INFORMATION HIDING -- AN ANNOTATED BIBLIOGRAPHY (4/10)

'Schutz der Vertraulichkeit des Aufenthaltsorts von Mobilfunkteilnehmern'

  • H Federrath, J Thees, Datenschutz und Datensicherheit (June 1995) pp 338-- 348 (in German)
  • The authors describe how direct sequence spread spectrum techniques can be used to make it harder for an eavesdropper to determine the phyical location of a trans- mitter. Low-probability-of-intercept systems use cryptographic rather than maximal length spreading sequences; a direction finding apparatus would need to do a autocor- relation or crosscorrelation analysis of the sequence in order to select the signal. By comparison, frequency agile systems are relatively easy to intercept. There are a num- ber of other engineering factors which need to be considered when designing anonymous networks, such as computer security measures to protect location information stored in the network, and dummy traffic to preventing active attacks through the network. Call setup is troublesome, particularly when initiated by the mobile station; the most elegant solution involves Rabin's beacons.

    'Cryptology and the origins of spread spectrum'

  • D Kahn, IEEE Spectrum v 21 no 9 (Sep 1984) pp 70 -- 80
  • This article describes SIGSALY, the first digital secure telephone, which was used by Roosevelt and Churchill during the war. It used a vocoder with 10 bands of 300 Hz, each sampled for amplitude every 20mS; the digital signal was Vernam encrypted (though since the samples had six levels, the arithmetic was modulo 6). The cables from the scrambling equipment to the users were pressurised and alarmed. Finally, the radio link used an early spread spectrum technique to reduce the likelihood of interception or jamming. One of the inventors of spread specturm was the actress Hedy Lamarr, who obtained a US patent in 1941 on a frequency agile torpedo control system.

    SPREAD SPECTRUM SYSTEMS WITH COMMERCIAL APPLICATIONS

  • RC Dixon, Wiley 1994, ISBN 0-471-59342-7
  • Spread spectrum techniques provide a number of benefits, most notably signal hid- ing, jamming margin, selective addressing, multiple access, interference rejection and high-resolution ranging. For nineteen years, Dixon was the standard reference work on spread spectrum, but was relatively unknown outside a circle of military specialists. Recent advances in integration, as well as a 1985 decision by the FCC to allow com- mercial use, have led to rapid growth in commercial applications ranging from GPS to digital cellular telephones; Dixon has just rewritten his book to take account of all this.

    The mechanics of spread spectrum systems can be quite complicated. The basic techniques -- direct sequence, frequency hopping and time hopping -- are simple enough in concept, but there are many complex tradeoffs between error rate, process gain, chip or hop rate, synchronisation, and the various strategies available to participants and opponents; and the linkage between coding, cryptographic and RF engineering aspects is uniquely complicated. Dixon provides a guide to the underlying theory which should be accessible to a graduate student, and goes on to discuss the engineering aspects of satellite uplinks, GPS, military tactical radios and modems, digital cellular radio and vehicle location. There are also hundreds of references which provide a lead into the research literature.

    METEOR BURST COMMUNICATIONS: THEORY AND PRACTICE

  • DL Schilling (editor), Wiley 1993, ISBN 0-471-52212-0
  • The ionised trails of micrometeors entering the earth's atmosphere reflect radio waves, especially in the low VHF band, and attempts have been made since the 1960's to use the phenomenon for communication. Although the initial interest in the subject faded with the introduction of satellites for the bulk of beyond-line-of-sight communi- cations, meteor burst communications are now used in a number of commercial and military roles. Their intermittent nature, and the relatively small ground footprint of each trail, make them inherently hard to monitor.

    This book is the first comprehensive guide to the subject to appear in modern times. Such a guide is welcome, as the subject spans a very wide range of subject matter: from the physics of the meteor trails themselves through the various coding and other techniques which are used to maximise the available channel capacity through to a number of issues arising from practical engineering experience. The context of the book is a US Air Force network in Alaska, which provides backup communications for early warning radars in the event that satellite communications are knocked out.