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IEEE Transactions on Information Forensics and Security

Exploiting Multi-Antenna Non-Reciprocal Channels for Shared Secret Key Generation

Dongrun Qin, Zhi Jun Ding

Published Jul 27, 2016• 43 citations
Publisher

Abstract

Traditional physical-layer (PHY)-based secret key generation schemes typically assume reciprocity of forward and reverse channels in order for facilitated shared secrecy. However, frequency-selective channels for frequency division duplex links as well as calibration difference of transceiver hardwares can invalidate reciprocity assumption and can cause severe key error floors between users. In this paper, we propose two practical PHY-based secret key generation schemes for multiple-input and multiple-output wireless links over non-reciprocal physical channels. One is pilot assistant secret key (PASKey) generation and the other is eigenvalue-based secret key (EVSKey) generation. In PASKey, both legitimate users estimate the same composite channel, while EVSKey exploits the properties of eigenvalues to generate secret bits. Both are effective against secrecy key theft by eavesdroppers. Furthermore, without adopting public training signals, EVSKey manages to generate secret bits even when nearby eavesdroppers collude. Our two schemes are shown to be more efficient and can achieve lower key error ratio than the existing methods.

DOI
10.1109/tifs.2016.2594143

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Traditional physical-layer (PHY)-based secret key generation schemes typically assume reciprocity of forward and reverse channels in order for facilitated shared secrecy. However, frequency-selective channels for frequency division duplex links as well as calibration difference of transceiver hardwares can invalidate reciprocity assumption and can cause severe key error floors between users. In this paper, we propose two practical PHY-based secret key generation schemes for multiple-input and multiple-output wireless links over non-reciprocal physical channels. One is pilot assistant secret key (PASKey) generation and the other is eigenvalue-based secret key (EVSKey) generation. In PASKey, both legitimate users estimate the same composite channel, while EVSKey exploits the properties of eigenvalues to generate secret bits. Both are effective against secrecy key theft by eavesdroppers. Furthermore, without adopting public training signals, EVSKey manages to generate secret bits even when nearby eavesdroppers collude. Our two schemes are shown to be more efficient and can achieve lower key error ratio than the existing methods.
Recently, shared secret key generation from wireless channel characteristics has attracted a lot of attention due to its low computational overhead. However, in a static environment, the secret key rate is relatively lower due to the lack of channel time-variation. In this paper, we propose a lightweight and efficient PLKG scheme for static channels. Different from current schemes, in the proposed scheme, the transmitter attaches the information of the secret key into the downlink pilot signals to the legitimate receiver by the proposed secret precoding. At the receiving end, the legitimate receiver can directly observe the correct information of secret key from the public downlink pilot signals, while keeping perfect secrecy of the generated key against eavesdroppers. The key principle lies in that the information of the secret key is coded in the index of the non-activated MIMO (multiple-input-multiple-output) sub-channel. Consequently, only the legitimate receiver can accurately observe the value of the key information. For any other eavesdroppers who suffer an independent channel from the legitimate transmitter and receiver, it is proven that she cannot observe any key information from her received signals. Therefore, the key rate can be improved for the static channels by increasing the symbol rate of the pilot signals. Simulations are conducted to validate the effectiveness of the proposed scheme.
This chapter presents a multi-user secret key generation in massive multiple-input multiple-output (MIMO) wireless networks. Physical-layer key generation (PKG) based on channel reciprocity has emerged as a new technique to establish secret keys between devices. However, in future wireless communications, massive antennas will be employed to support multiple users, which bring new challenges to PKG, e.g., low key agreements caused by long-time delay or strong inter-user interference. By harnessing the sparse characteristics of the beam domain channel model, we propose a general channel dimensionality reduction (CDR)-based secret key generation scheme and optimize the parameters of this scheme. The security of this key generation scheme under different user cases of non-overlapping and overlapping beams is then discussed. Some new directions that generalize and improve the basic scenarios for PKG are described. These new directions include multi-cell multi-user secret key generation, PKG in millimeter wave wireless communications, and PKG in frequency division duplexing systems.
Physical-layer secret key generation (PSKG) stands as a promising privacy protection technique, establishing shared encryption keys through the analysis of highly correlated wireless channel measurements. This approach relies on exploiting reciprocal channel characteristics between uplink and downlink transmissions. Nonetheless, the distinct carrier frequencies employed for uplink and downlink in frequency-division duplexing (FDD) systems pose a challenge in identifying common features. This paper presents a novel approach that exploits the inherent reciprocity between scattering parameters of passive two-port networks within same frequency ranges to overcome this obstacle. By capitalizing this reciprocity and considering closely situated FDD bands, a seamless continuity is anticipated in phase differences extracted form the corresponding S-parameters, between neighboring antennas of an antenna array from both uplink and downlink directions. This continuity, thereby ensures consistency in the generated keys from both transmission ends. Furthermore, a two-stage pre-processing method is proposed to enhance performance effectively. Additionally, the paper suggests the utilization of polynomial curve-fitting through measurement data to improve reciprocity and proposes a non-linear framework for quantizing the merging points of the two FDD bands. A statistical analysis employing multiple linear regression is provided to determine the error probability associated with the generated keys. Empirical results validate the feasibility and effectiveness of the proposed key generation scheme, affirming its attributes in terms of randomness, efficiency, key distribution uniformity, and key disagreement ratio (KDR).
Exploiting the inherent physical properties of wireless channels to complement or enhance the traditional security mechanisms has attracted prominent attention recently. However, the existing secret key generation schemes suffer from miscellaneous extracting procedure. Many PHY-layer authentication schemes assume that the knowledge of the shared key is preknown. In this paper, we propose PHY-layer secret key generation and authentication schemes for orthogonal frequency-division multiplexing (OFDM) systems. In the secret key generation scheme, to simplify the extracting procedure, only one legitimate party is chosen to probe the channel and quantize the measurements to obtain the preliminary key. The preliminary key is masked by the channel-phase after the mapping and before equalization and distributed to the other party. The final shared key is used for the PHY-layer authentication scheme in which random signals and the shared key masked by the channel-phase are exchanged at the PHY-layer. Then, a binary hypothesis test is formulated for authentication. Simulation results show that the proposed secret key generation scheme outperforms the existing schemes. For the PHY-layer authentication scheme, it is immune to various passive and active attacks and a high successful authentication rate is acquired even at low signal-to-noise ratio region.
Physical layer security key generation exploits unpredictable features from wireless channels to achieve high security, which requires high reciprocity in order to set up symmetric keys between two users. This paper investigates enhancing the channel reciprocity using a loop-back scheme with multiple frequency bands in time-division duplex (TDD) communication systems, in order to mitigate the effect of hardware fingerprint interference and synchronization offset. The scheme is evaluated to be robust to passive eavesdropping and active Man-in-the-Middle attack through both theoretical analyses and practical measurements. A secret key generation protocol is subsequently designed. The performance of the proposed secret key generation method is then evaluated through both numerical simulation and experiments. Results demonstrate that the proposed scheme can effectively mitigate non-reciprocity and outperforms the classical TDD scheme in both key disagreement rate and key generation rate.
The physical layer secret key generation exploiting wireless channel reciprocity has attracted considerable attention in the past two decades. On-going research have demonstrated its viability in various radio frequency (RF) systems. Most of existing work rely on quantization technique to convert channel measurements into digital binaries that are suitable for secret key generation. However, non-simultaneous packet exchanges in time division duplex systems and noise effects in practice usually create random channel measurements between two users, leading to inconsistent quantization results and mismatched secret bits. While significant efforts were spent in recent research to mitigate such non-reciprocity, no efficient method has been found yet. Unlike existing quantization-based approaches, we take a different viewpoint and perform the secret key agreement by solving a bipartite graph matching problem. Specifically, an efficient dual-permutation secret key generation method, DP-SKG, is developed to match the randomly permuted channel measurements between a pair of users by minimizing their discrepancy holistically. DP-SKG allows two users to generate the same secret key based on the permutation order of channel measurements despite the non-reciprocity over wireless channels. Extensive experimental results show that DP-SKG could achieve error-free key agreement on received signal strength (RSS) with a low cost under various scenarios.