13th June 2002 - Combined Lecture and Visit to Bletchley Park

This event combined a lecture on Enigma Machines, by Jon Paul of Scientific Conversions, with a visit to Bletchley Park, where the Enigma machine-coded messages were decoded by teams of experts during World War II. We had about 60 attendees, all of whom really enjoyed the visit. We started off at 2pm with a brief lecture given by Peter Wescombe, who was one of the three people responsible for saving Bletchley Park (BP) from demolition in the 1990's. Peter gave a brief history of BP and its ownership transitions. We then spent the next two hours touring the grounds and buildings, expertly guided by Peter. Amongst the numerous interesting sights were a Colussus valve computer, radio listening rooms, half a submarine used in the Enigma movie, many huts with famous previous occupants (for example Alan Turing), Enigma machines, Bombe decoding machines, a dispatch rider motorcycle, power generators, an example decoding room, and a gift shop.

We then had tea, and at 5pm, Jon presented his lecture. Jon began by stating that the coding machines and methods are important, but the really critical issue with encryption is the management of the keys. How keys are generated, passed, synchronised and destroyed is very important to maintaining security. Jon then discussed the differences between a code and a cipher, and then described how an Enigma machine works. The input mechanism is a keyboard with a key for each letter of the alphabet. When a key is pressed, an electrical contact is made. The resulting current flows through an entry rotor, three coding rotors, through a reversing rotor, back through the three coding rotors and then lights a lamp behind a particular alphabet letter display. There are five types of coding rotors, of which three are placed in the machine in any order. Each time a key is pressed, the right hand rotor rotates by one position, thereby altering the coding path used. Every 26 key strokes, the next rotor moves by one position. Every 26 squared key strokes, the left hand rotor moves by one position.

The German company that invented Enigma actually filed a US patent on its construction, which helped the BP team to break the codes. For each message, the starting position of the rotors was sent twice. This also helped to break the codes.

A plug board was added which gave an extra layer of routing, both from the keys to the entry rotor and from the exit rotor to the lamp display. The positions of the cables which were plugged into the plug board were changed periodically.

Various accessories were available, including extension lamp displays, and a printing attachment. In 1944 an "UHR" or hour board was added, which caused the positions of the plug board cables to be automatically changed every hour. This would have made the breaking of the codes much more difficult, however the war ended before this could have a serious effect.

Later machines, for example US Sigaba and Type X used a randomised movement of the rotors. An earlier, more complicated machine, the T-52d, was used for high command messages. This used a pseudo random movement of the rotors with exclusive OR coding. This is regarded as the mechanical analogue of every modern coding scheme.

Enigma weaknesses were mostly about how it was used, for example

• Messages were sent in the clear occasionally
• Sending "nothing is happening" (Nichts zu melden) regularly
• Messages sent in the normal order, no swapping around of words
• Keys and messages kept together

Several methods were used to break the codes, including sheets with punched holes placed over a light table, and motorised rotors (Bombe machines). During 1932 to 1945 analysis techniques included traffic analysis, Bombes, captured Enigma machines and code books, and various other pattern based tricks.

At the peak of BP activity, there were more than 10,000 people employed on site, and they were breaking the daily code typically within one hour of the code change. Tens of thousands of messages were intercepted and decoded.

Jon finished off his lecture by demonstrating how an Enigma machine was used, by operating the replica that was made for the Enigma film. At the end of the lecture, there was a lively question and answer session, and Jon was thanked in the usual way for his lecture. He is a world expert on Enigma machines, and his knowledge and passion were in evidence throughout his lecture. Also, I was left with a deep respect for the Bletchley Park people, and a sense of gratitude for the clear influence they had on the outcome of World War II.

For more information about Bletchley Park, look at www.bletchleypark.org.uk

Steven Harris