Unmanned aircraft measurement and control network security communication method, system, equipment and medium

Abstract

The invention relates to the technical field of unmanned aircraft measurement and control network communication, in particular to an unmanned aircraft measurement and control network safety communication method, system and device and a medium. The data of the unmanned aerial vehicle are prevented from being tampered in the transmission process through the Hash technology; through multi-factor identity authentication, whether the identity of the to-be-networked unmanned aerial vehicle is legal is verified, the security of the authentication process is improved, an encryption algorithm is called to encrypt unmanned aerial vehicle communication data, the security communication of an unmanned aerial vehicle measurement and control network is ensured, and the security, reliability and anti-interference capability of unmanned aerial vehicle communication are improved; the key technology for ensuring the safety of the unmanned aerial vehicle system not only can protect the flight safety of the unmanned aerial vehicle, but also can guarantee the safety of data carried by the unmanned aerial vehicle and the stability of a monitored environment.

Description

Technical Field

[0001] The present invention relates to the field of unmanned aerial vehicle safety communication technology, and in particular to an unmanned aerial vehicle measurement and control network safety communication method, system, equipment and medium. Background Art

[0002] With the rapid advancement of science and technology, drones have expanded from the military field to a wide range of civilian applications. They play an important role in remote sensing mapping, environmental monitoring, logistics distribution, and agricultural plant protection. Drones can efficiently collect data, perform complex tasks, and enter areas that are difficult for humans to reach. However, the security issues of these high-tech devices have also become prominent, especially in terms of network security.

[0003] Hacker attacks are one of the main threats facing drone systems. Since drones rely on wireless signals to operate, hackers can hijack drones by jamming signals or directly hacking into control systems, thereby carrying out illegal activities or causing damage to critical infrastructure. In addition, the data collected by drones is often highly sensitive, and once leaked, it may pose a serious risk to personal privacy, commercial secrets, and even national security.

[0004] Data leakage is not only about the confidentiality of information, but also about the integrity and availability of data. If the data transmitted by drones is maliciously tampered with or deleted, it may lead to incorrect decisions and evaluations, which in turn affects the normal operation of drones and the effectiveness of mission execution. At the same time, malicious manipulation of drones may lead to unpredictable consequences, such as using drones to carry out terrorist attacks or criminal activities, which poses a direct threat to public safety.

[0005] Therefore, in response to the above security challenges, it is urgent to conduct research on key technologies for drone measurement and control network security. Through in-depth research on drone communication encryption, anti-interference, identity authentication, access control, intrusion detection and defense technologies, the security protection capabilities of drone systems can be effectively improved to ensure that they can maintain stable and reliable operation in the face of various network threats. In addition, strengthening the security of drone software, promptly patching possible security vulnerabilities, and establishing a comprehensive security management and audit mechanism are also crucial to preventing potential risks. Summary of the invention

[0006] Aiming at the problem that data transmitted by drones can be easily maliciously tampered with or deleted, leading to erroneous decisions and evaluations, and affecting the normal operation of drones and the performance of mission execution, the present invention proposes a method, system, equipment and medium for secure communication of unmanned aerial vehicle measurement and control networks; uses hash technology to prevent drone data from being tampered with during transmission; uses multi-factor identity authentication to verify whether the identity of the drone to be networked is legal; and calls an encryption algorithm to encrypt drone communication data, thereby ensuring secure communication of the unmanned aerial vehicle measurement and control network and improving the confidentiality, reliability and anti-interference capability of drone communications.

[0007] The specific implementation contents of the present invention are as follows: A method for secure communication of unmanned aerial vehicle measurement and control network, specifically comprising: Initialize the system and generate system common parameters; Register the obtained drone identity information through the unmanned aerial vehicle measurement and control network, and obtain the drone private key; Generate a signature based on the private key of the drone to be networked and send an identity authentication message; Verify the legality of the drone identity to be networked based on the system public parameters and identity response message; Legal drones to be networked are added to the network, and encryption algorithms are used to encrypt drone communication data to ensure secure communication of the unmanned aerial vehicle measurement and control network.

[0008] In order to better implement the present invention, further, the initialization system and generation of security parameters specifically include: Generate m-order multiplicative cyclic group and symmetric linear map e: G1×G2→GT; Select a generator g from the multiplicative cyclic group G1; Select a random number, a hash function h1, a hash function h2, and a hash function h3; the hash function h2 is used to generate a hash chain; Use the random number as the system master key, and construct and publish the system public parameters based on the multiplication cyclic group, symmetric linear map e, hash function h1, hash function h2, and hash function h3.

[0009] In order to better implement the present invention, further, the identity information of the drone to be networked is registered through the unmanned aerial vehicle measurement and control network, and the drone private key is obtained, specifically including: Obtain the identity information of the drone to be networked and send it to the trusted key server through the unmanned aerial vehicle measurement and control network; Obtain the identity information of the drone to be networked from the trusted key server, and extract the identity ID of the drone to be networked; Map the identity ID of the drone to be networked to the elements of the multiplication cyclic group G1, and calculate the private key of the drone to be networked; Register the identity ID and private key of the drone to be networked offline to the drone to be networked.

[0010] In order to better implement the present invention, further, the generation of the signature of the drone to be networked according to the private key of the drone to be networked and sending the identity authentication message specifically includes: Generate a hash chain based on the private key of the drone to be networked, the hash function h2, and the set length, and obtain the tail element Hw of the hash chain; Calculate the signature information Q of the drone to be networked based on the hash chain tail element Hw, drone identity ID, and timestamp; Calculate the security parameter summary of the drone to be networked based on the signature information Q of the drone to be networked, the selected random number, and the system public parameters; Generate the signature of the drone to be networked according to the security parameter summary of the drone to be networked; According to the signature of the drone to be networked, an identity authentication message is generated and sent.

[0011] In order to better implement the present invention, further, the public key is calculated according to the system public parameters and the identity response message to verify whether the identity of the drone to be networked is legal, specifically including: According to the identity authentication message, obtain the timestamp and verify whether the timestamp is valid; If the timestamp is valid, obtain the identity ID of the drone to be networked, and calculate the signature information Q' of the drone to be networked; If the signature information Q' of the drone to be networked is consistent with the signature information Q of the drone to be networked, it is determined whether the identity of the drone to be networked is legal.

[0012] In order to better implement the present invention, further, the calling of the encryption algorithm to encrypt the unmanned sensitive data and the network interface of the ground equipment specifically includes: calling the AES encryption algorithm to encrypt the UAV communication data and the sensitive information carried by the UAV, and encrypting the network interface of the ground station equipment.

[0013] In order to better implement the present invention, further, the unmanned aerial vehicle measurement and control network security communication method also includes: calling the role-based access control model RBAC and the attribute-based access control model ABAC, regularly reviewing and monitoring user permissions, ensuring the rationality of permissions, and preventing potential internal threats.

[0014] Based on the unmanned aircraft measurement and control network security communication method proposed above, in order to better realize the present invention, further, an unmanned aircraft measurement and control network security communication system is proposed, which is used to execute the unmanned aircraft measurement and control network security communication method proposed above; it includes an initialization unit, an authentication unit, a verification unit, and an encryption unit; The initialization unit is used to initialize the system and generate system common parameters; The authentication unit is used to register the acquired identity information of the unmanned aerial vehicle to be networked through the unmanned aerial vehicle measurement and control network and obtain the private key of the unmanned aerial vehicle; generate a signature according to the private key of the unmanned aerial vehicle to be networked, and send an identity authentication message; The verification unit is used to verify whether the identity of the drone to be networked is legal based on the system public parameters and the identity response message; The encryption unit is used to connect the legal drones to the network and call the encryption algorithm to encrypt the drone communication data to ensure the secure communication of the unmanned aerial vehicle measurement and control network.

[0015] Based on the above-proposed unmanned aerial vehicle measurement and control network security communication method, in order to better realize the present invention, further, an electronic device is proposed, which includes a memory and a processor; a computer program is stored on the memory; when the computer program is executed on the processor, the above-proposed unmanned aerial vehicle measurement and control network security communication method is implemented.

[0016] Based on the above-proposed unmanned aerial vehicle measurement and control network secure communication method, in order to better realize the present invention, further, a computer-readable storage medium is proposed, on which computer instructions are stored; when the computer instructions are executed on the above-mentioned electronic device, the above-mentioned unmanned aerial vehicle measurement and control network secure communication method is implemented.

[0017] The present invention has the following beneficial effects: (1) The present invention improves the security of the authentication process through multi-factor identity authentication, and implements fine-grained permission management based on the role-based access control (RBAC) and attribute-based access control (ABAC) models. Regularly review and monitor user permissions to ensure the rationality of permissions and prevent potential internal threats.

[0018] (2) The present invention encrypts communication data and sensitive data through encryption algorithms to ensure confidentiality during transmission, and prevents drone data from being tampered with during transmission through hash technology; thus ensuring secure communication of the unmanned aerial vehicle measurement and control network between the drone and the ground control station. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 The present invention provides a schematic block diagram of the overall process. DETAILED DESCRIPTION

[0020] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. It should be understood that the described embodiments are only part of the embodiments of the present invention, not all of the embodiments, and therefore should not be regarded as limiting the scope of protection. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technical personnel in this field without making creative work are within the scope of protection of the present invention.

[0021] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "disposed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be the internal communication of two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0022] Embodiment 1: This embodiment provides a method for secure communication of an unmanned aerial vehicle measurement and control network. Figure 1 As shown, the specific steps include: Step S1: Initialize the system and generate system common parameters.

[0023] Step S1 specifically includes the following steps: Step S11: Generate an m-order multiplication cyclic group and a symmetric linear map e: G1×G2→GT; Step S12: Select a generator g from the multiplication cyclic group G1; Step S13: Select a random number, a hash function h1, a hash function h2, and a hash function h3; the hash function h2 is used to generate a hash chain; Step S13: Use the random number as the system master key, and construct and publish the system public parameters according to the multiplication cyclic group, the symmetric linear map e, the hash function h1, the hash function h2, and the hash function h3.

[0024] Step S2: Register the obtained identity information of the drone to be networked through the unmanned aerial vehicle measurement and control network, and obtain the drone private key.

[0025] Step S2 specifically includes the following steps: Step S21: Obtain the identity information of the drone to be networked, and send it to the trusted key server through the unmanned aerial vehicle measurement and control network; Step S22: Obtain the identity information of the drone to be networked from the trusted key server, and extract the identity ID of the drone to be networked; Step S23: Map the identity ID of the drone to be networked to an element of the multiplication cyclic group G1, and calculate the private key of the drone to be networked; Step S24: offline registering the identity ID of the drone to be networked and the private key of the drone to be networked to the drone to be networked.

[0026] Step S3: Generate a signature based on the private key of the drone to be networked, and send an identity authentication message.

[0027] Step S3 specifically includes the following steps: Step S31: Generate a hash chain according to the private key of the drone to be networked, the hash function h2, and the set length, and obtain the tail element Hw of the hash chain; Step S32: Calculate the signature information Q of the drone to be networked according to the hash chain tail element Hw, the drone identity ID, and the timestamp; Step S33: Calculate the security parameter summary of the drone to be networked according to the signature information Q of the drone to be networked, the selected random number, and the system public parameters; Step S34: Generate a signature of the drone to be networked according to the security parameter summary of the drone to be networked; Step S34: Generate and send an identity authentication message based on the signature of the drone to be networked.

[0028] Step S4: Verify the legality of the identity of the drone to be networked based on the system public parameters and identity response message.

[0029] Step S4 specifically includes the following steps: Step S41: Obtain a timestamp according to the identity authentication message and verify whether the timestamp is valid; Step S42: If the timestamp is valid, obtain the identity ID of the drone to be networked, and calculate the signature information Q' of the drone to be networked; Step S43: If the signature information Q' of the drone to be networked is consistent with the signature information Q of the drone to be networked, it is determined whether the identity of the drone to be networked is legal.

[0030] Step S5: Add the legal drones to be networked to the network, and call the encryption algorithm to encrypt the drone communication data to ensure the secure communication of the unmanned aerial vehicle measurement and control network.

[0031] Step S5 specifically includes the following steps: Step S51: Adding legal drones to be networked to the network; Step S52: Call the AES encryption algorithm to encrypt the drone communication data and sensitive information carried by the drone, and encrypt the network interface of the ground station equipment to ensure secure communication of the unmanned aerial vehicle measurement and control network.

[0032] Step S6: Call the role-based access control model RBAC and the attribute-based access control model ABAC to regularly review and monitor user permissions to ensure the rationality of permissions and prevent potential internal threats.

[0033] Working principle: The communication between the drone and the ground control station is an important part of the drone system. Communication security is crucial to ensure the normal operation of the drone system. This embodiment improves the security of the authentication process through multi-factor identity authentication, and implements fine-grained permission management based on role-based access control RBAC and attribute-based access control ABAC models. Regularly review and monitor user permissions to ensure the rationality of permissions and prevent potential internal threats.

[0034] A large amount of sensitive data is involved in the drone system, such as remote sensing images, flight data, etc. Data security is of great significance to protecting user privacy. This embodiment encrypts communication data and sensitive data through an encryption algorithm to ensure confidentiality during the transmission and storage processes, and prevents drone data from being tampered with during transmission through hash technology; it ensures secure communication of the unmanned aerial vehicle measurement and control network between the drone and the ground control station.

[0035] This embodiment ensures the key technology of drone system security, which can not only protect the flight safety of the drone itself, but also ensure the security of the data it carries and the stability of the monitored environment. It greatly enhances the security of drone systems, promotes the healthy and sustainable development of the drone industry, and expands its application prospects in various fields.

[0036] Embodiment 2: Based on the above-mentioned embodiment 1, this embodiment proposes an unmanned aerial vehicle measurement and control network security communication system, which is used to execute the above-mentioned unmanned aerial vehicle measurement and control network security communication method; it includes an initialization unit, an authentication unit, a verification unit, and an encryption unit; The initialization unit is used to initialize the system and generate system common parameters; The authentication unit is used to register the acquired identity information of the unmanned aerial vehicle to be networked through the unmanned aerial vehicle measurement and control network and obtain the private key of the unmanned aerial vehicle; generate a signature according to the private key of the unmanned aerial vehicle to be networked, and send an identity authentication message; The verification unit is used to verify whether the identity of the drone to be networked is legal based on the system public parameters and the identity response message; The encryption unit is used to connect the legal drones to the network and call the encryption algorithm to encrypt the drone communication data to ensure the secure communication of the unmanned aerial vehicle measurement and control network.

[0037] This embodiment also proposes an electronic device, including a memory and a processor; the memory stores a computer program; when the computer program is executed on the processor, the above-mentioned unmanned aerial vehicle measurement and control network security communication method is implemented.

[0038] This embodiment further proposes a computer-readable storage medium, on which computer instructions are stored; when the computer instructions are executed on the above-mentioned electronic device, the above-mentioned unmanned aerial vehicle measurement and control network secure communication method is implemented.

[0039] The other parts of this embodiment are the same as those of the above-mentioned embodiment 1, and thus will not be described in detail.

[0040] The processor involved in the embodiments of the present application may be a chip. For example, it may be a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), a system on chip (SoC), a central processor unit (CPU), a network processor (NP), a digital signal processor (DSP), a microcontroller unit (MCU), a programmable logic device (PLD), or other integrated chips.

[0041] The memory involved in the embodiments of the present application may be a volatile memory or a non-volatile memory, or may include both volatile and non-volatile memories. Among them, the non-volatile memory may be a read-only memory (ROM), a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), an electrically erasable programmable read-only memory (EEPROM), or a flash memory. The volatile memory may be a random access memory (RAM), which is used as an external cache. By way of example and not limitation, many forms of RAM are available, such as static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDR SDRAM), enhanced SDRAM (ESDRAM), synchlink DRAM (SLDRAM), and direct rambus RAM (DR RAM). It should be noted that the memory of the systems and methods described herein is intended to include, but is not limited to, these and any other suitable types of memory.

[0042] It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned processes does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0043] Those of ordinary skill in the art will appreciate that the modules and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0044] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working processes of the systems, devices and modules described above can refer to the corresponding processes in the aforementioned method embodiments and will not be repeated here.

[0045] In the several embodiments provided in the present application, it should be understood that the disclosed systems, devices and methods can be implemented in other ways. For example, the device embodiments described above are only schematic. For example, the division of the modules is only a logical function division. There may be other division methods in actual implementation, such as multiple modules or components can be combined or integrated into another device, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or modules, which can be electrical, mechanical or other forms.

[0046] The modules described as separate components may or may not be physically separated, and the components shown as modules may or may not be physical modules, that is, they may be located in one device or distributed on multiple devices. Some or all of the modules may be selected according to actual needs to achieve the purpose of the present embodiment.

[0047] In addition, each functional module in each embodiment of the present application may be integrated into one device, or each module may exist physically separately, or two or more modules may be integrated into one device.

[0048] In the above embodiments, it can be implemented in whole or in part by software, hardware, firmware or any combination thereof. When implemented using a software program, it can be implemented in whole or in part 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 process or function described in the embodiment of the present application is generated in whole or in part. The computer may be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions may be stored in a computer-readable storage medium, or transmitted from one computer-readable storage medium to another computer-readable storage medium, for example, the computer instructions may be transmitted from a website site, computer, server or data center by wired (e.g., coaxial cable, optical fiber, digital subscriber line (DSL)) or wireless (e.g., infrared, wireless, microwave, etc.) mode to another website site, computer, server or data center. The computer-readable storage medium may be any available medium that a computer can access or may include one or more servers, data centers and other data storage devices that can be integrated with the medium. The available medium may be a magnetic medium (eg, a floppy disk, a hard disk, a magnetic tape), an optical medium (eg, a DVD), or a semiconductor medium (eg, a solid state disk (SSD)).

[0049] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art who is familiar with the present technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A method for secure communication of unmanned aerial vehicle measurement and control network, characterized in that: Specifically include: Initialize the system and generate system common parameters; Register the obtained drone identity information through the unmanned aerial vehicle measurement and control network, and obtain the drone private key; Generate a signature based on the private key of the drone to be networked and send an identity authentication message; Verify the legality of the drone identity to be networked based on the system public parameters and identity response message; Legal drones to be networked are added to the network, and encryption algorithms are used to encrypt drone communication data to ensure secure communication of the unmanned aerial vehicle measurement and control network.

2. The method for secure communication of unmanned aerial vehicle measurement and control network according to claim 1, characterized in that: The initialization system and generation of security parameters specifically include: Generate m-order multiplicative cyclic group and symmetric linear map e: G1×G2→GT; Select a generator g from the multiplicative cyclic group G1; Select a random number, a hash function h1, a hash function h2, and a hash function h3; the hash function h2 is used to generate a hash chain; Use the random number as the system master key, and construct and publish the system public parameters based on the multiplication cyclic group, symmetric linear map e, hash function h1, hash function h2, and hash function h3.

3. The method for secure communication of unmanned aerial vehicle measurement and control network according to claim 1, characterized in that: The identity information of the drone to be networked is registered through the unmanned aerial vehicle measurement and control network, and the drone private key is obtained, specifically including: Obtain the identity information of the drone to be networked and send it to the trusted key server through the unmanned aerial vehicle measurement and control network; Obtain the identity information of the drone to be networked from the trusted key server, and extract the identity ID of the drone to be networked; Map the identity ID of the drone to be networked to the elements of the multiplication cyclic group G1, and calculate the private key of the drone to be networked; Register the identity ID and private key of the drone to be networked offline to the drone to be networked.

4. The method for secure communication of unmanned aerial vehicle measurement and control network according to claim 3, characterized in that: The generating of the signature of the drone to be networked according to the private key of the drone to be networked and sending of the identity authentication message specifically includes: Generate a hash chain based on the private key of the drone to be networked, the hash function h2, and the set length, and obtain the tail element Hw of the hash chain; Calculate the signature information Q of the drone to be networked based on the hash chain tail element Hw, drone identity ID, and timestamp; Calculate the security parameter summary of the drone to be networked based on the signature information Q of the drone to be networked, the selected random number, and the system public parameters; Generate the signature of the drone to be networked according to the security parameter summary of the drone to be networked; According to the signature of the drone to be networked, an identity authentication message is generated and sent.

5. The method for secure communication of unmanned aerial vehicle measurement and control network according to claim 4, characterized in that: The public key is calculated based on the system public parameters and the identity response message to verify whether the identity of the drone to be networked is legal, specifically including: According to the identity authentication message, obtain the timestamp and verify whether the timestamp is valid; If the timestamp is valid, obtain the identity ID of the drone to be networked, and calculate the signature information Q' of the drone to be networked; If the signature information Q' of the drone to be networked is consistent with the signature information Q of the drone to be networked, it is determined whether the identity of the drone to be networked is legal.

6. The method for secure communication of unmanned aerial vehicle measurement and control network according to claim 1, characterized in that: The calling of the encryption algorithm to encrypt the unmanned sensitive data and the network interface of the ground equipment specifically includes: calling the AES encryption algorithm to encrypt the UAV communication data and the sensitive information carried by the UAV, and encrypting the network interface of the ground station equipment.

7. The method for secure communication of unmanned aerial vehicle measurement and control network according to claim 6, characterized in that: The unmanned aerial vehicle measurement and control network security communication method also includes: calling the role-based access control model RBAC and the attribute-based access control model ABAC, regularly reviewing and monitoring user permissions, ensuring the rationality of permissions, and preventing potential internal threats.

8. An unmanned aircraft measurement and control network security communication system, used to execute the unmanned aircraft measurement and control network security communication method as claimed in claim 1; characterized in that: It includes an initialization unit, an authentication unit, a verification unit, and an encryption unit; The initialization unit is used to initialize the system and generate system common parameters; The authentication unit is used to register the acquired identity information of the unmanned aerial vehicle to be networked through the unmanned aerial vehicle measurement and control network and obtain the private key of the unmanned aerial vehicle; generate a signature according to the private key of the unmanned aerial vehicle to be networked, and send an identity authentication message; The verification unit is used to verify whether the identity of the drone to be networked is legal based on the system public parameters and the identity response message; The encryption unit is used to connect the legal drones to the network and call the encryption algorithm to encrypt the drone communication data to ensure the secure communication of the unmanned aerial vehicle measurement and control network.

9. An electronic device, characterized in that: It comprises a memory and a processor; the memory stores a computer program; when the computer program is executed on the processor, the unmanned aerial vehicle measurement and control network security communication method as described in any one of claims 1 to 7 is implemented.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores computer instructions; when the computer instructions are executed on the electronic device as described in claim 9, the unmanned aerial vehicle measurement and control network security communication method as described in any one of claims 1-7 is implemented.

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