Breaking Enigma


February 27, 2009 by D Stack

I mentioned in my into that I’m into history and I’m a software engineer. One can see the impact that computers had on World War II. One example of this is the breaking of the German Enigma code.

This is a complicated area and there are contradictory versions of it. This is not too surprising – for years after the war this remained a secret. What I’m posting here is primarily a summary of it with links to some sites with some pretty heavy details.

Basically, after WWI there was an awareness in Germany how important signals intelligence had become. For example, the British intercepting and leaking to the US the contents of the Zimmerman Telegram, a message from Germany to Mexico during WWI, encouraging a Mexican invasion of the US, helped bring the US into the war.

In 1919 Dr. Arthuer Scherbius of Germany developed the Enigma machine. This machine scrambled a typed in message and scrambled it by means of three to five rotors which displayed different letters of the alphabet. The initial values of these rotors functioned as a key. With every letter typed in, the lowest value rotor would turn one notch. Eventually this rotor would reach a value which would trigger the next rotor to turn and so on. The values on these rotors would determine the output value. This is far more secure than the typicall secret codes you may have made as a child – for instance, the first instance of the letter “A” may represent a “B”, the second a “Q”, and the third an “M”.  In your typical child’s cipher each letter always represents another cipher. A long enough message makes decoding the message fairly easy.

The Enigma machine was bidirectional. If you started with the proper key and had an Enigma machine of the same type (more rotors added greater complelxity), typing in a coded message would result in an output of the original message.

In 1931 a German agent of the French allowed his spymasters to photograph stolen Enigma operating manuals. However neither the French nor the British were able to take advantage of this. They did share the manuals with the Polish Cypher Bureau.

The Poles were able to reconstruct the internals of Enigma and determine how to decipher many German transmissions. This was far from a trivial task. Authorized users of the Enigma machine had access to a codebook which determined the base key value for a given period of time (originally monthly or weekly, but over time this shifted to daily or even many times per day). The beginning of any message was encrypted with this base key and contained the value the key should be set to.

The Polish cyphers used a variety of tools to aid them. They built “bombes”: primitive computers to aid in decoding. They took advantage of the fact that no letter would ever be encoded to itself. They took advantage of frequent phrases within messages(for example many messages ended with “Heil Hitler” or “Please Respond”).

Though Germany was unaware the Poles could decrypt Enigma, they continued to improve it, adding additional rotors, creating a sort of intelligence arms race.

During the German Invasion of Poland in Septermber of 1939, key Polish Cipher Personnel were evacuated, evantually settling in a facility named PC Bruno, located outside of Paris.  The PC  cryptologists began working with Bletchley Park in England, the headquarters of the UK’s cryptologists. One of the best known of these was the English Mathematician (and one of the founders of modern Computer Science) Alan Turing. Ironically, the telegraph line between the facilities was secured by use of Enigma!

After the fall of France the PC Bruno team relocated again, first to Vichy France and Algeria. After the Germans took over all of France they were forced to flee again, with some captured by the Germans, though none gave away the secret of Enigma decryption despite harsh interrogation. Some of the cryptologists made a harrowsing escape, eventually making it to Britain.

What impact did all this work have on the war? Germany never knew for certain that their Enigma coding was not secure. Many historians estimate that the aggregate effect of all this work was to shorten the war by up to two years.  For example, the exact intelligence provided by Ultra (the codename for the intelligence gained from decrypted Enigma transmissions) helped prevent Rommel from taking Egypt in 1942. If British Egypt had fallen to the Germans in 1942, it is hard to imagine D-Day happening as early as it did.



One thought on “Breaking Enigma

  1. publiusthegeek says:

    My own personal comment… If you crawl up my family tree, my ancestry winds up in Italy and Poland. Back in Elementary School I heard all the “Dumb Polish” jokes. In college I heard how easily Poland fell to Germany, using horse-mounted cavalry against the advancing German army. So its nice to be able to write about a major accomplishment of Poles. Actually, doing a little googling of the history of computer science will show a ton of Polish contributors.

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