention, a computer-readable storage medium may be any tangible medium containing or storing a program that can be used by or in conjunction with an instruction execution system, apparatus, or device. In the present invention, a computer-readable signal medium may include a data signal transmitted in baseband or as part of a carrier wave, which carries a computer-readable computer program. Such a propagated data signal may take any of a variety of forms, including but not limited to electromagnetic signals, optical signals, or any suitable combination thereof.

[0130] The flowcharts and block diagrams in the accompanying drawings illustrate the possible architectures, functions and operations of the systems, methods and computer program products according to various embodiments of the present invention. Each box in the flowchart or block diagram can represent a module, program segment, or part of the code, and the above-mentioned module, program segment, or part of the code contains one or more executable instructions for implementing the specified logical function. It should also be noted that in some alternative implementations, the functions marked in the boxes can also occur in an order different from that marked in the accompanying drawings. For example, two boxes shown in succession can actually be executed substantially in parallel, and they can sometimes be executed in the opposite order, depending on the functions involved. It should also be noted that each box in the block diagram or flowchart, and the combination of boxes in the block diagram or flowchart, can be implemented using a dedicated hardware-based system that performs the specified function or operation, or can be implemented using a combination of dedicated hardware and computer instructions.

[0131] As another aspect, the present invention further provides a computer-readable storage medium, which may be included in the system described in the above embodiments, or may exist independently and not incorporated into the system. The storage medium carries one or more computer programs, and when executed by a processor of a system, the system implements the methods provided in the above embodiments.

[0132] As described above, the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present application.

[0133] As used in the above embodiments, the term “when…” may be interpreted as “if…” or “after…” or “in response to determining…” or “in response to detecting…”, depending on the context. Similarly, the phrases “upon determining…” or “if (stated condition or event) is detected” may be interpreted as “if determining…” or “in response to determining…” or “upon detecting (stated condition or event)” or “in response to detecting (stated condition or event)”, depending on the context.

[0134] In the above embodiments, all or part of the embodiments can be implemented using software, hardware, firmware, or any combination thereof. When implemented using software, all or part of the embodiments can be implemented in the form of a computer program product. The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, the processes or functions described in the embodiments of the present application are generated in whole or in part. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions can be stored in a computer-readable storage medium or transmitted from one computer-readable storage medium to another. For example, the computer instructions can be transmitted from one website, computer, server, or data center to another website, computer, server, or data center via wired (e.g., coaxial cable, optical fiber, digital subscriber line) or wireless (e.g., infrared, wireless, microwave, etc.) means. The computer-readable storage medium can be any available medium that can be accessed by a computer or a data storage device such as a server or data center that integrates one or more available media. The available medium can be a magnetic medium (e.g., a floppy disk, hard disk, tape), an optical medium (e.g., a DVD), or a semiconductor medium (e.g., a solid-state drive).

[0135] Those skilled in the art will appreciate that all or part of the process steps in the above-described method embodiments can be implemented by a computer program instructing the relevant hardware. The program can be stored in a computer-readable storage medium, and when executed, the program can include the process steps in the above-described method embodiments. The aforementioned storage medium includes various media capable of storing program code, such as ROM or random access memory (RAM), magnetic disks, or optical disks.

Claims

1. A method for extracting a security key for soft information codeword reconstruction in rail transit, applied to a system, characterized in that: The system includes a sending end and a legal receiving end, and the method includes: The transmitting end and the legitimate receiving end alternately send pilot signals to each other through a preset channel, so that the transmitting end and the legitimate receiving end respectively calculate a first characteristic sequence and a second characteristic sequence based on the pilot signals; The transmitting end generates a master key from a preset random source and encodes the master key to obtain a codeword sequence; The transmitting end generates an information interaction sequence according to the first feature sequence and the codeword sequence, and sends the information interaction sequence to the legal receiving end. Specifically, the transmitting end maps the codeword sequence into a real number sequence through a mapping function, and the mapping function is: ； In the above function, is the real number sequence, the is the amplitude, is the codeword sequence, is the independent variable; The sending end calculates the distance between the real number sequence and the first characteristic sequence by using a preset first function to obtain an information interaction sequence, where the preset first function is: ； In the above function, is the information interaction sequence, the is the distance, is the real part or imaginary part of the first characteristic sequence, is the real number sequence, the is the independent variable; After receiving the information exchange sequence, the legal receiving end reconstructs the second feature sequence according to the information exchange sequence to obtain a codeword structure sequence, specifically including: The legal receiving end inputs the information interaction sequence and the second feature sequence into a preset second function to calculate a codeword structure sequence. The preset second function is: ； In the above function, is the codeword structure sequence, is the real part or imaginary part of the second characteristic sequence, is the information interaction sequence, the is the independent variable; The legal receiving end decodes the codeword structure sequence to obtain a final key; The legitimate receiving end calculates the channel change rate based on the time change of the characteristic sequence; The legitimate receiving end determines the key dynamic sharding parameters according to the channel change rate. The key dynamic sharding parameters include the number of shards and the validity period of each shard, specifically including: The legal receiving end obtains a preset channel change rate threshold set and a corresponding slice parameter mapping table; The legitimate receiving end compares the channel change rate with the preset channel change rate threshold set to determine the rate interval to which the channel change rate belongs; The legitimate receiving end obtains the corresponding number of fragments and the effective duration of each fragment from the fragment parameter mapping table according to the rate interval as the key dynamic fragment parameter; The legal receiving end divides the final key into multiple key fragments according to the key dynamic fragmentation parameter; During the communication process, the sending end and the legitimate receiving end switch to use different key fragments according to the validity period of each key fragment; When it is detected that the channel characteristic sequence changes to a preset degree, the legitimate receiving end sends a switching instruction to the sending end so that the sending end sends the next key fragment according to the switching instruction.

2. The method according to claim 1, characterized in that After the legitimate receiving end sends a switching instruction to the sending end so that the sending end sends the next key fragment according to the switching instruction, the method further includes: The legal receiving end performs a fast Fourier transform on the channel characteristic sequence to obtain a frequency domain distribution of the channel change; The legitimate receiving end divides the frequency domain distribution into high-frequency components and low-frequency components according to a preset frequency threshold, wherein the high-frequency components correspond to channel changes caused by multipath propagation, and the low-frequency components correspond to channel changes caused by physical location changes; The legitimate receiving end recalculates the channel change rate based on the low-frequency component to obtain a modified channel change rate; The legal receiving end re-determines the key dynamic sharding parameter according to the modified channel change rate.

3. The method according to claim 2, characterized in that The legitimate receiving end recalculates the channel change rate based on the low-frequency component, specifically including: The legal receiving end performs an inverse fast Fourier transform on the low-frequency component to obtain a filtered channel characteristic sequence; The legal receiving end calculates the variation range of the filtered channel characteristic sequence within a preset time window; The legal receiving end divides the change amplitude by the duration of the preset time window to obtain a corrected channel change rate.

4. A rail transit-oriented soft information codeword reconstruction security key extraction system, characterized by: The system comprises: One or more processors and a memory; the memory is coupled to the one or more processors, the memory is used to store computer program code, the computer program code includes computer instructions, and the one or more processors call the computer instructions to cause the system to execute the method according to any one of claims 1 to 3.

5. A computer-readable storage medium comprising instructions, characterized in that: When the instructions are executed on a system, the system is caused to perform the method according to any one of claims 1 to 3.

6. A computer program product, characterized in that When the computer program product is run on a system, the system is caused to perform the method according to any one of claims 1 to 3.

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