John Erik Hershey

Unconventional cryptographic keying variable management

We present three unconventional approaches to keying variable management. The first approach is based on using a public key cryptosystem (PKC) that is breakable in short, but on average less, time than it takes to set up an ultrawide bandwidth modem that is then used to transport a keying variable for a classical cryptosystem. The second concept proposes using the characteristics of an urban UHF radio channel, determined by mutual sounding, as the cryptovariable. The third concept encourages research into ill-conditioned problems as potentially fruitful ground for PKCs not based on finite field arithmetic. >

Doppler characterization for LEO satellites

Mobile ground-based terminals observe significant Doppler on the forward channel when communicating through low Earth orbit (LEO) satellites. This paper deals with the analytic derivation of the Doppler shift measured by a user on the surface of Earth on a signal transmitted by a circular orbit LEO satellite. Two simplifications are performed to obtain the analytical expression of the Doppler shift as a function of time. First, during the visibility duration of the satellite at a terminal, the trajectory of the satellite with respect to the Earth is approximated by a great circle arc. Second, the angular velocity of the satellite with respect to the user is assumed to be constant. Numerical results validate the approximations. Another result of our analysis is an expression for the visibility window duration of a satellite at a terminal as a function of the maximum elevation angle. An algorithm for estimating the parameters of the Doppler curve based on a couple of Doppler and Doppler-rate measurements is also presented.

Cryptographic Key Agreement for Mobile Radio

Abstract Hassan, A. A., Stark, W. E., Hershey, J. E., and Chennakeshu, S., Cryptographic Key Agreement for Mobile Radio, Digital Signal Processing 6 (1996), 207–212. The problem of establishing a mutually held secret cryptographic key using a radio channel is addressed. The performance of a particular key distribution system is evaluated for a practical mobile radio communications system. The performance measure taken is probabilistic, and different from the Shannon measure of perfect secrecy. In particular, it is shown that by using a channel decoder, the probability of two users establishing a secret key is close to one, while the probability of an adversary generating the same key is close to zero. The number of possible keys is large enough that exhaustive search is impractical.

Fast algorithm for computing the reliability of a communication network

This paper presents a fast algorithm for computing the reliability of a communication network when each link has the same probability of success. The computational complexity of existing algorithms increases exponentially as the number of links increases. Our algorithm is based on combinatorics and we conclude, for the case when links have the same survival probabilities, that the computational complexity grows subexponentially as the number of links increases. Furthermore, the algorithm is mostly algebraic in nature.

Spatial optical CDMA

A method for optical signaling is investigated that allows multiple transmitters to simultaneously use the same noncoherent receiving aperture. The signaling is based on the properties of laser speckle interference patterns. Capacity for an interference limited system is explored for a correlation receiver based on random coding. A synchronization scheme is presented that allows a practical implementation of the proposed optical multiple access system. >

Information assurance through Kolmogorov complexity

The problem of information assurance is approached from the point of view of Kolmogorov complexity and minimum message length criteria. Several theoretical results are obtained, possible applications are discussed and a new metric for measuring complexity is introduced. Utilization of Kolmogorov complexity like metrics as conserved parameters to detect abnormal system behavior is explored. Data and process vulnerabilities are put forward as two different dimensions of vulnerability that can be discussed in terms of Kolmogorov complexity. Finally, these results are utilized to conduct complexity-based vulnerability analysis.

Frequency-hopped spread spectrum in the presence of a follower partial-band jammer

A countermeasure to a partial-band follower jammer is proposed for frequency-hopped spread-spectrum communications. This technique randomizes the transmission technique used by the transmitter (and receiver). Either the information is carried by M tones which are transmitted in a frequency slot, or by M frequency slots which contain signal energy. As a counter-countermeasures, the jammer randomizes between jamming the same frequency slot being used by the communicator, or jamming a subset of the slots not being used by the communicator. The performance for randomized strategies for the communicator and jammer is investigated. It is shown that the proposed technique enhances the systems performance. >

A naturalness-preserving transform for image coding and reconstruction

We have developed a new image transform that possesses some unusual properties that may be useful for some special applications; especially for those instances in which the communications must pass through channels whose quality and characteristics are unpredictable due to nonstationary noise processes or jamming. We present an example of a use for the new transform that can be characterized as a man-in-the-loop error correction procedure. A stable and an efficient algorithm has been developed for the proposed correction scheme and the algorithm is amenable for an array processor. The algorithm is shown to be superior to the conventional least squares type algorithms and can operate in near real time.

Feature cueing in the discrete cosine transform domain

The demand for fast and cost-effective access to multiple compressed data sources is imminent. An interactive link between a distant user and the compressed data is needed such that data transmission is constrained to only a small subset of the compressed data that possesses specific features of interest to the user, hence avoiding transmission and decompression of other noninteresting data sources. We present the first step toward developing this user-compressed data link. We show that oriented line features can be detected in data that are transformed, prior to being quantized and coded, using the discrete cosine transform (DCT). Our work is based on the DCT performed on block sizes of 32 x 32 pixels. The choice of this particular block size is due to proprietary constraints imposed by the specific problem we were commissioned to solve. It involves a proprietary database structure. The DCT is the basis of many established image compression standards such as the Joint Photographic Experts Group (JPEG) and the Motion Picture Experts Group (MPEG). These standards are based on block sizes of 8 x 8 pixels. We also show that the effect we report is exploitable with this more familiar block size. Theoretical proof of the DCT line-feature detector and experimental results are provided. An extension of the approach for logical inference of the existence of specific objects of interest is also outlined. When used as a feature analytical tool in conjunction with its compression role, the DCT may serve as a smart compression procedure for intelligent data archiving, abstraction, and transmission.

Spectral Properties of the Thue-Morse Sequence

The Thue-Morse sequence (TMS) has been proposed as a vehicle for effecting comma-free synchronization. The purpose of this correspondence is twofold. First, we present and examine an expression which describes the TMS power spectral density. Second, we consider the autocorrelation of the TMS sequence and cast it as a random process.