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arXiv:1005.0712 · IEEE Journal on Selected Areas in Communications

Secure Key Generation in Sensor Networks Based on Frequency-Selective Channels

Matthias Wilhelm, Ivan Martinović, Jens Burkhard Schmitt

Published Aug 23, 2013• 81 citations
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Abstract

Key management in wireless sensor networks faces several unique challenges. The scale, resource limitations, and new threats such as node capture suggest the use of in-network key generation. However, the cost of such schemes is often high because their security is based on computational complexity. Recently, several research contributions justified experimentally that the wireless channel itself can be used to generate information-theoretic secure keys. By exchanging sampling messages during device movement, a bit string is derived known only to the two involved entities. Yet, movement is not the only option to generate randomness: the channel response strongly depends on the signal frequency as well. In this work, we introduce a key generation protocol based on the frequency-selectivity of multipath fading channels. The practical advantage of this approach is that it does not require device movement during key establishment. Thus the frequent case of a sensor network with static nodes is supported. We show the protocol's applicability by implementing it on MICAz motes, and evaluating its robustness and security through experiments and analysis. The error correction property of the protocol mitigates the effects of measurement errors and temporal effects, giving rise to an agreement rate of over 97 %.

DOI
10.1109/jsac.2013.130911

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Key management in wireless sensor networks faces several unique challenges. The scale, resource limitations, and new threats such as node capture suggest the use of in-network key generation. However, the cost of such schemes is often high because their security is based on computational complexity. Recently, several research contributions justified experimentally that the wireless channel itself can be used to generate information-theoretic secure keys. By exchanging sampling messages during device movement, a bit string is derived known only to the two involved entities. Yet, movement is not the only option to generate randomness: the channel response strongly depends on the signal frequency as well. In this work, we introduce a key generation protocol based on the frequency-selectivity of multipath fading channels. The practical advantage of this approach is that it does not require device movement during key establishment. Thus the frequent case of a sensor network with static nodes is supported. We show the protocol's applicability by implementing it on MICAz motes, and evaluating its robustness and security through experiments and analysis. The error correction property of the protocol mitigates the effects of measurement errors and temporal effects, giving rise to an agreement rate of over 97 %.
Key management in wireless sensor networks does not only face typical, but also several new challenges. The scale, resource limitations, and new threats such as node capture and compromise necessitate the use of an on-line key generation, where secret keys are generated by the nodes themselves. However, the cost of such schemes is high since their secrecy is based on computational complexity. Recently, several research contributions justified that the wireless channel itself can be used to generate information-theoretic secure keys between two parties. By exchanging sampling messages during movement, a bit string can be derived that is only known to the involved entities. Yet, movement is not the only possibility to generate randomness. The channel response is also strongly dependent on the frequency of the transmitted signal. In our work, we introduce a protocol for key generation based on the frequency-selectivity of channel fading. The great practical advantage of this approach is that we do not rely on node movement as the source of randomness. Thus, the frequent case of a sensor network with static motes is supported. Furthermore, the error correction property of the proposed protocol mitigates the effects of measurement errors and other temporal effects, giving rise to a key agreement rate of over 97%. We show the applicability of our protocol by implementing it on MICAz motes, and evaluate its robustness and secrecy through experiments and analysis.
Due to the broadcast nature of wireless channels, wireless communication is vulnerable to eavesdropping, message modification, and node impersonation. Securing the wireless communication requires the shared secret keys between the communicating entities. Traditional security schemes rely on public key infrastructures and cryptographic algorithms to manage secret keys. Recently, many physical-layer-based methods have been proposed as alternative solutions for key generation in wireless networks. These methods exploit the inherent randomness of the wireless fading channel to generate secret keys while providing information-theoretical security without intensive cryptographic computations. This article provides an overview of the existing PHY-based key generation schemes exploiting the randomness of the wireless channels. Specifically, we first introduce the fundamental and general framework of the PHY-based key generation schemes and then categorize them into two classes: received-signal-strength-based and channel- phase-based protocols. Finally, we present a performance comparison of them in terms of key disagreement probability, key generation rate, key bit randomness, scalability, and implementation issues.
A less expensive and more flexible solution to the problem of sharing secret keys between wireless nodes is to use the inherent randomness in the wireless channel between them as the source for extracting bits of the secret key between these nodes. We propose a new secret key distribution approach that utilizes the uniformly distributed phase information of channel responses to extract shared cryptographic keys under narrowband multipath fading models. Compared to existing approaches which only support pair wise key distribution, our scheme is highly scalable and can support efficient group key distribution. Various key distribution schemes are appraised to determine the level of security and connectivity. Simulated results show that key distribution exploiting channel phase has improved resilience against node capture. them as the source for extracting bits of the secret key between these nodes. Shared secret key distribution from channel measurements is an application which benefits from the randomnessof the multipath channel. It would not, for example, work in a truly free-space environment (such as deep space radio links). Secret sharing benefits from: • Reciprocity of the wireless radio channel: The multipath properties of the radio channel (gains, phase shifts, and delays) at any point in time and on any given frequency channel are identical on both directions of the link. • Temporal variations in the radio channel: Over time, the multipath channel changes due to movement of either end of the link, and any motion of people and objects in the environment near the link. An application may specifically request a user to move or shake the wireless device in order to generate more temporal variation. • Spatial variations: The properties of the radio channel are unique to the locations of the two endpoints of the link. An eavesdropper at a third location more than a few wavelengths from either endpoint will measure a different, uncorrelated radio channel. The proposed scheme is more flexible and can be applied in both static and mobile environments. Our scheme introduces phase randomness to bit distribution and removes the reliance on node mobility to obtain high entropy bits. We evaluate the proposed schemes through both analytical and simulation studies. The parameters of the scheme can be selected such that a desired level of key distribution accuracy and reliability is achieved with high efficiency.
Recently, there has been great interest in physical layer security techniques that exploit the randomness of wireless channels for securely extracting cryptographic keys. Several interesting approaches have been developed and demonstrated for their feasibility. The state-of-the-art, however, still has much room for improving their practicality. This is because i) the key bit generation rate supported by most existing approaches is very low which significantly limits their practical usage given the intermittent connectivity in mobile environments; ii) existing approaches suffer from the scalability and flexibility issues, i.e., they cannot be directly extended to support efficient group key generation and do not suit for static environments. With these observations in mind, we present a new secret key generation approach that utilizes the uniformly distributed phase information of channel responses to extract shared cryptographic keys under narrowband multipath fading models. The proposed approach enjoys a high key bit generation rate due to its efficient introduction of multiple randomized phase information within a single coherence time interval as the keying sources. The proposed approach also provides scalability and flexibility because it relies only on the transmission of periodical extensions of unmodulated sinusoidal beacons, which allows effective accumulation of channel phases across multiple nodes. The proposed scheme is thoroughly evaluated through both analytical and simulation studies. Compared to existing work that focus on pairwise key generation, our approach is highly scalable and can improve the analytical key bit generation rate by a couple of orders of magnitude.
Wireless sensor networks with a large number of cheap low-power interconnected devices bring up challenging tasks when considering the security of their communications. Our paper addresses the issue how cryptographic link keys between communicating sensor nodes can be continuously re-secured even in presence of an attacker who can read the memory of captured nodes. Distributed sensor systems enable us to use two different approaches to link key (re-)establishment - secrecy amplification and key extraction from radio channel fading. A secrecy amplification protocol lets a group of neighboring nodes cooperate together to re-secure previously compromised link keys using the non-compromised paths, and previous research showed the ability of secrecy amplification protocols to improve the security of a network from 50% of compromised link keys to 90% of secure keys. Key extraction then exploits radio channel properties to generate secret bits shared between two radio-enabled devices. We propose to combine secrecy amplification and key extraction into a dynamic protocol, where every node will dynamically decide what approach provides a greater benefit, considering the probability of getting the link key re-secured and the resources available. In principle, the more standard traffic messages are exchanged on a given link, the more beneficiary is the usage of key extractions. And less busy links then rely on secrecy amplification.
An efficient protocol for establishing secret keys for neighboring nodes in a wireless network is presented in this paper. The security of the key generated by our protocol is based on the unpredictability of the noises in binary symmetric communication channels. The two nodes which try to establish a common secret key receive messages from a common source of random bit strings. Due to noises in the communication channel, the messages received by them, and most importantly the messages received by the eavesdropper, are not the same. We show that the two nodes can modify their messages in an efficient way to obtain a common string which contains enough uncertainty to the eavesdropper. Then, a secure universal hash function is applied to compute a secret key for extracting randomness from the common string. We prove that the probability for the eavesdropper to know the value of the key is negligible. One advantage of our proposed protocol is that the secret key rate is better than the known scheme in other models, such as the bounded storage model. Furthermore, our proposed protocol needs only to perform exclusive-or operations and compute hash values. Thus, it is a computationally light-weight protocol and suitable for devices with limited computing resources, such as sensors in sensor networks.