The physical characteristics of wireless channels, in particular the channel impulse response (CIR), have become an attractive source of shared secrecy between two communicating parties for many wireless applications. However, ensuring high-entropy secret bits from the CIR may not be always feasible, including in IoT and/or indoor environments, where the coherence time may not be easy to obtain, or mobility is limited. Many techniques have been developed to resolve the above problems, including privacy amplification for higher entropy output, and pre-processing for data distillation. In this paper, we develop a framework for extracting entropy from sequentially arriving channel impulse response observation samples, and propose algorithms for both online and offline versions of the problem. We fully characterize the optimal solutions to these problems through a series of analytic results, and establish the optimality of our proposed online and offline algorithms, namely A_EMSON and A_EMSOFF to solve the problem. We also verify the performance of the algorithms through comparisons with the existing related techniques, and evaluate their capability to ensure high-entropy under different channel models using MATLAB simulations.
The physical characteristics of wireless channels, in particular the channel impulse response (CIR), have become an attractive source of shared secrecy between two communicating parties for many wireless applications. However, ensuring high-entropy secret bits from the CIR may not be always feasible, including in IoT and/or indoor environments, where the coherence time may not be easy to obtain, or mobility is limited. Many techniques have been developed to resolve the above problems, including privacy amplification for higher entropy output, and pre-processing for data distillation. In this paper, we develop a framework for extracting entropy from sequentially arriving channel impulse response observation samples, and propose algorithms for both online and offline versions of the problem. We fully characterize the optimal solutions to these problems through a series of analytic results, and establish the optimality of our proposed online and offline algorithms, namely A_EMSON and A_EMSOFF to solve the problem. We also verify the performance of the algorithms through comparisons with the existing related techniques, and evaluate their capability to ensure high-entropy under different channel models using MATLAB simulations.
The wireless radio communications channel between two legitimate user terminals has long been considered as a renewable source of shared secrecy. As characterization of the channel, the time-varying channel impulse response (CIR) is commonly used for this purpose due to its reciprocity and inherent difficulty in being captured by an adversary. While each terminal generates a secret key at its side, based on its own channel measurements combined with an independent noise component, mismatches may occur in the extracted information or in the obtained bit sequences after a sampling and quantization process. To mitigate such bit mismatches, we propose a method that utilizes the properties of the Golay code to correct or detect the bit mismatches for CIR-based secret key agreement. In our approach, the reconciliation information in the transmission is deliberately designed to hide the original shared information (perfect or imperfect). The correctness of our bit mismatch mitigation algorithm is thoroughly analyzed for all possible scenarios. In addition, the effectiveness of our algorithm is validated through simulations using MATLAB for generated CIR data, the results of which match our theoretical analysis. Our design is shown to be secure and optimal by security analysis and comparisons between our proposed algorithm and its variants. The comparisons with related techniques and the scenarios in which our design can benefit are also discussed.
We evaluate the effectiveness of secret key extraction, for private communication between two wireless devices, from the received signal strength (RSS) variations on the wireless channel between the two devices. We use real world measurements of RSS in a variety of environments and settings. Our experimental results show that (i) in certain environments, due to lack of variations in the wireless channel, the extracted bits have very low entropy making these bits unsuitable for a secret key, (ii) an adversary can cause predictable key generation in these static environments, and (iii) in dynamic scenarios where the two devices are mobile, and/or where there is a significant movement in the environment, high entropy bits are obtained fairly quickly. Building on the strengths of existing secret key extraction approaches, we develop an environment adaptive secret key generation scheme that uses an adaptive lossy quantizer in conjunction with Cascade-based information reconciliation [7] and privacy amplification [14]. Our measurements show that our scheme, in comparison to the existing ones that we evaluate, performs the best in terms of generating high entropy bits at a high bit rate. The secret key bit streams generated by our scheme also pass the randomness tests of the NIST test suite [21] that we conduct.
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.
In wireless decentralized networks, security and privacy become more challenging due to stringent architecture and sometimes low-complexity constraints. Confidentiality of the transmissions is usually achieved by symmetric cryptography, which implies the distribution of identical secret keys to the legitimate nodes. Classical solutions for symmetric key distribution can be too complex from a computational or infrastructural point of view. An alternative would be physical layer key generation, which exploits the radio channel as a source of common information. This paper looks into the generation of secret keys from Impulse Radio - Ultra Wideband (IR-UWB) channels using low-power integrated devices. We investigate the possibility to generate identical secret bit sequences in practical environments (e.g., possibly occupied indoor rooms) and operating conditions (e.g., mobility) under specific hardware limitations inherent to device integration (e.g., time resolution and signal dynamics). The main challenge is to obtain reciprocal and random bits at an acceptable rate. Taking into account these three criteria (i.e., reciprocity, randomness, and key length), we describe and evaluate a new quantization scheme adapted to the channel impulse responses acquired by real devices.
We evaluate the effectiveness of secret key extraction, for private communication between two wireless devices, from the received signal strength (RSS) variations on the wireless channel between the two devices. We use real world measurements of RSS in a variety of environments and settings. The results from our experiments with 802.11-based laptops show that in certain environments, due to lack of variations in the wireless channel, the extracted bits have very low entropy making these bits unsuitable for a secret key, an adversary can cause predictable key generation in these static environments, and in dynamic scenarios where the two devices are mobile, and/or where there is a significant movement in the environment, high entropy bits are obtained fairly quickly. Building on the strengths of existing secret key extraction approaches, we develop an environment adaptive secret key generation scheme that uses an adaptive lossy quantizer in conjunction with Cascade-based information reconciliation and privacy amplification. Our measurements show that our scheme, in comparison to the existing ones that we evaluate, performs the best in terms of generating high entropy bits at a high bit rate. The secret key bit streams generated by our scheme also pass the randomness tests of the NIST test suite that we conduct. We also build and evaluate the performance of secret key extraction using small, low-power, hand-held devices-Google Nexus One phones-that are equipped 802.11 wireless network cards. Last, we evaluate secret key extraction in a multiple input multiple output (MIMO)-like sensor network testbed that we create using multiple TelosB sensor nodes. We find that our MIMO-like sensor environment produces prohibitively high bit mismatch, which we address using an iterative distillation stage that we add to the key extraction process. Ultimately, we show that the secret key generation rate is increased when multiple sensors are involved in the key extraction process.
Physical layer secret key generation is a novel research direction that has attracted many researchers in recent years due to its low computational capacity and ability to obtain information-theoretic security. In this paper, we propose two methods based on the modulus and the real and imaginary parts of the channel impulse response (CIR) matrix to generate a secret key between two legitimate devices in massive Multiple-Input Multiple-Output (MIMO) wireless communication systems. For these methods, the legitimate devices first estimate the channel by using a probing signal to find their CIRs. Next, the CIR matrices are transformed into single-row vectors. The first method uses a simple quantization algorithm to convert the modulus of the complex components of the vector into the final key bit sequence. The second method uses the same quantization algorithm to transform the highest points of the real and imaginary parts into two sequences. The XOR operation between these two sequences generates the final key sequence. The key samples with different lengths generated from the system can be assessed by the most crucial performance metric of randomness according to the National Institute of Standards and Technology (NIST) version 800-22REV1A. The numerical results show that the first method has a higher pass rate and more randomness than the second method. Moreover, our proposed methods are superior to existing methods.