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URSI General Assembly and Scientific Symposium

Impact of direct-path wave on Imac in secret key agreement system using ESPAR antennas

Yoshida Tadafumi, Takafumi Saito, Fujiki Katsuhiro, Kazumasa Uematsu, Hideyuki Uehara, Takashi Ohira

Published Aug 1, 2011• 2 citations
Publisher

Abstract

Current cryptography may be potentially decrypted by an extremely high performance computer. More secure key sharing schemes are expected. One possible solution is secret key agreement system which makes use of Electrically Steerable Parasitic Array Radiator (ESPAR) antenna. In this scheme ideally, by using wave propagation characteristics, eavesdropper cannot guess secret key from regular terminals. Actually however, eavesdropper has chances to guess the key if he is located on the line of direct-path wave between regular terminals. In our study, we evaluate the impact of direct-path wave on Information mutual anti-tapping condition (Imac) under a noisy channel.

DOI
10.1109/ursigass.2011.6050502

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Current cryptography may be potentially decrypted by an extremely high performance computer. More secure key sharing schemes are expected. One possible solution is secret key agreement system which makes use of Electrically Steerable Parasitic Array Radiator (ESPAR) antenna. In this scheme ideally, by using wave propagation characteristics, eavesdropper cannot guess secret key from regular terminals. Actually however, eavesdropper has chances to guess the key if he is located on the line of direct-path wave between regular terminals. In our study, we evaluate the impact of direct-path wave on Information mutual anti-tapping condition (Imac) under a noisy channel.
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.
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.
The potential of automatically generating secret keys by randomly changing the state of a reconfigurable antenna is investigated through direct measurement. Measurements are performed with a 5×5 parasitic reconfigurable aperture (RECAP) antenna at 2.54 GHz in an indoor laboratory environment, illustrating that keys can be generated with physical-layer reciprocal channel key generation (RCKG) even in static or line-of-sight (LOS) conditions. Details of the parasitic array, varactor-diode reconfigurable elements, and network-analyzer based measurement setup are presented. Analysis of the data shows that secure key bits is lower when an eavesdropper is in the LOS path between legitimate nodes as opposed to a direction perpendicular to that path. It is also illustrated that the statistics of fading induced with the reconfigurable array are not strictly Gaussian for LOS channels. Encouragingly, measurements show that around 80% of generated key bits are secure even in the worst case conditions.
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 considers the problem of wireless secret key agreement based on radio propagation characteristics, where two legitimate parties generate and share a secret key by exploiting the radio propagation characteristics between them in the presence of an eavesdropper. We developed an experimental system using array antennas to implement the wireless secret key agreement. In this paper, we present the experimental system and validate basic characteristics that are fundamental for the wireless secret key agreement, such as the reciprocity and position dependence of radio propagation characteristics, with the developed system.