This paper presents a novel physical-layer approach to information security. Unlike conventional mathematical-based encryptors, a secret key is generated from a natural phenomenon, i.e., the multi-path fading of a radio wave. The reciprocity in wave propagation is exploited to share the key with a party on the air. An array of electronically steered parasitic radiators provides time variation in the faded amplitude. This variation exhibits strong spatio-randomness which can never be practically solved by any other party. We show proof-of-concept experiments carried out in an actual indoor environment.
This paper presents a novel physical-layer approach to information security. Unlike conventional mathematical-based encryptors, a secret key is generated from a natural phenomenon, i.e., the multi-path fading of a radio wave. The reciprocity in wave propagation is exploited to share the key with a party on the air. An array of electronically steered parasitic radiators provides time variation in the faded amplitude. This variation exhibits strong spatio-randomness which can never be practically solved by any other party. We show proof-of-concept experiments carried out in an actual indoor environment.
We describe a secure communication scheme that uses the random fluctuation of the natural environment of communication channels. Only the transmitter and the receiver share the communication channel characteristics. From reciprocity between a transmitter and a receiver, it is possible for them to share one-time information of their fluctuating channel. This can provide a secret key agreement scheme without key management and key distribution processes. In this paper, we propose a new secret key generation and agreement scheme that uses the fluctuation of channel characteristics with an electronically steerable parasitic array radiator (ESPAR) antenna. This antenna, which has been proposed and prototyped, is a smart antenna designed for consumers. Using the beam-forming technique of the ESPAR antenna, we can increase the fluctuation of the channel characteristics. From experimental results, we conclude that the proposed scheme has the ability to generate secret keys from the received signal strength indicator (RSSI) profile with sufficient independence.
Physical-layer secret key generation enables information-theoretic security by extracting secret keys from channel reciprocity. A key challenge, however, is the limited secret-key rate (SKR) in slow-fading environments. To address this, we propose a movable antenna–aided scheme in which the legitimate parties independently randomize their antenna positions at each probe, inducing channel variation even when the underlying channel parameters are fixed. We show that, with a sufficiently large movement region and many propagation paths, the resulting channel effectively emulates a Gaussian model even in a static setting. Simulations further demonstrate substantial gains in SKR outage performance, with outage probability decreasing as the number of channel probes increases.
In order to establish a secure connections in the wireless environment, cryptographic methods may require an exchange of a key or secret. Fortunately, the environment provides randomness due to multi-path fading that can be exploited by physical-layer security algorithms to help establish this shared secret. However, in some cases, multi-path fading might be absent or negligible; therefore, we look for artificial ways to increase randomness. In this thesis, we explore antenna radiation variation by altering the phase between two antennas as a means of creating artificial fading. We construct a model of the antenna gain variation by analyzing the radiation pattern and run Monte-Carlo simulations to compare our approach to a base case with only multi-path fading. We then empirically collect data in order to confirm our analysis. Finally, we incorporate this model in a prominent security algorithm to demonstrate the improvements in security possible through such an approach.
As a way to ensure security on wireless communication, a secret key agreement scheme exploiting the unique properties of the wireless medium, the reciprocity of radio propagation and the irregularity of multipath fading, is being proposed [1-3]. The method enables two legitimate users to share a secret key without distributing the actual key, by using the radio propagation channel between the two as a common and exclusive source of randomness upon which a secret key could be derived. As an application of this scheme to environments where the radio propagation characteristics are static, a method employing a variable directivity antenna called the ESPAR antenna [4] has been known to be effective. In this method, secret keys are generated from received signal strength indicator (RSSI) sequences of legitimate users. One known problem of this method is that in environments with simple radio propagation characteristics such as a rectangular room of concrete with no fixtures, the strength of the direct wave becomes the dominant factor affecting the fluctuation of the RSSI, thus increasing the chance for eavesdroppers located near the path of such waves to estimate the secret key. Since all previous considerations of the aforementioned scheme has been done with environments where the legitimate users are in line-of-sight, this paper aims to illustrate how the location of eavesdroppers affect the secrecy of keys generated with the scheme, in a radio propagation environment with no clearly dominant wave. The key capacity in respect to the position of the eavesdropper in such environment is shown using computer simulations.
A few years ago, physical layer based techniques have started to be considered as a way to improve security in wireless communications. A well known problem is the management of ciphering keys, both regarding the generation and distribution of these keys. A way to alleviate such difficulties is to use a common source of randomness for the legitimate terminals, not accessible to an eavesdropper. This is the case of the fading propagation channel, when exact or approximate reciprocity applies. Although this principle has been known for long, not so many works have evaluated the effect of radio channel properties in practical environments on the degree of randomness of the generated keys. To this end, we here investigate indoor radio channel measurements in different environments and settings at either 2.4625 GHz or 5.4 GHz band, of particular interest for WIFI related standards. Key bits are extracted by quantizing the complex channel coefficients and their randomness is evaluated using the NIST test suite. We then look at the impact of the carrier frequency, the channel variability in the space, time, and frequency degrees of freedom used to construct a long secret key, in relation to the nature of the radio environment such as the LOS/NLOS character.
Generating a secret key between two parties by extracting the shared randomness in the wireless fading channel is an emerging area of research. Previous works focus mainly on single-antenna systems. Multiple-antenna devices have the potential to provide more randomness for key generation than single-antenna ones. However, the performance of key generation using multiple-antenna devices in a real environment remains unknown. Different from the previous theoretical work on multiple-antenna key generation, we propose and implement a shared secret key generation protocol, Multiple-Antenna KEy generator (MAKE) using off-the-shelf 802.11n multiple-antenna devices. We also conduct extensive experiments and analysis in real indoor and outdoor mobile environments. Using the shared randomness extracted from measured Received Signal Strength Indicator (RSSI) to generate keys, our experimental results show that using laptops with three antennas, MAKE can increase the bit generation rate by more than four times over single-antenna systems. Our experiments validate the effectiveness of using multi-level quantization when there is enough mutual information in the channel. Our results also show the trade-off between bit generation rate and bit agreement ratio when using multi-level quantization. We further find that even if an eavesdropper has multiple antennas, she cannot gain much more information about the legitimate channel.