SECURE COMMUNICATION METHOD

Abstract

Disclosed is a method for a secure communication method having a secret key generation technique. The novelty of the proposed method stems from enhancing physical layer security (PHY) by using channel-adaptive keys, after manipulating a channel by introducing an artificial component into the channel. An adaptively designed artificial component is cascaded with the legitimate user’s channel. In an orthogonal frequency division multiplexing (OFDM) system, subcarriers corresponding to a channel gain higher than a threshold value are selected to extract the keys. Since the number of the selected subcarriers is adaptive, the length of the generated key sequences is changing adaptively as well. Thus, the channel reciprocity property in a time division duplexing (TDD) system is utilized.

Claims

1. A secure communication method, wherein a receiver (B) sends a reference signal (S.sub.ref) to a transmitter (A) for channel estimation and wherein N corresponds to a total number of complex data symbols; the method comprising the steps of: a. Selecting the a first m point out of M number of peak-points from the a frequency selective channel between the transmitter (A) and the receiver (B), where in points correspond to subcarriers and where in M < N, b. Creating an artificial channel, F.sub.b ∈ ℂ.sup.Nx1, by using the selected m points, c. Creating a new channel, H.sub.b ∈ ℂ.sup.Nx1, by cascading receiver’s (B) channel, A.sub.b ∈ ℂ.sup.Nx1, with the artificial channel, F.sub.b; as H.sub.b = A.sub.b ⊙ F.sub.b, d. Selecting peak points from the cascaded channel, H.sub.b, e. Quantization of the selected subcarriers which their gains are corresponds to the peak points from cascaded channel by the transmitter (A) and the receiver (B) to construct a binary key (B.sub.b), f. Converting the binary key (B.sub.b) into a complex key (C.sub.b),and g. Reshaping the complex key (C.sub.b) to the closest multiplication of N.

2. The secure communication method according to claim 1, wherein M number of peak-points are selected where channel gain (G) is above average gain (G) of all the frequency (f) indices from cascaded channel, H.sub.b, considered by the transmitter (A) to extract keys.

3. The secure communication method according to claim 1, wherein if the length of a last key block is less than N, key samples from the head are added as a suffix to reshape the complex key (C.sub.b).

4. The secure communication method according to claim 1; wherein after reshaping the complex key (C.sub.b), transmitted signal, x, is sent to the receiver (B) by applying cyclic prefix (CP) to the time-domain encrypted symbols as y.sub.b = h.sub.b ∗ x + n.sub.b; where y.sub.b is received signal at receiver (B), h.sub.b is the cascaded channel in time-domain, and n.sub.b is the zero-mean complex additive white-Gaussian noise (AWGN) at the receiver’s (B) side.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 shows proposed system model for secure communication in presence of a passive eavesdropper.

[0015] FIG. 2 shows OFDM transceiver structure of the proposed method

[0016] FIG. 3 shows the structure of the proposed channel-based key generation technique

[0017] FIG. 4a shows frequency response graph of receiver’s channel

[0018] FIG. 4b shows frequency response graph of receiver’s newly created channel

[0019] FIG. 5 shows an illustration of reshaping the key for symbol encryption

BRIEF DESCRIPTION OF REFERENCE NUMBERS

[0020] A. Transmitter [0021] B. Receiver [0022] E. Eavesdropper [0023] S.sub.ref. Reference signal [0024] h.sub.b. Cascaded channel [0025] h.sub.e. Eavesdropper channel [0026] b.sub.t. Transmitted bits [0027] b.sub.r. Received bits [0028] M. Modulation [0029] DM. Demodulation [0030] DC. Decoding [0031] K. Key [0032] S/P. Serial to parallel [0033] P/S. Parallel to serial [0034] IFFT. Inverse fast Fourier transform [0035] FFT. Fast Fourier transform [0036] CP. Cyclic prefix [0037] CPR. Cyclic prefix removal [0038] f. Frequency [0039] G. Gain [0040] KS. Key stream [0041] B.sub.b. Binary key [0042] C.sub.b. Complex key [0043] EQ. Channel equalization

DETAILED DESCRIPTION

[0044] FIG. 1 shows proposed system model for secure communication in presence of a passive eavesdropper. Particularly, the purpose of a transmitter (A) is to send a secret data and communicate confidentially with a legitimate user, or receiver (B) in the presence of a passive eavesdropper (E). Eavesdropper’s (E) aim is to access the secret data content from the communication link between transmitter (A) and receiver (B) by its own observations of the signals it received. Besides, it is stronger than the receiver (B) by having multiple antennas, better signal processing skills, more power, hardware capabilities, processing capability, distributed reception capability, and offline processing. Its antennas are distributed where their locations are not known by the transmitter (A). It is assumed that the channels of both the receiver (B) and eavesdropper (E) are independent and uncorrelated of each other, i.e. eavesdropper (E) is located at least half wavelength away from the receiver (B). Furthermore, all received signals experience Rayleigh fading frequency selective channel. It is assumed that the transmitter (A) knows channel state information (CSI) of the receiver (B) by using the reciprocity property in a time division duplexing (TDD) system, but it doesn’t have any knowledge about eavesdropper’s (E) channel, since it is passive. Therefore, both channels, transmitter-to-receiver and receiver-to-transmitter, are assumed to be estimated as correlated and same with each other in TDD mode as proposed in Zhou X. et al. (2016).

[0045] In the proposed communication method, the receiver (B) sends a reference signal (S.sub.ref) to transmitter (A) for channel estimation. Assuming N corresponds to total number of complex data symbols; the proposed method fundamentally comprises the steps of: [0046] a. Selecting the first m point out of M number of peak-points from the frequency selective channel between transmitter (A) and receiver (B), where in points correspond to subcarriers and where in M < N, [0047] b. Creating an artificial channel, F.sub.b ∈ C.sup.Nx1, by using the selected m points, [0048] c. Creating a new channel, H.sub.b ∈ C.sup.Nx1, by cascading receiver’s (B) channel, .sub.Ab ∈ C.sup.Nx1, with the artificial channel, F.sub.b; as H.sub.b = .sub.Ab ⊙ F.sub.b, [0049] d. Selecting peak points from the cascaded channel, H.sub.b, [0050] e. Quantization of the selected subcarriers which their gains are corresponds to the peak points from cascaded channel by transmitter (A) and receiver (B) to generate a binary key (B.sub.b), [0051] f. Converting the binary key (B.sub.b) into a complex key (C.sub.b), [0052] g. Reshaping the complex key (C.sub.b) to the closest multiplication of N.

[0053] In a preferred embodiment, M number of peak-points are selected where channel gain (G) is above average channel gaina (G) of all the frequency (f) indices considered by the transmitter (A) to extract keys.

[0054] In another embodiment, if the length of last key block is less than N, key samples from the head are added as a suffix to reshape the complex key (C.sub.b).

[0055] Yet in another embodiment, after reshaping the complex key (C.sub.b), transmitted signal, x, is sent to the receiver (B) by applying cyclic prefix (CP) to the time-domain encrypted symbols as y.sub.b = h.sub.b ∗ x + n.sub.b; where y.sub.b is received signal at receiver (B) , h.sub.b is the cascaded channel in time-domain, and n.sub.b is the zero-mean complex additive white-Gaussian noise (AWGN) at the receiver’s (B) side.

AN EXAMPLE IMPLEMENTATION

[0056] In this example implementation, firstly the receiver (B) transmits a reference signal (S.sub.ref) to the transmitter (A) for channel estimation. Thus, as an advantage of the channel reciprocity property in TDD mode, the downlink channel is obtained from its uplink as suggested in Goldsmith A. (2005).

[0057] The proposed OFDM transceiver structure of the proposed method is depicted in FIG. 2. Frequency-domain complex data symbols having the length of N is represented by:

$S=\left[S_1,S_2,.Math.,S_N\right]$

where S ∈ (ℂ.sup.1xN. These symbols, obtained by using BPSK modulation, are going to be send by the transmitter (A) to the receiver (B) in the presence the eavesdropper (E).

[0058] To encrypt data, a secret key is used. Generation of this key at the transmitter is illustrated in FIG. 3. The frequency response of the channel experienced by the receiver (B) is denoted by .sub.Ab ∈ ℂ.sup.Nx1 and by the eavesdropper (E) is by A.sub.e ∈ ℂ.sup.Nx1 where N is the channel length of each. M selected subcarriers (M < N) out of N points are selected from the frequency response of the receiver’s (B) channel,

$A_b={\left[A_{b_1},A_{b_2},.Math.,A_{b_N}\right]}^T$

of length N. These M subcarriers correspond to the points where the channel gain is above the average gain of all the frequency indices are considered by the transmitter (A) to extract the secret keys. Both to increase the number of the subcarriers corresponding to a channel gain higher than a threshold value and ensure that the channel is more selective, an artificial channel is designed by using the selected M points. The values of .sub.Ab at selected M frequency values are copied till the length of the artificial channel, F.sub.b ∈ C.sup.Nx1, equals to the length of the receiver’s (B) channel, .sub.Ab ∈ ℂ.sup.Nx1. A new channel for the receiver (B), H.sub.b ∈ C.sup.Nx1, is created by cascading the receiver’s (B) channel, .sub.Ab ∈ ℂ.sup.Nx1, with the artificial channel, F.sub.b ∈ C.sup.Nx1 as expressed:

$H_b=A_b\odot F_b$

[0059] The number of selected points corresponding to the frequency indices where the channel gain values are above the average gain of all values is shown in FIGS. 4a and 4b. As presented in FIG. 4a, the number of the selected subcarriers of the receiver’s (B) channel is very less to extract keys securely. Therefore, the transmitter (A) adds an artificial component to manipulate the channel. Thus, the number of the selected parts of the cascaded channel given in FIG. 4b is greater than the number of selected points in FIG. 4a. After selecting subcarriers of H.sub.b, the receiver’s (B) enhanced channel is quantized by the transmitter (A) and the receiver (B) to construct a key. The cascaded channel gain measurements are equally divided into regions and each region is quantized into multi-bit quantization levels by the transmitter (A) and the receiver (B). Each of the highest peak points corresponds to a bit stream. Therefore, length of the generated key depends on the number of the selected peak points of the receiver’s (B) cascaded channel. Since our proposed encryption is based on symbol level, the binary key for the receiver (B), B.sub.b ∈ {0,1}, is converted into a complex key, C.sub.b, as i(2xB.sub.b - 1). Since the length of the key is longer than the number of symbols, the key is reshaped to the closest multiplication of the total number of symbols, N.

[0060] As it is seen in FIG. 5, the key stream (KS), K.sub.b, which has an adaptive length, is divided into multiple blocks and length of each block should be equal to the number of symbols, N. If the length of the last key block is less than N, the key samples from the head are added as a suffix. At transmitter’s (A) side, after reshaping the key, the encrypted symbols, E.sub.b, can be written as the multiplication of each key block of with the symbols, S as:

$E_b=K_{b_i}\odot S^T$

where i = 1, ..., n and n is the number of key blocks. E.sub.b is reshaped to obtain a vector of encrypted symbols of length (N×n)×1. The transmitted signal, x, of having an adaptive length is sent to the receiver (B), after applying cyclic prefix (CP) to the time-domain encrypted symbols to avoid inter symbol interference (ISI). The received signal at the receiver’s (B) side is defined as:

$y_b=h_b\ast x+n_b$

where h.sub.b is the receiver’s (B) cascaded channel in time-domain, x is the transmitted signal, and n.sub.b is the zero-mean complex additive white Gaussian noise (AWGN) at the receiver (B). Since the length of x is adaptive, the length of the received signal, y.sub.b, is adaptive as well.

[0061] After removing cyclic prefix (CP) and then applying S/P conversion on the time-domain received signal, y.sub.b, the receiver uses FFT on the resulted signal. A zero-forcing channel equalization process is performed to reduce the effects of noise from the channel for a better decoding. Thus, the received signal at the receiver’s (B) side after channel equalization process is found by element-wise division of the received signal and his channel is expressed as:

${\widehat{Y}}_b=Y_b\varnothing H_b$

Where H.sub.b is the receiver’s (B) cascaded channel in frequency-domain and y.sub.b is the frequency-domain received signal after S/P conversion shown in FIG. 2. After P/S conversion, the receiver (B) generates the decoded data by using the key stream (KS), K.sub.b, that it extracted from its own channel and the decoded data is observed as:

${\widehat{X}}_b={\widehat{Y}}_b\varnothing K_b$

[0062] The eavesdropper has access to the transmitted signal, x, as well. As it has stronger skills and a more versatile receiver than the receiver (B), it follows the same steps with the receiver (B) as shown in FIG. 2 to generate its own keys from its channel. The eavesdropper (E) also designs an artificial component as the same way of the receiver (B) and cascades it with its channel, A.sub.e, to obtain its cascaded channel, H.sub.e. The signal it captures from the channel is defined as:

$y_e=h_e\ast x+n_e$

where h.sub.e is the eavesdropper’s (E) cascaded channel in time-domain and n.sub.e is the zero-mean complex AWGN at the eavesdropper (E). The received signal at the eavesdropper’s (E) side after channel equalization process is expressed as:

${\widehat{Y}}_e=Y_e\varnothing H_e$

where Y.sub.e is the frequency-domain received signal after S/P conversion and H.sub.e is the eavesdropper’s (E) cascaded channel. The eavesdropper (E) generates the decoded data by using the key, K.sub.e, it extracted from its channel and the decoded data is expressed as:

${\widehat{X}}_e={\widehat{Y}}_e\varnothing K_e$

[0063] It is important to note that both the receiver (B) and the eavesdropper (E) follow the same steps as shown in FIG. 2 in order to decode the secret data they received from the transmitter (A). Since the transmitter (A) transmits the data using the keys she extracted from the receiver’s (B) channel, the receiver (B) can securely receive this signal in the presence of the eavesdropper (E). Although the eavesdropper (E) has multiple antennas and more skills than the receiver (B), it cannot decode the data correctly even if it extracts its own keys after manipulating its channel by adding an artificial component into it.

REFERENCES

[0064] K. Ren, H. Su, and Q. Wang, “Secret key generation exploiting channel characteristics in wireless communications,” IEEE Wireless Commun., vol. 18, no. 4, pp. 6-12, 2011.

[0065] K. Zeng, “Physical layer key generation in wireless networks: challenges and opportunities,” IEEE Commun. Mag., vol. 53, no. 6, pp. 33-39, 2015.

[0066] J. Zhang, T. Q. Duong, A. Marshall, and R. Woods, “Key generation from wireless channels: A review,” IEEE Access, vol. 4, pp. 614-626, 2016.

[0067] J. M. Hamamreh, H. M. Furqan, and H. Arslan, “Classifications and applications of physical layer security techniques for confidentiality: A comprehensive survey,” IEEE Commun. Surveys & Tut., 2018.

[0068] A. Badawy, T. Elfouly, T. Khattab, C.-F. Chiasserini, A. Mohamed, and D. Trinchero, “Robust secret key extraction from channel secondary random process,” Wireless Commun. Mob. Comp., vol. 16, no. 11, pp. 1389-1400, 2016.

[0069] J. Zhang, A. Marshall, R. Woods, and T. Q. Duong, “Efficient key generation by exploiting randomness from channel responses of individual OFDM subcarriers,” IEEE Trans. Commun., vol. 64, no. 6, pp. 2578-2588, 2016.

[0070] G. Li, A. Hu, L. Peng, and C. Sun, “The optimal preprocessing approach for secret key generation from OFDM channel measurements,” in IEEE Global Commun. Conf. (Globecom), 2016, pp. 1-6.

[0071] G. Li, A. Hu, J. Zhang, and B. Xiao, “Security analysis of a novel artificial randomness approach for fast key generation,” in IEEE Global Commun. Conf. (Globecom), 2017, pp. 1-6.

[0072] N. Patwari, J. Croft, S. Jana, and S. K. Kasera, “High-rate uncorrelated bit extraction for shared secret key generation from channel measurements,” IEEE Trans. Mob. Comp., vol. 9, no. 1, pp. 17-30, 2010.

[0073] Y. Wu, H. Xia, and C. Cheng, “Improved mult-bit adaptive quantization algorithm for physical layer security based on channel charscteristics,” in Int. Conf. Sys. Inf. (ICSAI), 2018, pp. 807-811.

[0074] Y. S. Khiabani and S. Wei, “ARQ-based symmetric-key generation over correlated erasure channels,” IEEE Trans. Inf. Forensics Sec., vol. 8, no. 7, pp. 1152-1161, 2013.

[0075] J. Zhang, R. Woods, A. Marshall, and T. Q. Duong, “An effective key generation system using improved channel reciprocity,” in IEEE Int. Conf. Acoust., Speech Sig. Proc. (ICASSP), 2015, pp. 1727-1731.

[0076] S. Fang, I. Markwood, and Y. Liu, “Manipulatable wireless key establishment,” in IEEE Conf. Commun. Net. Sec. (CNS), 2017, pp. 1-9.

[0077] S. Fang, “Channel camouflage and manipulation techniques in wireless networks,” PHD theses and dissertations, 2018, https://scholarcommons.usf.edu/etd/7284.

[0078] L. Jiao, N. Wang, P. Wang, A. Alipour-Fanid, J. Tang, and K. Zeng, “Physical layer key generation in 5G wireless networks,” IEEE Wireless Commun. Mag., 2019.

[0079] X. Zhou, L. Song, and Y. Zhang, Physical Layer Security in Wireless Communications. CRC Press, 2016.

[0080] A. Goldsmith, Wireless Communications. Cambridge University Press, 2005.