CN114337905A - Wi-Fi backscattering covert communication method and system by using FCS (binary system clock) characteristics - Google Patents
Wi-Fi backscattering covert communication method and system by using FCS (binary system clock) characteristics Download PDFInfo
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Abstract
The invention discloses a method and a system for carrying out Wi-Fi backscattering covert communication by using FCS characteristics, belonging to the field of wireless communication. The implementation method of the invention comprises the following steps: the transmitting terminal label codes the hidden information into position information, intentional errors are generated in the process of backscattering the Wi-Fi signals based on the position information containing the hidden information codes, after the receiving terminal demodulates the Wi-Fi backscattering signals, the positions of the intentional errors are determined by using the FCS characteristics, the hidden information is demodulated according to the codes containing the hidden information, and therefore Wi-Fi backscattering hidden communication is achieved by using the FCS characteristics. The information quantity of the hidden information carried in the Wi-Fi data packet containing the hidden information is increased by encoding the hidden information into the intentional error position information at the transmitting end, so that the throughput of hidden communication is improved. The invention also discloses a system for realizing the Wi-Fi backscattering concealed communication method by using the FCS characteristic.
Description
Technical Field
The invention relates to a Wi-Fi backscattering hidden communication method and system by using FCS characteristics, belonging to the field of wireless communication.
Background
The backscattering communication system consists of a radio frequency signal source, a label and a receiver. The radio frequency signal source provides energy for the label and transmits carrier signals used by information; tags are generally passive, passing information by modulating and reflecting radio frequency signals generated by a radio frequency signal source; the receiver receives and demodulates the backscatter signal containing the tag information. In short, in the backscatter communication system, the tag does not generate a carrier signal, but transmits information by modulating a signal from a radio frequency signal source, and the backscatter communication system has the characteristics of low power consumption and low cost.
Different from the traditional backscattering communication system, the Wi-Fi backscattering communication system does not need a special radio frequency signal source, the tag transmits information by modulating and reflecting commercial Wi-Fi signals in the environment, so that the power consumption of the transmission system is further reduced, and compared with the traditional backscattering communication system, the reflected signals generated by the Wi-Fi backscattering communication system are similar to the Wi-Fi signals in the environment in signal characteristics, the signal strength is lower, and the concealment is higher.
In the Wi-Fi backscattering covert communication transmission system, a label transmits covert information by utilizing commercial Wi-Fi signals. The commercial Wi-Fi signal conforms to an 802.11 protocol, and a 32-bit cyclic check code is added at the end of a frame to be used as a Frame Check Sequence (FCS) so as to judge whether the Wi-Fi data packet has errors in the transmission process.
Disclosure of Invention
The invention aims to provide a method for carrying out Wi-Fi backscattering concealed communication by using FCS characteristics. Based on the position, the hidden information is coded and demodulated according to the contained hidden information, namely, the Wi-Fi backscattering hidden communication is realized by using FCS characteristics. The invention has the advantages of strong concealment, simple structure, high realizability, low power consumption, low cost and the like.
The purpose of the invention is realized by the following technical scheme.
The invention discloses a method for carrying out Wi-Fi backscattering concealed communication by using FCS characteristics, wherein a transmitting terminal label encodes concealed information into position information, intentional errors are generated in the process of backscattering Wi-Fi signals based on the position information containing the concealed information code, after a receiving terminal demodulates the Wi-Fi backscattering signals, the position of the intentional errors is determined by using the FCS characteristics, and the concealed information is demodulated according to the position information containing the concealed information code, namely the Wi-Fi backscattering concealed communication is realized by using the FCS characteristics. The information quantity of the hidden information carried in the Wi-Fi data packet containing the hidden information is increased by encoding the hidden information into the intentional error position information at the transmitting end, so that the throughput of hidden communication is improved.
The invention discloses a method for carrying out Wi-Fi backscattering concealed communication by using FCS characteristics, which comprises the following steps:
step one, the transmitting terminal label encodes the hidden information into position information.
Step 1.1, when the hidden information acquisition and storage module register of the transmitting terminal label is not full, continuously acquiring the hidden information to be transmitted from the environment or an upper computer, and storing the hidden information in the label register.
And step 1.2, the microprocessor of the transmitting terminal label encodes and frames the hidden information, converts the content of the hidden information after encoding and framing into preset delay time based on a mapping relation, and determines the intentional error position through the preset delay time.
And secondly, generating intentional errors in the process of backscattering the Wi-Fi signals based on the position information containing the hidden information codes.
And 2.1, after the transmission state control module captures that the Wi-Fi signal exists in the environment, transmitting a 'transmission start' instruction to a hidden information modulation module of the transmitting terminal label.
And 2.2, after the hidden information modulation module of the transmitting terminal label receives the 'start transmission' instruction, the hidden information modulation module reflects the Wi-Fi signal and carries out preset frequency shift on the Wi-Fi signal so as to avoid the original signal from interfering the backscatter signal. Meanwhile, based on the time delay after the coding and framing in the step 1.2, correspondingly, after the time delay is specified, the information in the Wi-Fi data section is destroyed, so that the reflected Wi-Fi signal generates an intentional error at the specified position.
The damaged Wi-Fi signal containing the concealment information and the normal Wi-Fi signal in the step 2.2 have no obvious difference in signal characteristics, and an intentional error appears in a corresponding specified area after demodulation on signal content, and the intentional error has no obvious difference from the normal Wi-Fi signal which is damaged due to environment and generates an error code, and has strong concealment.
Preferably, in step 2.2, after delaying for a specified time, the method for destroying the information in the Wi-Fi data segment, thereby causing the Wi-Fi signal after reflection to generate an intentional error at the specified position includes amplitude keying, phase shift keying, and frequency shift keying.
For the sake of simple hardware structure, as a further preference, in step 2.2, the information in the Wi-Fi data segment is destroyed by phase shift keying.
And step three, after the receiving end demodulates the Wi-Fi backscattering signal, determining an intentional error position by using the FCS characteristic, and demodulating the concealment information according to the code containing the concealment information, namely realizing Wi-Fi backscattering concealment communication by using the FCS characteristic.
And 3.1, receiving and demodulating the Wi-Fi signal by the covert receiver, and extracting the Wi-Fi data packet with the transmission error.
Step 3.2, the covert receiver extracts a Wi-Fi data packet containing covert information from the Wi-Fi data packet extracted in the step 3.1 by using the characteristic of the FCS, and determines the position of an intentional error in the data packet;
and 3.3, correspondingly demodulating the hidden information transmitted by the label by the hidden receiver based on the error position and the mapping relation between the predetermined error position and the hidden information, namely realizing hidden communication.
Further, the specific steps of the microprocessor in step 1.2 encoding and framing the hidden information, and converting the content of the hidden information after encoding and framing into a certain delay time are as follows:
step 1.2-1, the microprocessor sets the number N of encoding bits in advance based on the requirement and determines the corresponding delay time T when the hidden information is transmitted to '1' in advance based on the requirement with the hidden receiving end0+ Δ T. Wherein, T0Is the time delay for avoiding destroying fields such as preamble, service and the like in the Wi-Fi frame.
Step 1.2-2, the microprocessor combines the hidden information in the register into an N-bit 2-system number (B) according to the encoding bit number N0,B1,B2......)。
Step 1.2-3, based on the N-bit 2-system number and the predetermined delay time T corresponding to the hidden information' 10+ Δ T to compute the 2-ary number B after transmission of the combination0,B1,B2.... correspond to the delays that should be performed, respectively: t is0+BkΔT，k＝1,2,3......。
Further, in step 2.2, a specific method for destroying the Wi-Fi data packet by the hidden information modulation module through phase shift keying is as follows:
in a backscatter system, the tag antenna is impedance ZAAnd a load impedance ZLExpressed in the following form:
the reflection coefficient of the label is then expressed in the form:
therefore, the load impedance Z of the label can be changed by switching the open and close states of the radio frequency switchLThe value of (2) to generate a square wave signal for moving the central frequency point of the backscatter signal, thereby avoiding the interference of the original signal to the backscatter signal. Meanwhile, the information such as frequency, amplitude, phase and the like of the square wave is changed, so that the method can realize the method based on frequency shift keying, amplitude keying or phase shift keyingIs generated.
Therefore, in step 2.2, the number of intentionally erroneous bits N generated by the transmitting end tag is first preset1. And then, when the Wi-Fi signals exist in the environment, the radio frequency switch continuously switches the on-off state according to a certain frequency to generate square waves with corresponding frequency, the antenna reflects the Wi-Fi signals, and the reflected Wi-Fi signals are shifted by a certain frequency band compared with the original signals, so that the interference of the original signals on the reflected Wi-Fi signals is avoided. Meanwhile, after the time delay set in the step 1.2 is carried out, the radio frequency switch changes the phase of the square wave signal generated by the radio frequency switch, so that part of information carried by the original Wi-Fi signal is damaged, and the information is damaged according to the damage digit N1After a period of time, the phase of the square wave signal is restored to the original state, and the information is not damaged any more.
The radio frequency switch can destroy part of information carried by the original Wi-Fi signal in the mode.
Further, in step 3.2, the specific steps of distinguishing, by the covert receiver, the Wi-Fi frame containing the covert information from the received data frame with the transmission error by using the characteristics of the FCS, and determining the position of the error generated by the transmitting end tag in the Wi-Fi frame containing the covert information are as follows:
the end of the commercial Wi-Fi signal is appended with an FCS for error detection, which is related only to transmission errors of the received packet, and not to the specific contents of the packet.
Therefore, if a sequence S with the same length as the received data exists, the sequence S is 0 at the error-free position of the received data, and the sequence S is 1 at the error-free position; and there is an all-zero sequence S of equal length to the received data0And the corresponding cyclic check bits are also all 0. Then take S as S0The result is when a transmission error occurs, and the position where the transmission error occurs is the same as the position where the received data error occurs. In this case, if the received data is P, the FCS check of S and P will obtain the same result.
In summary, if the received data packet P contains the hidden informationRNo error exists in the transmission processBy mistake, but only by deliberate error at a given location by the tag, the equivalent length sequence S generated in the manner described aboveRIs expressed in the following form:
SR＝{0,0,......0,SR0,0,0,......0,SR1,0,0,......0,SRk,0,0,......},k＝0,1,2......
wherein S isRkAre areas where intentional errors may occur. The intentional error is generated by the transmitting end backscattering tag destroying originally transmitted Wi-Fi information through phase modulation during the process of reflecting the Wi-Fi signal.
The transmitting end tag in step 2.2 reflects the Wi-Fi signal based on the delayed signal while destroying only the content of a certain part of the signal by phase modulation. Therefore, S in Wi-Fi data packet containing hidden information in case of no transmission errorRkThere is and only one position other than 0. Based on the preset damaged digit N of the transmitting terminal and the modulation mode of the received Wi-Fi signal, the maximum value N of the damaged digit of the receiving terminal can be determined, and S isRkWill be less than or equal to N (the number of bits of N ≦ FCS).
In this case, the error code is first made to appear in
Then the corresponding SRIs expressed in the following form:
wherein the content of the first and second substances,
correspondingly solving FCS check result R based on the received Wi-Fi data packet, and then SRThe corresponding check result is also R.
From R, S 'is obtained by correlation'RThe corresponding check result R'.
At this time, is composed of S'RIs characterized in that it is a combination of (A) and (B),
the value of (D) is R'. If the assumption is true, the number of R' bits will be equal to
The same is true, namely less than or equal to N, otherwise, the situation is not true.
Thereby, the location of the intentional error produced by the tag can be determined.
The invention also discloses a system for realizing Wi-Fi backscattering concealed communication by using the FCS characteristic, which comprises a concealed information transmitting terminal label and a concealed information receiving terminal. The hidden information transmitting terminal label comprises a transmission state control module, a microprocessor and a hidden information modulation module; the hidden information receiving end is a common commercial Wi-Fi receiver.
The transmission state control module of the hidden information transmitting end comprises a comparator and a radio frequency detector. The radio frequency detector outputs the average value of signals received by the antenna, the comparator compares the average value with a set threshold, and a high-low level is output to judge whether the Wi-Fi signals exist in the environment.
Specifically, the threshold V is set in advancerefWhen the average value V of the input signal is greater than the threshold VrefWhen the Wi-Fi signal exists in the environment, the comparator outputs a high level; when the average value V of the input signal is less than the threshold VrefWhen the Wi-Fi signal does not exist in the environment, the comparator outputs low level.
The microprocessor is used for storing the hidden information to be transmitted and controlling the hidden information modulation module to transmit the hidden information based on the information fed back by the transmission state control module. When the transmission state control module outputs high level, the microprocessor considers that Wi-Fi signals exist in the environment and sends a 'transmission start' instruction to the hidden information modulation module.
The hidden information modulation module is composed of a radio frequency switch. After receiving a 'start transmission' instruction from the microprocessor, the hidden information modulation module changes the reflection coefficient by changing the on-off state of the radio frequency switch under the control of the microprocessor. The module generates square waves with specified frequency by constantly switching the on-off state of the switch with specified frequency, and is used for changing the central frequency point of the reflected Wi-Fi signals so as to avoid the interference of the reflected Wi-Fi signals by the Wi-Fi signals in the environment.
Meanwhile, by changing the frequency, amplitude, phase and other information of the square wave, the intentional error generation based on frequency shift keying, amplitude keying or phase shift keying can be realized. In the system, a phase shift keying mode is adopted.
Advantageous effects
1. The invention discloses a method and a system for carrying out Wi-Fi backscattering concealed communication by using FCS characteristics.
2. The method and the system for carrying out Wi-Fi backscattering covert communication by utilizing the FCS characteristics improve the information quantity of covert information carried in a Wi-Fi data packet containing the covert information by coding the covert information into intentionally wrong position information at a transmitting end, thereby improving the throughput of covert communication.
3. The invention discloses a Wi-Fi backscattering covert communication method and a system by using FCS characteristics, which carry out covert communication by adopting a high backscattering commercial Wi-Fi signal mode. The adopted Wi-Fi signals are the same as the original Wi-Fi signals in signal characteristics and have lower signal strength, so that the transmitted covert signals are difficult to distinguish and identify by a monitoring end and common Wi-Fi signals in the environment. Therefore, the invention has higher concealment;
4. the Wi-Fi backscattering covert communication method and the system thereof by using the FCS characteristics have the advantages that the Wi-Fi signal carrying covert information has good concealment performance, and an interference end does not need to be added. The hidden signal receiving end is commercial Wi-Fi receiving equipment such as a wireless network card and the like, and a special hidden signal receiver does not need to be designed. Therefore, compared with the existing covert communication transmission system, the transmission system has low hardware complexity and higher realizability.
5. The invention discloses a method and a system for Wi-Fi backscattering concealed communication by using FCS characteristics.A receiving end completes the demodulation of information by using the FCS characteristics and only adopting a single receiver, and the existing Wi-Fi backscattering communication system adopts two receivers in different frequency bands to demodulate the information. Therefore, compared with the existing Wi-Fi backscatter communication system, the invention reduces the complexity of the receiving end.
6. The method and the system for Wi-Fi backscattering covert communication by using the FCS characteristics can realize a mode of generating intentional errors at a designated position only through the passive analog device on the label, and the label has the characteristics of simple structure and high realizability.
Drawings
FIG. 1 is a flow chart of a method of Wi-Fi backscatter covert communication using FCS characteristics according to the invention;
FIG. 2 is a schematic diagram of a basic backscatter communications system architecture;
FIG. 3 is a block diagram of a Wi-Fi backscatter covert communication device module architecture using FCS features;
FIG. 4 is a schematic diagram of a tag structure of a transmitting end of a Wi-Fi backscatter covert communication device using FCS characteristics according to an embodiment of the invention;
fig. 5 is a schematic diagram of a bit error rate of a receiving end according to an embodiment of the present invention.
Detailed Description
The present invention will be described in detail below with reference to the accompanying drawings and examples. The technical problems and the advantages solved by the technical solutions of the present invention are also described, and it should be noted that the described embodiments are only intended to facilitate the understanding of the present invention, and do not have any limiting effect.
In the embodiment, a commercial wireless network card Qualcomm Atheros AR938x is used as a transmitting end to generate Wi-Fi signals, and a MacBook Air notebook computer is matched with the commercial wireless network card to be used as a receiving end to demodulate the received commercial Wi-Fi signals into Wi-Fi data packets.
To verify the universality of the invention, the ambient Wi-Fi signal selection for backscattering was: the invention is verified by Wi-Fi signals with a transmission rate of 1Mbps and an 802.11b protocol, Wi-Fi signals with a transmission rate of 6Mbps and an 802.11g protocol, and Wi-Fi signals with a transmission rate of 6.5Mbps and an 802.11n protocol respectively.
The FCS of the 802.11 protocol described in this embodiment is a 32-bit cyclic check code, and the check code is generated as follows:
first, 32 zeros are padded to the end of the information to be transmitted.
Then, a polynomial generated according to the following formula is used as a divisor, modulo-2 division is performed on the information to be transmitted after zero padding, the obtained 32-bit remainder is a cyclic check code, and the cyclic check code is added to the end of a Wi-Fi frame and is transmitted, so that generation of the 32-bit cyclic check code is realized.
G(x)＝x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1
At the receiving end, the above-mentioned generating polynomial is also used as divisor to make modulo-2 division on the received data, if the transmission is error-free, the remainder is 0, otherwise, the remainder is not 0.
For the Wi-Fi transmission signal conforming to the 802.11b/g/n protocol, as shown in fig. 1, the specific implementation steps of the FCS-based Wi-Fi backscatter covert communication system are as follows:
step one, the transmitting terminal label encodes the hidden information into position information.
Step 1.1, when the hidden information acquisition and storage module register of the transmitting terminal label is not full, the hidden information to be transmitted is continuously acquired from the environment or an upper computer and stored in the label register.
And step 1.2, the microprocessor of the transmitting terminal label encodes and frames the hidden information, converts the content of the hidden information after encoding and framing into preset delay time based on the mapping relation, and determines the intentional error position through the preset delay time.
In this embodiment, the microprocessor sets the number N of intentionally wrong bits generated by the tag at the transmitting end to 4 in advance based on the requirement, and determines the corresponding delay time T in advance when the hidden information is transmitted as "1"0+ Δ T. Wherein, T040 μ s is the time delay performed to avoid destroying fields such as preamble and service in the Wi-Fi frame, and Δ T8 μ s is;
then, the microprocessor synthesizes the hidden information in the register into a 4-bit 2-system number (B) according to the coding bit array0,B1,B2......)。
Finally, the 2-bit number B after the combination is transmitted is calculated based on the 4-bit 2-bit number and the corresponding delay time of the predetermined hidden information 10,B1,B2.... correspond to the delays that should be performed, respectively: (40+ 8B)k) μ s, where k is 0,1, 2.
And secondly, generating intentional errors in the process of backscattering the Wi-Fi signals based on the position information containing the hidden information codes.
And 2.1, after the transmission state control module captures that the Wi-Fi signal exists in the environment, transmitting a 'transmission start' instruction to a hidden information modulation module of the transmitting terminal label.
And 2.2, after the hidden information modulation module of the transmitting terminal label receives the 'start transmission' instruction, the hidden information modulation module reflects the Wi-Fi signal and carries out preset frequency shift on the Wi-Fi signal so as to avoid the original signal from interfering the backscatter signal. Meanwhile, based on the time delay after the coding and framing in the step 1.2, correspondingly, after the time delay is specified, the information in the Wi-Fi data section is destroyed, so that the reflected Wi-Fi signal generates an intentional error at a specified position;
in this embodiment, in order to ensure that the transmitting end tag hardware has a simple structure, the information in the Wi-Fi data segment is destroyed in a phase shift keying manner, the specific method is as follows:
impedance Z of tag antenna at transmitting end used in this embodimentA50 Ω, when the switch is closed, the load impedance Z L0 Ω, and when the switch is off, the load impedance ZL→+∞。
In this embodiment, the reflection coefficient of the tag can be expressed as follows:
thus, when the switch is off, the reflection coefficient Γ of the transmitting end tag in this embodiment is 1, and when the switch is off, the reflection coefficient Γ of the transmitting end tag in this embodiment is-1. Therefore, by continuously switching the open and closed states of the rf switch, a square wave signal with an amplitude of 1 can be generated, and by changing the phase of the square wave signal, an intentional error can be generated by phase shift keying.
In this embodiment, the frequency of the square wave signal is 20MHz, so that the central frequency point of the WiFi signal reflected by the tag antenna at the transmitting end is shifted by 20MHz compared with the original signal. Meanwhile, in this embodiment, after the delay time set in step 1.2 is performed, the phase of the square wave signal generated by the radio frequency switch is changed, so as to destroy part of the information carried by the original Wi-Fi signal, and according to the preset destruction bit number N1After a period of time, the phase of the square wave signal is restored to the original state and no further information is destroyed.
And step three, after the receiving end demodulates the Wi-Fi backscattering signal, determining an intentional error position by using the FCS characteristic, and demodulating the concealment information according to the code containing the concealment information, namely realizing Wi-Fi backscattering concealment communication by using the FCS characteristic.
And 3.1, receiving and demodulating the Wi-Fi signal by the covert receiver, and extracting the Wi-Fi data packet with the transmission error.
And 3.2, the covert receiver extracts the Wi-Fi data packet containing covert information from the Wi-Fi data packet extracted in the step 3.1 by utilizing the characteristics of the FCS, and determines the position of an intentional error in the data packet.
In this embodiment, based on the delay and the number of bits to be destroyed set by the transmitting end and the transmission rate of the Wi-Fi data packet, the time at which an intentional error may occur is as follows:
when the Wi-Fi signal of the environment, in which the tag at the transmitting end performs backscattering, is a Wi-Fi signal with a transmission rate of 1Mbps and according with an 802.11b protocol, the intentional error position is as follows: (40+40B) μ s to (64+40B) μ s, where B is a decimal representation of a 4-bit binary number after coding framing, and B is 0,1, 2. When the Wi-Fi signal of the environment, in which the tag at the transmitting end performs backscattering, is a Wi-Fi signal with a transmission rate of 6Mbps and according with an 802.11g protocol, the intentional error position is as follows: (40+40B) μ s to (44+40B) μ s, where B is a decimal representation of a 4-bit binary number after coding framing, and B is 0,1, 2. When the Wi-Fi signal of the environment, in which the tag at the transmitting end performs backscattering, is a Wi-Fi signal with a transmission rate of 6.5Mbps and according with an 802.11n protocol, the intentional error position is as follows: (40+40B) μ s to (44+40B) μ s, where B is a decimal representation of a 4-bit binary number after coding framing, and B is 0,1, 2.
Thus, the intentional error location may be determined based on the FCS check result of a received packet with an intentional error.
And 3.3, correspondingly demodulating the hidden information transmitted by the label by the hidden receiver based on the error position and the mapping relation between the predetermined error position and the hidden information, namely realizing hidden communication.
Specifically, the present embodiment was tested in the line-of-sight range, and the error rate curve of the test results is shown in fig. 5. As can be seen from the figure, transmission within 20 meters is realized, but since the wireless signal has a high packet loss rate in the transmission process, the test result has a high error rate, and in the subsequent process, the performance can be improved by increasing the retransmission times of the hidden information of the transmitting terminal and the like.
In the above experiment, the microprocessor part in the hidden information storage module and the hidden information modulation module of the tag adopts Igllo Nano AGLN250 FPGA;
the radio frequency switch part in the hidden information modulation module adopts an ADG902 radio frequency switch of ANALOG DEVICES company;
the transmission state control module uses LT5534 of ANALOG DEVICES company as a rectifier and NCS2200 of ONSEMI company as a comparator.
Under the experimental conditions, the maximum transmission distance of the sending device for sending the hidden information is greater than or equal to 20 m. It should be emphasized that, in the implementation, the transmission distance can be further increased by increasing the number of retransmissions, increasing the antenna gain, and the like. The hidden information can be further encrypted by means of encryption modes such as link encryption, node encryption, end-to-end encryption and the like, so that the security of transmission of the hidden information is guaranteed.
The transmission of the covert information can be realized by the backscattering covert communication method of the Wi-Fi signal based on the 802.11b/g/n protocol. Since the Wi-Fi signal has the same signal characteristics as the original signal and has lower self energy after reflection in the transmission process, the possibility of being discovered by the hidden transmission system is remarkably reduced.
The above description is only for the preferred embodiments of the present invention and is not intended to limit the embodiments of the present invention. Variations and modifications in other variations will occur to those skilled in the art upon reading the foregoing description. It is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed, and obviously, many modifications and variations are possible in light of the above teaching.
Claims (10)
1. The method for carrying out Wi-Fi backscattering concealed communication by using FCS characteristics is characterized by comprising the following steps: the method comprises the following steps:
step one, a transmitting terminal label encodes hidden information into position information;
generating intentional errors in the process of backscattering the Wi-Fi signals based on the position information containing the hidden information codes;
and step three, after the receiving end demodulates the Wi-Fi backscattering signal, determining an intentional error position by using the FCS characteristic, and demodulating the concealment information according to the code containing the concealment information, namely realizing Wi-Fi backscattering concealment communication by using the FCS characteristic.
2. The method for Wi-Fi backscatter cloaking communication using FCS characteristics of claim 1, wherein: the first implementation method comprises the following steps of,
step 1.1, when the hidden information acquisition and storage module register of the transmitting terminal label is not full, continuously acquiring hidden information to be transmitted from the environment or an upper computer and storing the hidden information in the label register;
and step 1.2, the microprocessor of the transmitting terminal label encodes and frames the hidden information, converts the content of the hidden information after encoding and framing into preset delay time based on a mapping relation, and determines the intentional error position through the preset delay time.
3. The method for Wi-Fi backscatter cloaking communication using FCS characteristics of claim 2, wherein: the second step is realized by the method that,
step 2.1, after the transmission state control module captures that a Wi-Fi signal exists in the environment, transmitting a 'start transmission' instruction to a hidden information modulation module of a transmitting terminal label;
2.2, after the hidden information modulation module of the transmitting terminal tag receives the command of 'start transmission', the hidden information modulation module reflects the Wi-Fi signal and carries out preset frequency shift on the Wi-Fi signal so as to avoid the original signal from interfering the backscatter signal; meanwhile, based on the time delay after the coding and framing in the step 1.2, correspondingly, after the time delay is specified, the information in the Wi-Fi data section is destroyed, so that the reflected Wi-Fi signal generates an intentional error at a specified position;
the damaged Wi-Fi signal containing the concealment information and the normal Wi-Fi signal in the step 2.2 have no obvious difference in signal characteristics, and an intentional error appears in a corresponding specified area after demodulation on signal content, and the intentional error has no obvious difference from the normal Wi-Fi signal which is damaged due to environment and generates an error code, and has strong concealment.
4. A method for Wi-Fi backscatter cloaking communication using FCS characteristics as claimed in claim 3, wherein: the third step is to realize the method as follows,
step 3.1, the covert receiver receives and demodulates the Wi-Fi signals, and extracts Wi-Fi data packets with transmission errors;
step 3.2, the covert receiver extracts a Wi-Fi data packet containing covert information from the Wi-Fi data packet extracted in the step 3.1 by using the characteristic of the FCS, and determines the position of an intentional error in the data packet;
and 3.3, correspondingly demodulating the hidden information transmitted by the label by the hidden receiver based on the error position and the mapping relation between the predetermined error position and the hidden information, namely realizing hidden communication.
5. The method for Wi-Fi backscatter cloaking communication using FCS characteristics of claim 4, wherein: in step 2.2, after delaying for a specified time, information in the Wi-Fi data segment is destroyed, so that the method for deliberately making errors on the Wi-Fi signal at the specified position after reflection comprises amplitude keying, phase shift keying and frequency shift keying.
6. The method for Wi-Fi backscatter cloaking communication using FCS characteristics of claim 5, wherein: in step 2.2, the information in the Wi-Fi data segment is destroyed by phase shift keying.
7. The method for Wi-Fi backscatter cloaking communication using FCS characteristics of claim 6, wherein: the specific steps of the microprocessor in step 1.2 encoding and framing the hidden information, and converting the content of the hidden information after encoding and framing into a certain delay are as follows:
step 1.2-1, the microprocessor sets the number N of encoding bits in advance based on the requirement and determines the corresponding delay time T when the hidden information is transmitted to '1' in advance based on the requirement with the hidden receiving end0+ Δ T; wherein, T0The delay is carried out for avoiding damaging fields such as preamble, service and the like in the Wi-Fi frame;
step 1.2-2, the microprocessor combines the hidden information in the register into an N-bit 2-system number (B) according to the encoding bit number N0,B1,B2......)；
Step 1.2-3, based on the N-bit 2-system number and the predetermined delay time T corresponding to the hidden information' 10+ Δ T to compute the 2-ary number B after transmission of the combination0,B1,B2.... correspond to the delays that should be performed, respectively: t is0+BkΔT，k＝1,2,3......。
8. The method for Wi-Fi backscatter cloaking communication using FCS characteristics of claim 7, wherein: in the step 2.2, the specific method for destroying the Wi-Fi data packet by the hidden information modulation module through phase shift keying is as follows:
in a backscatter system, the tag antenna is impedance ZAAnd a load impedance ZLExpressed in the following form:
the reflection coefficient of the label is then expressed in the form:
therefore, the load impedance Z of the label can be changed by switching the open and close states of the radio frequency switchLGenerating a square wave signal for moving the central frequency point of the backscatter signal, thereby avoiding the interference of the original signal to the backscatter signal; meanwhile, by changing the frequency, amplitude, phase and other information of the square wave, the intentional error generation based on frequency shift keying, amplitude keying or phase shift keying can be realized;
therefore, in step 2.2, the number of intentionally erroneous bits N generated by the transmitting end tag is first preset1(ii) a Thereafter, when a loop is detectedWhen Wi-Fi signals exist in the environment, the radio frequency switch continuously switches to be switched off and switched on according to a certain frequency to generate square waves with corresponding frequency, the antenna reflects the Wi-Fi signals, and the reflected Wi-Fi signals are shifted to a certain frequency band compared with original signals, so that the interference of the original signals on the reflected Wi-Fi signals is avoided; meanwhile, after the time delay set in the step 1.2 is carried out, the radio frequency switch changes the phase of the square wave signal generated by the radio frequency switch, so that part of information carried by the original Wi-Fi signal is damaged, and the information is damaged according to the damage digit N1After a period of time, the phase of the square wave signal is restored to the original state, and information is not damaged any more;
the radio frequency switch can destroy part of information carried by the original Wi-Fi signal through the mode.
9. The method for Wi-Fi backscatter cloaking communication using FCS characteristics of claim 8, wherein: in step 3.2, the concealment receiver uses the characteristics of the FCS to distinguish a Wi-Fi frame containing concealment information from a received data frame with transmission error, and determines the position of an error generated by a transmitting end tag in the Wi-Fi frame containing concealment information, which includes the following specific steps:
adding FCS at the end of the commercial Wi-Fi signal for error detection, wherein the error detection result is only related to the transmission error of a received data packet and is not related to the specific content of the data packet;
therefore, if a sequence S with the same length as the received data exists, the sequence S is 0 at the error-free position of the received data, and the sequence S is 1 at the error-free position; and there is an all-zero sequence S of equal length to the received data0The corresponding cyclic check bits are also all 0; then take S as S0A result when a transmission error occurs, and a position where the transmission error occurs is the same as a position where the received data error occurs; under the condition, if the received data is P, FCS (FCS check) is respectively carried out on S and P, and the same result is obtained;
in summary, if the received data packet P contains the hidden informationRThere is no error in the transmission process, and only the specified position generates intentional error by the labeledIf it is wrong, the equal length sequence S generated by the above method is adoptedRIs expressed in the following form:
SR＝{0,0,......0,SR0,0,0,......0,SR1,0,0,......0,SRk,0,0,......},k＝0,1,2......
wherein S isRkFor areas where intentional errors may occur; the intentional error is generated by a transmitting terminal backscattering tag in the process of reflecting a Wi-Fi signal by destroying originally transmitted Wi-Fi information through phase modulation;
in step 2.2, the transmitting end tag reflects the Wi-Fi signal based on the delay signal, and destroys only the content of a certain part of the signal through phase modulation; therefore, S in Wi-Fi data packet containing hidden information in case of no transmission errorRkAnd only one position is not 0; based on the preset damaged digit N of the transmitting terminal and the modulation mode of the received Wi-Fi signal, the maximum value N of the damaged digit of the receiving terminal can be determined, and S isRkThe number of bits of (A) will be less than or equal to N (N ≦ the number of bits of FCS);
in this case, the error code is first made to appear in
Then the corresponding SRIs expressed in the following form:
wherein the content of the first and second substances,
correspondingly solving FCS check result R based on the received Wi-Fi data packet, and then SRThe corresponding check result is also R;
from R, S 'is obtained by correlation'RA corresponding verification result R';
at this time, is composed of S'RIs characterized in that it is a combination of (A) and (B),
the value of (d) is R'; if the assumption is true, the number of R' bits will be equal to
The same, namely less than or equal to N, otherwise, the same is not true;
thereby, the location of the intentional error produced by the tag can be determined.
10. A Wi-Fi backscatter covert communication system implemented using FCS characteristics, for implementing the method for Wi-Fi backscatter covert communication using FCS characteristics of claim 1,2, 3, 4, 5, 6, 7, 8, or 9, wherein: the device comprises a hidden information transmitting end label and a hidden information receiving end; the hidden information transmitting terminal label comprises a transmission state control module, a microprocessor and a hidden information modulation module; the hidden information receiving end is a common commercial Wi-Fi receiver;
the transmission state control module of the hidden information transmitting end comprises a comparator and a radio frequency detector; the radio frequency detector outputs an average value of signals received by an antenna, the comparator compares the average value with a set threshold, and a high-low level is output to judge whether a Wi-Fi signal exists in the environment;
presetting a threshold VrefWhen the average value of the input signal
Greater than a threshold VrefWhen the Wi-Fi signal exists in the environment, the comparator outputs a high level; when the average value of the input signal
Less than threshold VrefWhen the Wi-Fi signal does not exist in the environment, the comparator outputs a low level;
the microprocessor is used for storing the hidden information to be transmitted and controlling the hidden information modulation module to transmit the hidden information based on the information fed back by the transmission state control module; when the transmission state control module outputs a high level, the microprocessor considers that a Wi-Fi signal exists in the environment and sends a 'start transmission' instruction to the hidden information modulation module;
the hidden information modulation module is composed of a radio frequency switch; after receiving a 'start transmission' instruction from the microprocessor, the hidden information modulation module changes the reflection coefficient by changing the on-off state of the radio frequency switch under the control of the microprocessor; the module generates square waves with specified frequency by constantly switching the on-off state of the switch with specified frequency, and is used for changing the central frequency point of the reflected Wi-Fi signals so as to avoid the interference of the reflected Wi-Fi signals by the Wi-Fi signals in the environment;
meanwhile, by changing the frequency, amplitude, phase and other information of the square wave, the intentional error generation based on frequency shift keying, amplitude keying or phase shift keying can be realized.
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