The reduced Enigma

Harold Thimbleby

September 2003 (updated 2011)

This web site describes a simplified cryptographic machine, based closely on the World War II Enigma. This 'reduced Enigma' exposes some of the design flaws of the original Enigma in a new way. Had the Axis powers built a reduced Enigma, the outcome of the war might have been different.

A fully working reduced Enigma has been used very successfully in numerous public lectures, in school talks, and in university seminars. Hands-on demonstrations of the reduced Enigma dramatically brings alive ideas about design, codes, permutations and groups. As a working trapdoor function, the reduced Enigma also provides an unusually clear introduction to public key cryptography. We provide background information, lecture suggestions, and details for building it.

Full paper in PDF

Full paper in Word (.doc file)

Full paper in HTML -- easier for browsing etc, but the HTML was written by Microsoft Word, so there will be formatting problems!

Brief biography

Harold Thimbleby is Gresham Professor of Geometry, and a Royal Society Wolfson Research Merit Award Holder. He is Director of the FIT Lab and was previously Director of UCLIC, the UCL Interaction Centre, a multidisciplinary centre at University College London. He is particularly interested in the human understanding of computers and complex systems, whether to use them more effectively and enjoyably or to understand them in a more technical sense — rather than be uncritical and passive consumers of the latest products. He gives many public and schools lectures.

See for more information.

Full size figures

Figure 1 (jpeg) Enigma in use

Figure 2 (jpeg) Enigma rotors

Figure 3 (jpeg) Enigma plugboard

Figure 4 (jpeg) Navy manual

Figure 5 (jpeg) Reduced enigma

Figure 6 (jpeg) Lamp board

Figure 7 (Freehand) (EPS PostScript) Keys and lamps

Figures 8 and 9 (Freehand) (EPS PostScript) Connector wires

Figure 10 (Freehand) (EPS PostScript) Lamp board

Figure 11 (Freehand) (EPS PostScript) Rotor wiring

Figure 12 (Freehand) (EPS PostScript) Initial key setting

Figure 13 (Freehand) (EPS PostScript) Reset circuit

Figure 14 (Freehand) (EPS PostScript) Reciprocal wiring