Background

This ugly house was the center of a huge wartime establishment devoted to codebreaking. Almost 9000 people worked here around the clock, in the utmost secrecy, even today few will admit more than that they worked there.

Many of the codebreakers were quite excentric, but unquestionably brilliant.

Enigma

Most of the effort centered around breaking Enigma which was used for short tactical messages by the German armed forces.

The key changed each day, and each service used different keys. Thus there was never any shortage of work for the code breakers. They were aided by some sloppy user behavior, a fundamental flaw in the machine (by relfecting the signal back through the rotors, it was impossible for a letter to be encoded as itself), and captured code books which could provide the machine settings for a month.

Buildings

The mansion itself was never big enough to hold everyone. The Cottage, and stables were used, and later primitive huts were put up everywhere.

Machines

A number of machines were developed to help find the keys. Unfortunately these were all destroyed after the war along with all their designs.

The secrets of Bletchley Park were well kept. A BBC documentary and book on The Secret War in the late '70s touched on the Enigma story. Then in 2000 more details came in Michael Smith's book Station X. Even today, I was told, most veterans of the Park will admit to working there, and in which hut, but little more.

Because all the machines and designs were destroyed after the war, it took 15 years to reconstruct a working replica of the bombe from the scraps of information that survived.

A rebuild of Colossus was aided by its designer still being available. See the Colossus rebuild for details of that project.

Colossus

Colossus (at right) was the worlds first (semi) programable computer, it was used to help break fish, a much stronger cipher used for longer messages.

The Lorenz machine (at left) used 12 cipher wheels (compared to 3-4 for Enigma) and works similar to modern stream ciphers. The machine generated a pseudorandom stream of data which was XOR'd with the plain text to produce cipher text which was sent out using the standard teleprinter Baudot code.

Bombe

Aparently named for the ticking noise it makes.

The Bombe simulates 36 separate Enigma machines working in parallel to find the correct wheel settings that would result in decoding a bit of cipher text to an assumed plain text.

The fact that the Enigma machine could not encode a letter as itself was invaluable in helping determine the menu for the bombes.

The plaque on the back of the machine aparently refers a member of the royal family who almost managed to touch the live 200+ volts DC inards of the machine, which would most certainly have been fatal.

The docents gave a rather lucid description of how the menu was derrived from a crib, and how the bombe was then configured.

Each simulated enigma machine worked on one transition represented by the menu. Whenever its wheels were such that the desired transition would result, it attempted to stop the machine by opening a circuit. The bombe only actually stopped when all the target transitions were achieved simultaneously - refered to as a stop. The indicated wheel settings were then tried on a check machine to see if plain (German) text resulted from the cipher text, if not the bombe was set to continue its search.

Operational failures

The fact that a letter could not be encoded as itself was a serious weakness in the system, but operational failures were far more significant.

Too many rules constraining the daily key - such as no rotor set could be used more than once per month, no rotor could be used in the same position two days in a row, and always using the same number of plugboard leads, served only to reduce the effective keyspace and thus the effort needed to attack the system. Considering everyone was using a code book for the daily settings, all those rules were completely unnecessary.

After configuring the enigma machine per the day's settings, the base key or Grundstellung, the operator was responsible for choosing tree random letters for the initial setting of the rotors. These were first transmitted using the base key, before setting the rotors to that setting for the rest of the message. The receiver decoded the session setting using the base key wheel settings, then set his wheels accordingly and decoded the rest of the message.

Many operators were lazy and used the same three letter random sequence repeatedly, AAA, ABC, BER, LIN etc. Also the original procedure was to send that indicator message twice - to ensure against transmission errors. Thus ABCABC might be received as BJEGSM. That gave the codebreakers the same plaintext (albeit short) sent twice with the same key. Later, the session setting was only encoded and sent once.

The German navy were careful to avoid common plaintexts in their messages, their manual of operation for the Enigma even recommended padding messages with random words, varying abreviations and more. Special messages were enciphered twice, once by an officer before giving it to the normal cipher team. Breaking the Naval traffic was thus quite an achievement.

For the other services it was not uncommon for a field operator to send the same NOTHING TO REPORT message every day, or starting the daily weather report with WEATHER REPORT, both gifts for the codebreakers. One of the most famous gifts was a long message which did not contain a single L in the cipher text, it was immediately obvious to one of the codebreakers that the plaintext had been a long series of LLLLLL.

References

The web is full of useful information about these machines and their history, and Google makes finding it easy. Some of the sites that I found useful/interesting are listed below.

http://en.wikipedia.org/wiki/Enigma_machine

Wikipedia provided virtually all the information I needed for my Enigma emulator

http://www.codesandciphers.org.uk/

Lots of pages by Tony Sale curator of the Bletchley Park Museum. The site doesn't appear to be maintained, but there is lots of interesting information.

http://www.ellsbury.com/enigmabombe.htm

Graham Ellsbury pages describing the bombe

http://w1tp.com/enigma/

Lots of photos of various Enigma machines

http://cromwell-intl.com/security/history/enigma.html

Analysis of enigma strength and operational weakness.