Alfred Scherbius was one of many people in the early twentiethcentury to patent the basic idea of a mechanical or electromechanical codemachine based on rotors. With cryptography, we have the added advantage that for an excitingand important period of history, the real devices in life-and-death use weremechanical too. Unlikecomputer programs, you can see how they work, and using them allowsparticipants to attempt more interesting activities than can be done purelymanually. Mechanical devices are ideal for lectures. In an hour's typical lecture, the reduced Enigma can illustratedramatic weaknesses in the real Enigma. However, the main point for the present that is conveyed,and that should be conveyed forcibly to all programmers and system developers,is that complex systems - particularly ones involving any interaction or humanelement - are hard to design to be effective and reliable under the pressuresof real use, and one should first build simplified systems to fully understandthe principles. Elsewhere we have discussed how to introduce the underlyingcryptographic concepts to audiences of any age or skill, from school childrento postgraduate specialists: see Bell et al. It is highly suitable for mixed audiences, typified bygrandparents bringing along enthusiastic grandchildren! Likewise, but due tolimitations of space rather than time, this paper, too, raises tantalising issues(e.g., Where do some of the numbers come from? What about different versions ofthe Enigma? What did people, on both sides, learn as the war progressed?) andnumerous fascinating potential discursions (e.g., What did Alan Turingcontribute? How do we do things better today? What about public keys?) thatwill stimulate the reader - or an audience - into unlimited further activityand reflection. The Enigma provides anendless source of historical interest, mathematical interest, human factorsinterest, and so on. This paper, then, should be of equalinterest to readers interested in cryptography and to lecturers who wish to communicatethe ideas clearly, particularly with hands-on participation.Īny good lecture provides ideas that at different pointsdraw the attention of different members of the audience. Design details for a simplified Engima aregiven, as well as relevant historical and technical context, particularly ashighlighted through the simplified design. This paper describes a simplecryptographic machine, closely based on the World War II Enigma: the machinehas been used very successfully in lectures, both to public and schoolaudiences, and to researchers. The paradoxical concept of "sharing secrets" has a natural appeal, andcan be used as a motivation to discuss history, mathematics and computerscience, or to help motivate many concepts such as key distribution, public keycryptography and digital signatures. KeywordsĮducation, Enigma, Public Understanding of Cryptography,"reduced Enigma" IntroductionĬryptography is a fascinatingsubject. This article provides background information, lecturesuggestions, and details for building it. As a working trapdoorfunction, the reduced Enigma also provides an unusually clear introduction topublic key cryptography. Hands-on demonstrations of the reduced Enigma dramatically bringsalive ideas about design, codes, permutations and groups. Had the Axis powers builta reduced Enigma, the outcome of the war might have been different.Ī fully working reduced Enigma has been used verysuccessfully in numerous public lectures, in school talks, and in universityseminars. This 'reduced Enigma' exposes some ofthe design flaws of the original Enigma in a new way. This article describes a simplified cryptographic machine,based closely on the World War II Enigma. Harold Thimbleby * Gresham Professor of Geometry
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