A method and system for covert Wi-Fi backscattering using FCS characteristics

Abstract

This invention discloses a method and system for Wi-Fi backscatter covert communication utilizing the FCS (Focus Cross Section) characteristic, belonging to the field of wireless communication. The implementation method of this invention is as follows: the transmitting tag encodes covert information into location information, and based on the location information containing the covert information encoding, generates an intentional error during the backscattering of the Wi-Fi signal. After demodulating the Wi-Fi backscattered signal, the receiving end uses the FCS characteristic to determine the location of the intentional error, and demodulates the covert information according to the covert information encoding, thus realizing Wi-Fi backscatter covert communication using the FCS characteristic. By encoding the covert information into intentional error location information at the transmitting end, the amount of covert information carried in the Wi-Fi data packet containing the covert information is increased, thereby improving the throughput of covert communication. This invention also discloses a system for implementing the aforementioned method for Wi-Fi backscatter covert communication utilizing the FCS characteristic.

Description

Technical Field

[0001] This invention relates to a method and system for Wi-Fi backscatter covert communication utilizing FCS characteristics, belonging to the field of wireless communication. Background Technology

[0002] A backscatter communication system consists of three parts: a radio frequency (RF) signal source, a tag, and a receiver. The RF signal source provides the carrier signal that powers the tag and transmits information. The tag is typically passive, transmitting information by modulating and reflecting the RF signal generated by the RF signal source. The receiver receives and demodulates the backscattered signal containing the tag's information. In short, in this backscatter communication system, the tag itself does not generate a carrier signal; instead, it transmits information by modulating the signal from the RF signal source, resulting in low power consumption and low cost.

[0003] Unlike traditional backscatter communication systems, Wi-Fi backscatter communication systems do not require a dedicated radio frequency signal source. Tags transmit information by modulating and reflecting commercial Wi-Fi signals in the environment, which further reduces the power consumption of the transmission system. Moreover, compared with traditional backscatter communication systems, the reflected signals generated by Wi-Fi backscatter communication systems are similar to Wi-Fi signals in the environment in terms of signal characteristics, but the signal strength is lower, resulting in higher concealment.

[0004] In the Wi-Fi backscatter covert communication transmission system, the tag uses commercial Wi-Fi signals to transmit covert information. The commercial Wi-Fi signal conforms to the 802.11 protocol and adds a 32-bit cyclic checksum as a frame check sequence (FCS) at the end of the frame to determine whether the Wi-Fi data packet has encountered errors during transmission. Summary of the Invention

[0005] The purpose of this invention is to provide a method for covert Wi-Fi backscattering communication utilizing the Frame Check Sequence (FCS) characteristic. The transmitting tag encodes covert information into location information. Based on this location information, the tag intentionally generates an error during the backscattering of the Wi-Fi signal. After demodulating the Wi-Fi backscattered signal, the receiving end uses the FCS characteristic to determine the location of the intentional error. Based on this location, the covert information is demodulated according to the encoding containing the covert information, thus achieving covert Wi-Fi backscattering communication using the FCS characteristic. This invention has advantages such as strong concealment, simple structure, high implementability, low power consumption, and low cost.

[0006] The objective of this invention is achieved through the following technical solution.

[0007] This invention discloses a method for covert Wi-Fi backscattering communication utilizing the FCS (Focused Cross-Sectional) characteristic. The transmitting tag encodes covert information into location information. Based on this location information, an intentional error is generated during the backscattering of the Wi-Fi signal. The receiving end demodulates the Wi-Fi backscattered signal, uses the FCS characteristic to determine the location of the intentional error, and then demodulates the covert information according to the encoded covert information. This achieves covert Wi-Fi backscattering communication using the FCS characteristic. By encoding covert information into intentional error location information at the transmitting end, the amount of covert information carried in the Wi-Fi data packet containing the covert information is increased, thereby improving the throughput of covert communication.

[0008] The method for Wi-Fi backscatter covert communication utilizing FCS characteristics disclosed in this invention includes the following steps:

[0009] Step 1: The transmitting tag encodes the concealed information into location information.

[0010] Step 1.1: When the register of the covert information acquisition and storage module of the transmitting end tag is not full, continuously acquire the covert information to be transmitted from the environment or the host computer and store it in the tag register.

[0011] Step 1.2: The microprocessor of the transmitting tag encodes and frames the covert information, and converts the content of the encoded and framed covert information into a predetermined delay based on a mapping relationship, and determines the intentional error location through the predetermined delay.

[0012] Step 2: Based on the location information containing the hidden information encoding, an intentional error is generated during the backscattering of the Wi-Fi signal.

[0013] Step 2.1: When the transmission status control module detects the presence of a Wi-Fi signal in the environment, it transmits the "start transmission" command to the covert information modulation module of the transmitting tag.

[0014] Step 2.2: After receiving the "Start Transmission" command, the covert information modulation module of the transmitting tag reflects the Wi-Fi signal and performs a predetermined frequency shift on it to avoid interference between the original signal and the backscattered signal. Simultaneously, based on the delay after encoding and framing in Step 1.2, the information in the Wi-Fi data segment is destroyed after a specified delay, thereby causing an intentional error in the reflected Wi-Fi signal at a designated location.

[0015] The Wi-Fi signal containing hidden information after being damaged in step 2.2 is not significantly different from ordinary Wi-Fi signals in terms of signal characteristics. However, after demodulation, the signal content will show intentional errors in the corresponding designated area. These intentional errors are not significantly different from ordinary Wi-Fi signals that have been damaged due to environmental factors, and thus have strong concealment.

[0016] Preferably, in step 2.2, the method of destroying the information in the Wi-Fi data segment after a specified delay, thereby causing the reflected Wi-Fi signal to produce an intentional error at a specified location, includes amplitude shift keying, phase shift keying, and frequency shift keying.

[0017] For the sake of hardware simplicity, as a further preferred option, in step 2.2, the information in the Wi-Fi data segment is destroyed by phase shift keying.

[0018] Step 3: After demodulating the Wi-Fi backscatter signal at the receiving end, the intentionally erroneous location is determined using the FCS characteristic. The covert information is then encoded and demodulated according to the covert information contained therein. In other words, the covert Wi-Fi backscatter communication is realized using the FCS characteristic.

[0019] Step 3.1: The covert receiver receives and demodulates the Wi-Fi signal and extracts the Wi-Fi data packets that have transmission errors.

[0020] Step 3.2: The covert receiver extracts the Wi-Fi data packet containing covert information from the Wi-Fi data packet extracted in step 3.1 by utilizing the characteristics of FCS, and determines the location of the intentional error in the data packet;

[0021] Step 3.3: Based on the error location and the pre-determined mapping relationship between the error location and the covert information, the covert receiver demodulates the covert information transmitted by the tag, thus realizing covert communication.

[0022] Furthermore, the specific steps in step 1.2 where the microprocessor encodes and frames the covert information, and then converts the content of the encoded and framed covert information into a certain time delay are as follows:

[0023] Step 1.2-1: The microprocessor pre-sets the encoding bit length N based on requirements and the covert receiver pre-determines the delay time T0+ΔT corresponding to when the covert information transmission is "1". Here, T0 is the delay to avoid disrupting fields such as preamble and service in the Wi-Fi frame.

[0024] Step 1.2-2: The microprocessor combines the hidden information in the register into an N-bit binary number (B0, B1, B2, ...) according to the number of bits N in the encoding.

[0025] Steps 1.2-3: Based on the above N-bit binary number and the predetermined delay time T0+ΔT corresponding to the hidden information "1", calculate the delay time that should be performed for transmitting the combined binary numbers B0, B1, B2... respectively: T0+B k ΔT, k = 1, 2, 3, ...

[0026] Furthermore, in step 2.2, the specific method by which the covert information modulation module disrupts Wi-Fi data packets using phase-shift keying is as follows:

[0027] In a backscattering system, the tag antenna impedance Z A and load impedance Z L It can be expressed in the following form:

[0028]

[0029] The reflectance coefficient of the label can be expressed in the following form:

[0030]

[0031] Therefore, by switching the open and closed states of the RF switch, the tag load impedance Z can be changed. L The value of the signal is used to generate a square wave signal, which is then used to shift the center frequency of the backscattered signal, thereby avoiding interference from the original signal. Furthermore, by changing the frequency, amplitude, and phase of this square wave, it is possible to generate intentional errors based on frequency shift keying, amplitude shift keying, or phase shift keying.

[0032] Therefore, in step 2.2, the number of intentionally erroneous bits N1 generated by the transmitting tag is first preset. Then, when a Wi-Fi signal is detected in the environment, the RF switch continuously switches between open and closed states at a certain frequency, generating a square wave of the corresponding frequency. The antenna reflects this Wi-Fi signal and shifts the reflected Wi-Fi signal by a certain frequency band compared to the original signal to avoid interference from the original signal. Simultaneously, after the delay set in step 1.2, the RF switch changes the phase of the square wave signal generated by the RF switch, thereby destroying part of the information carried by the original Wi-Fi signal. Based on the number of destruction bits N1, after a period of time, the phase of the square wave signal is restored to its original state, and the information is no longer destroyed.

[0033] The radio frequency switch can destroy some of the information carried by the original Wi-Fi signal in the above way.

[0034] Furthermore, in step 3.2, the covert receiver utilizes the characteristics of FCS to distinguish Wi-Fi frames containing covert information from the received data frames with transmission errors, and determines the location of the error generated by the transmitter tag in the Wi-Fi frames containing covert information. The specific steps are as follows:

[0035] Commercial Wi-Fi signals are appended with an FCS (Error Detection System) for error detection. The result of this error detection is only related to the transmission errors of the received data packets, and is not related to the specific content of the data packets.

[0036] Therefore, if there exists a sequence S of the same length as the received data, where the received data is 0 where there are no errors and 1 where there are errors; and there also exists a sequence S0 of all zeros of the same length as the received data, where all corresponding cyclic parity bits are 0, then S is taken as the result of S0 when a transmission error occurs, and the location of this transmission error is the same as the location of the error in the received data. In this case, if the received data is P, then performing FCS check on S and P will yield the same result.

[0037] In summary, if the received data packet P contains hidden information... R If no errors occur during transmission, but intentional errors are introduced by the tag only at designated locations, then the equal-length sequence S generated in the above manner is... R It is expressed in the following form:

[0038] S R ={0,0,......0,S R0 ,0,0,......0,S R1 ,0,0,......0,S Rk ,0,0,......},k=0,1,2......

[0039] Among them, S Rk This area is where intentional errors may occur. These intentional errors are caused by the backscattering tag at the transmitting end disrupting the original transmitted Wi-Fi information through phase modulation during the reflection of the Wi-Fi signal.

[0040] In step 2.2, the transmitting tag, while reflecting the Wi-Fi signal based on the delayed signal, simultaneously disrupts, and only disrupts, a portion of the signal content through phase modulation. Therefore, in the absence of transmission errors, the S in the Wi-Fi data packet containing the hidden information... Rk There is exactly one non-zero position. Based on the pre-set number of bits N to be destroyed at the transmitting end and the modulation scheme of the received Wi-Fi signal, the maximum value N of the number of bits destroyed at the receiving end can be determined, then S Rk The number of bits will be less than or equal to N (N ≤ the number of bits in FCS).

[0041] In the aforementioned case, let the error first occur at Then the corresponding S R It is expressed in the following form:

[0042]

[0043] in,

[0044] Based on the received Wi-Fi data packets, calculate the corresponding FCS check result R, then S R The corresponding verification result is also R.

[0045] From R, we can find S′. R The corresponding verification result R′.

[0046] At this time, by S′ R Its characteristics The value of is R′. If the assumption holds, then the number of digits in R′ will be the same as . The same, meaning less than or equal to N, and vice versa.

[0047] This allows us to pinpoint the location of the intentional error caused by the label.

[0048] This invention also discloses a Wi-Fi backscatter covert communication system utilizing FCS characteristics to implement the aforementioned Wi-Fi backscatter covert communication method, comprising a covert information transmitting tag and a covert information receiving end. The covert information transmitting tag includes a transmission state control module, a microprocessor, and a covert information modulation module; the covert information receiving end is a common commercial Wi-Fi receiver.

[0049] The transmission status control module of the covert information transmitter includes a comparator and a radio frequency detector. The radio frequency detector outputs the average value of the signal received by the antenna, and the comparator compares this average value with a set threshold to output a high or low level to determine whether a Wi-Fi signal exists in the environment.

[0050] Specifically, a threshold V is set in advance. ref When the average value of the input signal Greater than threshold V ref When the average value of the input signal is 100000, the comparator outputs a high level, assuming the presence of a Wi-Fi signal in the environment; when the average value of the input signal is 1000000, the comparator outputs a high level. Less than threshold V ref When the signal is low, it is assumed that there is no Wi-Fi signal in the environment, and the comparator outputs a low level.

[0051] The microprocessor stores the covert information to be transmitted and controls the covert information modulation module to transmit the covert information based on the information fed back from the transmission status control module. When the transmission status control module outputs a high level, the microprocessor assumes that a Wi-Fi signal exists in the environment and sends a "start transmission" command to the covert information modulation module.

[0052] The covert information modulation module consists of a radio frequency switch. Upon receiving a "start transmission" command from the microprocessor, the covert information modulation module, under the control of the microprocessor, changes the reflection coefficient by altering the on / off state of the radio frequency switch. This module continuously switches the on / off state of the switch at a specified frequency to generate a square wave of that frequency, which is used to change the center frequency of the reflected Wi-Fi signal, thus preventing interference from ambient Wi-Fi signals.

[0053] Furthermore, by altering the frequency, amplitude, and phase of the square wave, it is possible to generate intentional errors based on frequency shift keying (FPS), amplitude shift keying (APS), or phase shift keying (PSK). This system employs phase shift keying.

[0054] Beneficial effects

[0055] 1. The present invention discloses a method and system for covert Wi-Fi backscattering communication using FCS characteristics. The transmitting end tag encodes covert information into location information, and generates an intentional error during the backscattering of the Wi-Fi signal based on the location information containing the covert information encoding. After the receiving end demodulates the Wi-Fi backscattered signal, it uses the FCS characteristics to determine the location of the intentional error, and demodulates the covert information according to the covert information encoding, that is, it realizes covert Wi-Fi backscattering communication using FCS characteristics.

[0056] 2. The method and system for covert Wi-Fi communication using FCS characteristics disclosed in this invention increases the amount of covert information carried in Wi-Fi data packets containing covert information by encoding covert information as intentionally erroneous location information at the transmitting end, thereby increasing the throughput of covert communication.

[0057] 3. The method and system for covert Wi-Fi communication utilizing FCS characteristics disclosed in this invention employs high backscattering commercial Wi-Fi signals for covert communication. The Wi-Fi signal used is identical in signal characteristics to the original Wi-Fi signal but has lower signal strength. Therefore, the transmitted covert signal is difficult for the eavesdropping end to distinguish and identify from common Wi-Fi signals in the environment. Thus, this invention possesses high covertness.

[0058] 4. The Wi-Fi backscatter covert communication method and system disclosed in this invention utilizes FCS characteristics. The Wi-Fi signal carrying covert information inherently possesses excellent covertness, eliminating the need for additional interference. Furthermore, the covert signal receiver described in this invention is a commercially available Wi-Fi receiving device such as a wireless network card, eliminating the need for a dedicated covert signal receiver. Therefore, compared to existing covert communication transmission systems, the transmission system of this invention has lower hardware complexity and higher feasibility.

[0059] 5. The present invention discloses a method and system for covert Wi-Fi backscatter communication utilizing FCS characteristics. The receiving end, by leveraging FCS characteristics, can complete information demodulation using only a single receiver, whereas existing Wi-Fi backscatter communication systems all employ two receivers operating in different frequency bands for demodulation. Therefore, compared to existing Wi-Fi backscatter communication systems, the present invention reduces the complexity of the receiving end.

[0060] 6. The Wi-Fi backscatter covert communication method and system disclosed in this invention utilizes FCS characteristics. It can generate intentional errors at a specified location using only passive analog devices on the tag. The tag has the characteristics of simple structure and high feasibility. Attached Figure Description

[0061] Figure 1 This is a flowchart of the Wi-Fi backscatter covert communication method utilizing FCS characteristics according to the present invention;

[0062] Figure 2 This is a schematic diagram of a basic backscatter communication system.

[0063] Figure 3 This is a module architecture diagram of a Wi-Fi backscatter covert communication device that utilizes FCS characteristics;

[0064] Figure 4 This is a schematic diagram of the transmitter tag structure of the Wi-Fi backscatter covert communication device utilizing FCS characteristics according to an embodiment of the present invention;

[0065] Figure 5 This is a schematic diagram of the bit error rate at the receiving end provided in an embodiment of the present invention. Detailed Implementation

[0066] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. The technical problems solved by the present invention and its beneficial effects are also described. It should be noted that the described embodiments are only intended to facilitate understanding of the present invention and do not constitute any limitation thereof.

[0067] This embodiment uses a commercial wireless network card, Qualcomm Atheros AR938x, as the transmitter to generate a Wi-Fi signal, and a MacBook Air laptop with the commercial wireless network card as the receiver to demodulate the received commercial Wi-Fi signal into Wi-Fi data packets.

[0068] To verify the universality of the invention, the following Wi-Fi signals were selected for backscattering: a Wi-Fi signal with a transmission rate of 1 Mbps conforming to the 802.11b protocol, a Wi-Fi signal with a transmission rate of 6 Mbps conforming to the 802.11g protocol, and a Wi-Fi signal with a transmission rate of 6.5 Mbps conforming to the 802.11n protocol.

[0069] The FCS of the 802.11 protocol described in this embodiment is a 32-bit cyclic checksum, which is generated as follows:

[0070] First, add 32 zeros to the end of the information to be transmitted.

[0071] Then, using the generator polynomial described in the following formula as the divisor, the zero-padded information to be transmitted is divided modulo 2, and the 32-bit remainder obtained is the cyclic check code. This remainder is added to the end of the Wi-Fi frame for transmission, thus realizing the generation of the 32-bit cyclic check code.

[0072] G(x)=x 32 +x 26 +x 23 +x 22 +x 16 +x 12 +x 11 +x 10 +x 8 +x 7 +x 5 +x 4 +x 2 +x+1

[0073] At the receiving end, the same generator polynomial is used as the divisor to perform modulo-2 division on the received data. If the transmission is error-free, the remainder is 0; otherwise, the remainder is non-zero.

[0074] For the Wi-Fi transmission signals that comply with the 802.11b / g / n protocol, such as Figure 1 As shown, the specific implementation steps of the FCS-based Wi-Fi backscatter covert communication system are as follows:

[0075] Step 1: The transmitting tag encodes the concealed information into location information.

[0076] Step 1.1: When the register of the covert information acquisition and storage module of the transmitting end tag is not full, the covert information to be transmitted is continuously acquired from the environment or the host computer and stored in the tag register.

[0077] Step 1.2: The microprocessor of the transmitting tag encodes and frames the covert information, and converts the content of the encoded and framed covert information into a predetermined delay based on a mapping relationship, and determines the intentional error location through the predetermined delay.

[0078] In this embodiment, the microprocessor pre-sets the number of intentionally erroneous bits generated by the transmitter tag to N=4 based on requirements, and pre-determines the delay time T0+ΔT corresponding to when the covert information transmission is "1". Here, T0=40μs is the delay to avoid disrupting fields such as preamble and service in the Wi-Fi frame;

[0079] Then, the microprocessor synthesizes a 4-bit binary number (B0, B1, B2...) from the hidden information in the register according to the encoded bit array.

[0080] Finally, based on the above 4-bit binary number and the predetermined delay time corresponding to the hidden information "1", the delay time corresponding to the transmission of the above combined binary numbers B0, B1, B2... is calculated as follows: (40 + 8B k μs, where k = 0, 1, 2, ...

[0081] Step 2: Based on the location information containing the hidden information encoding, an intentional error is generated during the backscattering of the Wi-Fi signal.

[0082] Step 2.1: When the transmission status control module detects the presence of a Wi-Fi signal in the environment, it transmits the "start transmission" command to the covert information modulation module of the transmitting tag.

[0083] Step 2.2: After receiving the "Start Transmission" command, the covert information modulation module of the transmitting tag reflects the Wi-Fi signal and performs a predetermined frequency shift on it to avoid interference between the original signal and the backscattered signal. Simultaneously, based on the delay after encoding and framing in Step 1.2, the information in the Wi-Fi data segment is destroyed after a specified delay, thereby causing an intentional error in the reflected Wi-Fi signal at a designated location.

[0084] In this embodiment, to ensure the simplicity of the transmitter tag hardware structure, phase shift keying is used to destroy the information in the Wi-Fi data segment. The specific method is as follows:

[0085] The transmitter tag antenna impedance Z used in this embodiment is... A=50Ω, when the switch is closed, the load impedance Z L =0Ω, and when the switch is open, the load impedance Z L →+∞.

[0086] Furthermore, in this embodiment, the reflectance coefficient of the tag can be expressed in the following form:

[0087]

[0088] Therefore, when the switch is open, the reflection coefficient Γ of the transmitting tag in this embodiment is 1, while when the switch is closed, the reflection coefficient Γ of the transmitting tag in this embodiment is -1. Thus, by continuously switching the open and closed states of the RF switch, a square wave signal with an amplitude of 1 can be generated. By changing the phase of this square wave signal, intentional errors can be generated through phase shift keying.

[0089] In this embodiment, the frequency of the square wave signal is 20MHz, thereby shifting the center frequency of the WiFi signal reflected by the transmitting tag antenna by 20MHz compared to the original signal. Simultaneously, after the delay set in step 1.2, this embodiment changes the phase of the square wave signal generated by the RF switch, thereby destroying some of the information carried by the original Wi-Fi signal. Based on a preset destruction bit depth N1 = 24, after a period of time, the phase of the square wave signal is restored to its original state, and the information is no longer destroyed.

[0090] Step 3: After demodulating the Wi-Fi backscatter signal at the receiving end, the intentionally erroneous location is determined using the FCS characteristic. The covert information is then encoded and demodulated according to the covert information contained therein. In other words, the covert Wi-Fi backscatter communication is realized using the FCS characteristic.

[0091] Step 3.1: The covert receiver receives and demodulates the Wi-Fi signal and extracts the Wi-Fi data packets that have transmission errors.

[0092] Step 3.2: The covert receiver extracts the Wi-Fi data packet containing covert information from the Wi-Fi data packet extracted in step 3.1 by utilizing the characteristics of FCS, and determines the location of the intentional error in the data packet.

[0093] In this embodiment, based on the delay, number of bits to destroy, and the Wi-Fi data packet transmission rate set at the transmitter, the possible times when intentional errors may occur are as follows:

[0094] When the ambient Wi-Fi signal backscattered by the transmitting tag is a Wi-Fi signal with a transmission rate of 1 Mbps conforming to the 802.11b protocol, the intentionally incorrect position is: (40+40B)μs~(64+40B)μs, where B is the decimal representation of the 4-bit binary number after encoding and framing, B=0,1,2,......; when the ambient Wi-Fi signal backscattered by the transmitting tag is a Wi-Fi signal with a transmission rate of 6 Mbps conforming to the 802.11g protocol, the intentionally incorrect position is: ( 40+40B)μs~(44+40B)μs, where B is the decimal representation of the 4-bit binary number after encoding and framing, B=0,1,2,......; When the ambient Wi-Fi signal that the transmitting tag backscatters is a Wi-Fi signal with a transmission rate of 6.5Mbps conforming to the 802.11n protocol, the intentionally incorrect position is: (40+40B)μs~(44+40B)μs, where B is the decimal representation of the 4-bit binary number after encoding and framing, B=0,1,2,........

[0095] Therefore, the location of the intentional error can be determined based on the FCS verification result of the received data packet containing the intentional error.

[0096] Step 3.3: Based on the error location and the pre-determined mapping relationship between the error location and the covert information, the covert receiver demodulates the covert information transmitted by the tag, thus realizing covert communication.

[0097] Specifically, this embodiment conducts experiments within the line-of-sight range, and the test result bit error rate curve is as follows. Figure 5 As shown in the figure, transmission within 20 meters was achieved. However, due to the high packet loss rate during wireless signal transmission, the test results showed a high bit error rate. In future tests, performance can be improved by increasing the number of times the concealed information is retransmitted at the transmitting end.

[0098] In the above experiment, the microprocessor part of the covert information storage module and the covert information modulation module of the tag adopts Igllo Nano AGLN250 FPGA;

[0099] The radio frequency switch section of the covert information modulation module uses the ADG902 radio frequency switch from ANALOG DEVICES.

[0100] The transmission status control module uses ANALOG DEVICES' LT5534 as a rectifier and ONSEMI's NCS2200 as a comparator.

[0101] Under the above experimental conditions, the maximum transmission distance for the concealed information transmitted by the transmitting device is greater than or equal to 20m. It should be emphasized that in practical implementation, the transmission distance can be further increased by increasing the number of retransmissions or increasing antenna gain. Furthermore, encryption methods such as link encryption, node encryption, and end-to-end encryption can be used to further encrypt the concealed information and ensure its secure transmission.

[0102] The covert communication method based on the backscattering of Wi-Fi signals using the 802.11b / g / n protocol described above can achieve the transmission of covert information. Because the reflected Wi-Fi signal has the same signal characteristics as the original signal and its energy is low, the likelihood of this covert transmission system being detected is significantly reduced.

[0103] The above description is merely a preferred embodiment of the present invention and is not intended to limit the implementation of the present invention. Those skilled in the art can make other variations and modifications based on the above description. It is impossible to exhaustively list all possible embodiments here; any obvious variations and modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A method for covert Wi-Fi backscattering communication utilizing FCS characteristics, characterized in that: Includes the following steps: Step 1: The transmitting tag encodes the covert information into location information; The implementation method for step one is as follows: Step 1.1: When the register of the covert information acquisition and storage module of the transmitting end tag is not full, continuously acquire the covert information to be transmitted from the environment or the host computer and store it in the tag register; Step 1.2: The microprocessor of the transmitting tag encodes and frames the covert information, and converts the content of the encoded and framed covert information into a predetermined delay based on a mapping relationship, and determines the intentional error location through the predetermined delay; Step 2: Based on the location information containing the hidden information encoding, an intentional error is generated during the backscattering of the Wi-Fi signal; The second step is implemented as follows: Step 2.1: When the transmission status control module detects the presence of a Wi-Fi signal in the environment, it transmits the "start transmission" command to the covert information modulation module of the transmitting tag. Step 2.2: After receiving the "Start Transmission" command, the covert information modulation module of the transmitting tag reflects the Wi-Fi signal and performs a predetermined frequency shift on the Wi-Fi signal to avoid interference between the original signal and the backscattered signal. At the same time, based on the delay after encoding and framing in Step 1.2, the information in the Wi-Fi data segment is destroyed after a specified delay, thereby causing the reflected Wi-Fi signal to produce an intentional error at a specified location. The Wi-Fi signal containing hidden information after being damaged in step 2.2 is not significantly different from ordinary Wi-Fi signals in terms of signal characteristics. However, after demodulation, the signal content will show intentional errors in the corresponding designated area. These intentional errors are not significantly different from ordinary Wi-Fi signals that have been damaged due to environmental factors, and thus have strong concealment. Step 3: After demodulating the Wi-Fi backscatter signal at the receiving end, the intentionally erroneous location is determined using the FCS characteristic. The covert information is then encoded and demodulated according to the covert information contained therein. In other words, the covert Wi-Fi backscatter communication is realized using the FCS characteristic.

2. The method for Wi-Fi backscatter covert communication using FCS characteristics as described in claim 1, characterized in that: The method for implementing step three is as follows: Step 3.1: The covert receiver receives and demodulates the Wi-Fi signal, and extracts the Wi-Fi data packets that have transmission errors; Step 3.2: The covert receiver extracts the Wi-Fi data packet containing covert information from the Wi-Fi data packet extracted in step 3.1 by utilizing the characteristics of FCS, and determines the location of the intentional error in the data packet; Step 3.3: Based on the error location and the pre-determined mapping relationship between the error location and the covert information, the covert receiver demodulates the covert information transmitted by the tag, thus realizing covert communication.

3. The method for Wi-Fi backscatter covert communication using FCS characteristics as described in claim 2, characterized in that: In step 2.2, the method of deliberately erroneously destroying information in the Wi-Fi data segment after a specified delay, thereby causing the reflected Wi-Fi signal to produce an error at a specified location, includes amplitude shift keying, phase shift keying, and frequency shift keying.

4. The method for Wi-Fi backscatter covert communication using FCS characteristics as described in claim 3, characterized in that: In step 2.2, phase shift keying is used to destroy the information in the Wi-Fi data segment.

5. The method for Wi-Fi backscatter covert communication using FCS characteristics as described in claim 4, characterized in that: In step 1.2, the microprocessor encodes and frames the hidden information, and then converts the content of the encoded and framed hidden information into a certain delay. The specific steps are as follows: Step 1.2-1: The microprocessor pre-sets the number of encoding bits N based on the requirements and the covert receiver pre-determines the delay time T0+ΔT corresponding to when the covert information is transmitted as "1" based on the requirements; where T0 is the delay to avoid destroying the preamble and service fields in the Wi-Fi frame. Step 1.2-2: The microprocessor combines the hidden information in the register into an N-bit binary number (B0, B1, B2...) according to the number of bits N. Steps 1.2-3: Based on the above N-bit binary number and the predetermined delay time T0+ΔT corresponding to the hidden information "1", calculate the delay time that should be performed for transmitting the combined binary numbers B0, B1, B2... respectively: T0+B k ΔT, k = 1, 2, 3, ...

6. The method for Wi-Fi backscatter covert communication using FCS characteristics as described in claim 5, characterized in that: In step 2.2, the specific method by which the covert information modulation module disrupts Wi-Fi data packets using phase shift keying is as follows: In a backscattering system, the tag antenna impedance Z A and load impedance Z L It can be expressed in the following form: The reflectance coefficient of the label can be expressed in the following form: Therefore, by switching the open and closed states of the RF switch, the tag load impedance Z can be changed. L The value of is used to generate a square wave signal, which is used to shift the center frequency of the backscattered signal, thereby avoiding interference from the original signal to the backscattered signal; at the same time, by changing the frequency, amplitude and phase information of the square wave, it is possible to generate intentional errors based on frequency shift keying, amplitude shift keying or phase shift keying. Therefore, in step 2.2, the number of intentionally erroneous bits N1 generated by the transmitting tag is first preset; then, when a Wi-Fi signal is detected in the environment, the RF switch generates a square wave of the corresponding frequency by continuously switching between open and closed states at a certain frequency. The antenna reflects the Wi-Fi signal and shifts the reflected Wi-Fi signal by a certain frequency band compared to the original signal to avoid interference from the original signal to the reflected Wi-Fi signal; at the same time, after the delay set in step 1.2, the RF switch changes the phase of the square wave signal generated by the RF switch, thereby destroying part of the information carried by the original Wi-Fi signal, and according to the number of destroyed bits N1, after a period of time, the phase of the square wave signal is restored to the original state, and the information is no longer destroyed; The radio frequency switch can destroy some of the information carried by the original Wi-Fi signal in the above way.

7. The method for Wi-Fi backscatter covert communication using FCS characteristics as described in claim 6, characterized in that: In step 3.2, the covert receiver utilizes the characteristics of FCS to distinguish Wi-Fi frames containing covert information from the received data frames with transmission errors, and determines the location of the error generated by the transmitter tag in the Wi-Fi frames containing covert information. The specific steps are as follows: Commercial Wi-Fi signals are appended with an FCS (Error Detection System) for error detection. The result of this error detection is only related to the transmission errors of the received data packets, and is not related to the specific content of the data packets. Therefore, if there exists a sequence S of the same length as the received data, it is 0 where there are no errors in the received data and 1 where there are errors; There exists a sequence S0 of all zeros of the same length as the received data, and the corresponding cyclic parity bits are also all 0; then S is taken as the result of S0 when a transmission error occurs, and the position where the transmission error occurs is the same as the position where the error occurred in the received data. In the above case, if the received data is P, then S and P will obtain the same result after performing FCS verification respectively; In summary, if the received data packet P contains hidden information... R If no errors occur during transmission, but intentional errors are introduced by tags only at designated locations, then the equal-length sequence S generated using the above method is... R It is expressed in the following form: S R ={0,0,......0,S R0 ,0,0,......0,S R1 ,0,0,......0,S Rk ,0,0,......},k=0,1,2...... Among them, S Rk This is an area where intentional errors may occur; these intentional errors are caused by the backscattering tag at the transmitting end disrupting the original transmitted Wi-Fi information through phase modulation during the reflection of the Wi-Fi signal. In step 2.2, the transmitting tag, while reflecting the Wi-Fi signal based on the delayed signal, simultaneously disrupts and only disrupts a portion of the signal content through phase modulation; therefore, in the absence of transmission errors, the S in the Wi-Fi data packet containing hidden information... Rk There is one and only one non-zero bit; based on the number of bits N to be destroyed preset at the transmitter and the modulation scheme of the received Wi-Fi signal, the maximum value N of the number of bits destroyed at the receiver can be determined, then S Rk The number of bits will be less than or equal to N, where N ≤ the number of bits in FCS; In the aforementioned case, let the error first occur at Then the corresponding S R It is expressed in the following form: in, Based on the received Wi-Fi data packets, calculate the corresponding FCS check result R, then S R The corresponding verification result is also R; From R, we can find S′. R The corresponding verification result R′; At this time, by S′ R Its characteristics The value of is R′; if the assumption holds, then the number of digits in R′ will be the same as . The same, meaning less than or equal to N, and the converse is not true; This allows us to pinpoint the location of the intentional error caused by the label.

8. A Wi-Fi backscatter covert communication system utilizing FCS characteristics, used to implement the method for Wi-Fi backscatter covert communication utilizing FCS characteristics as described in claims 1, 2, 3, 4, 5, 6, or 7, characterized in that: It includes a covert information transmitting tag and a covert information receiving end; the covert information transmitting tag includes a transmission status control module, a microprocessor, and a covert information modulation module; the covert information receiving end is a commercial Wi-Fi receiver; The transmission status control module of the covert information transmitter includes a comparator and a radio frequency detector; the radio frequency detector outputs the average value of the signal received by the antenna, and the comparator compares the average value with a set threshold to output a high or low level to determine whether a Wi-Fi signal exists in the environment; Preset threshold V ref When the average value of the input signal Greater than threshold V ref When the average value of the input signal is 100000, the comparator outputs a high level, assuming the presence of a Wi-Fi signal in the environment; when the average value of the input signal is 10000000, the comparator outputs a high level. Less than threshold V ref When the signal is low, it is assumed that there is no Wi-Fi signal in the environment, and the comparator outputs a low level. The microprocessor is used to store the covert information to be transmitted and to control the covert information modulation module to transmit the covert information based on the information fed back by the transmission status control module. When the transmission status control module outputs a high level, the microprocessor assumes that there is a Wi-Fi signal in the environment and sends a "start transmission" command to the covert information modulation module. The covert information modulation module is composed of a radio frequency switch. When a "start transmission" command is received from the microprocessor, the covert information modulation module, under the control of the microprocessor, changes the reflection coefficient by changing the on / off state of the radio frequency switch. The covert information modulation module continuously switches the on / off state of the switch at a specified frequency to generate a square wave of a specified frequency, which is used to change the center frequency of the reflected Wi-Fi signal to avoid interference from Wi-Fi signals in the environment. At the same time, by changing the frequency, amplitude, and phase information of the square wave, it is possible to generate intentional errors based on frequency shift keying, amplitude shift keying, or phase shift keying.

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