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Digest - IEEE Antennas and Propagation Society. International Symposium

Random number generation from multipath propagation: MIMO-based encryption key establishment

Chan Chen, Michael A. Jensen

Published Jun 1, 2009• 5 citations
Publisher

Abstract

This paper discusses the concept of using the physics associated with multipath propagation and multiple antennas to establish secure encryption keys between two wireless nodes in a network. The presentation includes a proposed algorithm for generating uncorrelated keys despite the fact that the random numbers generated by the propagation channel exhibit spatial and temporal correlation. It further briefly discusses some of the measures that must be taken to make the algorithm implementation practical as well as the theoretically-achievable performance of the algorithm. Simulation results using a physical path-based time-variant MIMO model demonstrate that the length of the established key grows as antennas are added. Furthermore, for high SNR environments, the algorithm is able to achieve a key length versus SNR curve whose slope matches that of the theoretically achievable bound.

DOI
10.1109/aps.2009.5172006

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This paper discusses the concept of using the physics associated with multipath propagation and multiple antennas to establish secure encryption keys between two wireless nodes in a network. The presentation includes a proposed algorithm for generating uncorrelated keys despite the fact that the random numbers generated by the propagation channel exhibit spatial and temporal correlation. It further briefly discusses some of the measures that must be taken to make the algorithm implementation practical as well as the theoretically-achievable performance of the algorithm. Simulation results using a physical path-based time-variant MIMO model demonstrate that the length of the established key grows as antennas are added. Furthermore, for high SNR environments, the algorithm is able to achieve a key length versus SNR curve whose slope matches that of the theoretically achievable bound.
An emerging area of research in wireless communications is the generation of secret encryption keys based on the shared (or common) randomness of the wireless channel between two legitimate nodes in a communication network. However, to date, little work has appeared on methods to use the increased randomness available when the network nodes have multiple antennas. This paper provides theoretical performance bounds associated with using multi-antenna communications and proposes two practical methods for generating secret keys exploiting the increased randomness. Performance simulations reveal the efficiency of the methods.
Encryption of confidential data with a secret key has become a widespread technique for securing wireless transmissions. However, existing key distribution methods that either deliver the secret key with a key distribution center or exchange the secret key using public-key cryptosystems are unable to establish perfect secret keys necessary for symmetric encryption techniques. This research considers secret key establishment, under the broad research area of information theoretic security, using the reciprocal wireless channel as common randomness for the extraction of perfect secret keys in multiple-input multiple-output (MIMO)communication systems. The presentation discusses the fundamental characteristics of the time-variant MIMO wireless channel and establishes a realistic channel simulation model useful for assessing key establishment algorithms. Computational examples show the accuracy and applicability of the model. The discussion then turns to an investigation of the influence of the spatial and temporal correlation of the channel coefficients on the bound of the key size generated from the common channel, and it is found that a sampling approach exists that can generate a key using the minimum sampling time. A practical key generation protocol is then developed based on an enhancement of a published channel coefficient quantization method that incorporates flexible quantization levels, public transmission of the correlation eigenvector matrix and low-density parity-check (LDPC) coding to improve key agreement. This investigation leads to the development of improved channel quantization techniques that dynamically shift the quantization boundaries at one node based on the information provided by the other node. Analysis based on a closed-form bound for the key error rate (KER) and simulations based on the channel model and measurement data show that the enhanced algorithms are able to dramatically reduce key mismatch and asymptotically approach the KER bound. Finally, other secret key generation algorithms based on channel-encryption rather than quantization are discussed, leading to a new concept for secret key generation using the common wireless channel.
While secret encryption keys are commonly-used for securing wireless transmissions, establishment of such keys has traditionally been accomplished using computational techniques at the higher layers of the wireless communication protocol stack. However, the multipath electromagnetic propagation typical of many environments can also be an excellent source of random variables useful for encryption key generation. Furthermore, antennas play an important role in how the transmitter and receiver excite or sense the channel and therefore become a critical component in the performance of techniques that exploit the multipath channel for key establishment. This paper focuses on how using multiple antennas can dramatically improve the performance of such key establishment methods.
An emerging area of interest in wireless security is the generation of secret encryption keys by leveraging the features of the physical propagation channel. The shared randomness of the wireless channel between two legitimate nodes in a communication network is exploited to provide strong secrecy. The secret keys are generated based on fluctuations or evolution of the wireless channel state. However, to the best of our knowledge, little work has been performed on algorithms to exploit the increased randomness available to wireless nodes equipped with MIMO antennas. In this paper, we present a practical key generation scheme for MIMO channels based on quantization performed on blind channel estimates. Our schemes generate random keys for transmitter and receiver by simultaneously quantizing the blind channel estimates. Our schemes are simulated for a realistic fading channel model, and performance metrics for error rate and efficiency are provided.
This paper investigates the establishment of secret encryption keys from temporally and spatially correlated MIMO channel matrices. The discussion first examines the influence of correlation on the secret key rate with the help of a simple SISO channel model that generates samples with correlation behavior that can be described in closed-form. This analysis reveals that an optimal sampling approach can be devised that minimizes the total time required to achieve a key with a specified length. The presentation also examines the decorrelation of measured channel samples, and demonstrates that independently removing spatial and temporal correlations fails to produce random variables that are truly decorrelated. Finally, attention turns to a method for quantizing the uncorrelated channel samples. Simulations using real channel measurements reconfirm the observations made using the simple channel model and demonstrate that while existing algorithms for channel quantization can be effective, their performance falls short of that predicted by the upper bound.
This paper proposes a practical protocol useful for establishing encryption keys between nodes in a wireless network. The method is designed to enable creation of long keys by extracting the increased randomness inherent in reciprocal time-variant multiple-input multiple-output wireless channels. The presentation provides a strategy for exploiting the degrees of freedom existing in the protocol to decorrelate the information in the public discussion from the key information as well as the channel state information. The paper further discusses several error reduction techniques that can efficiently enhance key agreement between the multiple-antenna nodes. Simulations based on a realistic time-variant channel model demonstrate the efficiency of the key establishment as well as the probability of key agreement for the protocol.