Physical layer key generation techniques aim to extract secret keys from the information contained in wireless channels. However, existing key generation schemes often rely on time-frequency domain waveforms for channel estimation, which not only makes secret extraction less reliable but may also compromise the confidentiality of the extracted secret information. This paper presents physical layer key generation methods relying on the Orthogonal Time Frequency and Space (OTFS) waveform. We present analysis showing that the delay-Doppler domain channel estimates obtained using OTFS are conducive to more secure and reliable secret extraction than time-frequency domain channel estimates obtained using the prevalent Orthogonal Frequency Division Multiplexing (OFDM). This analysis provides theoretical guarantees under certain simple assumptions. We then relax those assumptions in extensive time-division duplex (TDD) simulations and show that under realistic settings, OTFS offers the expected benefits to reliability and security. Our simulations show that the introduced OTFS schemes can reliably extract secret keys from channel estimates in scenarios where time-frequency domain methods deteriorate.
Physical layer key generation techniques aim to extract secret keys from the information contained in wireless channels. However, existing key generation schemes often rely on time-frequency domain waveforms for channel estimation, which not only makes secret extraction less reliable but may also compromise the confidentiality of the extracted secret information. This paper presents physical layer key generation methods relying on the Orthogonal Time Frequency and Space (OTFS) waveform. We present analysis showing that the delay-Doppler domain channel estimates obtained using OTFS are conducive to more secure and reliable secret extraction than time-frequency domain channel estimates obtained using the prevalent Orthogonal Frequency Division Multiplexing (OFDM). This analysis provides theoretical guarantees under certain simple assumptions. We then relax those assumptions in extensive time-division duplex (TDD) simulations and show that under realistic settings, OTFS offers the expected benefits to reliability and security. Our simulations show that the introduced OTFS schemes can reliably extract secret keys from channel estimates in scenarios where time-frequency domain methods deteriorate.
Physical layer key generation (PLKG) has emerged as a promising strategy for securing wireless communication using shared channel knowledge between two legitimate users. Traditional PLKG systems rely on time-frequency domain channels using metrics such as Received Signal Strength Indicator (RSSI) or Channel State Information (CSI) to generate secure keys. As a result of the rapid variation in the time-frequency domain channels, these systems are prone to error and suffer from a high reconciliation overhead requirement. Conversely, the same channels have a sparse and static representation in the delay-Doppler domain, which is utilized by the recently introduced Orthogonal Time Frequency Space (OTFS) modulation scheme. Furthermore, the channel representation in the delay-Doppler domain is uniquely determined by the scattering environment between the transmitter-receiver pair. In this paper, we leverage the sparsity of the delay-Doppler domain channel as well as the uniqueness of the channel representation to introduce two OTFS based PLKG schemes. We show through extensive simulations that the introduced OTFS schemes can get < 10% bit disagreement ratio (BDR) in scenarios where time-frequency domain based RSSI methods break down.
One of the basic problems in cryptography is the generation of a common secret key between two legitimate parties in order to prevent eavesdropping. In this paper, we develop an information-theoretic secret key generation method that exploits the physical layer properties of the multipath channel in orthogonal frequency division multiplexing (OFDM) systems with time division duplexing (TDD), and in particular the amplitude and phase of the estimated channel frequency response (CFR). Both the fast space decorrelation and the independency of the amplitude and phase of the multipath fading channel are used to generate the secret key, leading to increased secrecy bit rate and negligible secrecy leakage. The effect of the channel estimation error, delay and mobility are investigated for the presented method. Simulation results corroborate the increased performance of the proposed method over existing ones.
One of the fundamental problems in cryptography is the generation of a common secret key between two legitimate parties to prevent eavesdropping. In this paper, we propose an information-theoretic secret key generation (SKG) method for time division duplexing (TDD)-based orthogonal frequency-division multiplexing (OFDM) systems over multipath fading channels. By exploring physical layer properties of the wireless medium, i.e., the reciprocity, randomness, and privacy features of the radio channel, an SKG method is proposed to maximize the number of secret bits given a target secret key disagreement ratio (SKDR). In the proposed SKG method, the phase information of the estimated channel state information (CSI) is distilled for SKG, and a special guard band (GB) scheme is designed to achieve the target SKDR with a small phase information loss. The proposed GB consists of both the amplitude GB (AGB) and phase GB (PGB), where the AGB is determined by the average signal-to-interference plus noise ratio (SINR), whereas the PGB adapts itself to the instantaneous SINR and thus incurs a smaller phase information loss in the higher SINR region. Analyses show that this GB scheme trades off a small loss of channel phase information for a better SKDR performance, and achieves a much larger number of quantization levels for a given SKDR due to the fact that the PGB decreases quickly as the SINR increases. Based on the performance analysis on the SKDR, the average secret key length, the phase information loss percentage (PILP), and the optimal GB and quantization level of the adaptive quantizor are derived for a given target SKDR. Both analytical and simulation results are presented to demonstrate the superiority of the proposed scheme for TDD-OFDM systems over frequency-selective fading channels.
ABSTRACT This work proposes a secret key extraction technique from the complex impulse response (CIR) for secure orthogonal frequency division multiplexing (OFDM) wireless communication systems. Using time division duplexing (TDD), the legitimate transceivers exchange pilot tones within the channel coherence time to estimate their CIRs, from which the dominant peaks are selected and quantised into binary sequences using a mean‐referenced Heaviside rule. Simulations are conducted in MATLAB for OFDM systems over Additive White Gaussian Noise (AWGN) and Rayleigh fading channels with Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK) modulation schemes. The proposed method is evaluated in terms of randomness, key disagreement rate (KDR), and key generation rate (KGR). For key lengths of 128 bits, 100,000 bits, and 1,000,000 bits, the generated key sequences pass 8, 9 and 15 tests, respectively, in the National Institute of Standards and Technology (NIST) statistical test suite, with all p ‐values greater than 0.01. Since the simulations are performed under ideal reciprocal channel conditions, the keys generated by the two legitimate users are identical, resulting in a KDR of 0 prior to any reconciliation process. In addition, because 16 dominant CIR peaks are extracted from each realisation and each peak contributes one bit, the proposed method achieves a constant KGR of 16 bits per CIR realisation. The results show that the proposed scheme provides a scalable and deterministic key generation mechanism with strong randomness, zero disagreement in simulation, and controllable generation efficiency.
Physical layer key generation from the wireless channel is an emerging area of interest to provide confidentiality and authentication. One of the main challenges in this domain is to increase the length of the generated secret key while maintaining its randomness and uniformity. In this letter, new dimensions for wireless channel-based key generation are proposed for orthogonal frequency division multiplexing (OFDM) systems. The novel perspective of the proposed work lies in the generation of key bits not only from the magnitudes of OFDM subchannels as it has conventionally been done but also from the number and positions/indices of those subchannels, whose channel gains are above the mean of the respective subblock. The effectiveness of the proposed algorithms is evaluated in terms of key generation rate and key mismatch rate. It is shown in the simulation results that the involvement of the proposed dimensions can double the key generation rate compared to conventional algorithms.
In this paper, a physical layer security scheme is proposed for index modulation aided orthogonal time frequency space (IM-OTFS) modulation systems working in Frequency Division Duplex (FDD) mode. In FDD systems, the channel reciprocity, which is the basic assumption for most physical layer security (PLS) techniques, is not applicable. To deal with the security challenge in FDD IM-OTFS systems, Chaos sequence is generated by exploiting the angular reciprocity of the legitimate link to construct the secure mapping rule between the information bits and the modulation symbols as well as the symbol indices. A PLS scheme based on the proposed secure mapping is introduced. The secrecy performance of the proposed PLS scheme is analyzed in terms of Bit Error Rate (BER) and the ergodic secrecy rate. Furthermore, a close-form approximated ergodic secrecy rate is derived to reduce the computational complexity. A truncation method is proposed to obtain robust secure transmissions when the angles of the uplink and downlink legitimate channels are estimated with errors. Simulation results show that the theoretical BER curve approaches the simulated BER curve closely for medium and high signal to noise ratio (SNR) regions, and the eavesdropper’s BER is always around 0.5 for all SNRs. The closed-form approximated ergodic secrecy rate fits the ergodic secrecy rate exactly in low and high SNR regions, and can work as a lower bound of the ergodic secrecy rate in medium SNR region. The secure mapping at the legitimate users is not affected by the angle estimation errors by using the truncation method, while the eavesdropper’s BER is influenced and the influence is analyzed.