How Bletchley Park broke the German Enigma code
In part 2 of our series we look at how Enigma was broken and how the first computer was created in the process.
THE PARK GROWS
As the successes with Enigma continued to pile up, so did the requirement for more staff and more room at the Park and increased levels of security. Space was relatively easy to come by and huts were quickly constructed from wood (later brick ones were built) to house teams of cryptographers, each designated with breaking a different type of Enigma code.
Hut 6 was run by Gordon Welchman and was designated with breaking the Army and Air Force Enigma machines. Hut 8 was run by Alan Turing and was designed to break the Naval Enigma encryption. This was made harder by the German Navy’s worry that Enigma was being broken. As a result they used a more secure system of handing out message indicators and switched to using a four-rotor Enigma machine. Fortunately, the Germans also made a crucial mistake.
The four-rotor machine had a lockout wheel on it that meant it could send and receive traditional three-wheel Enigma messages. One day, before the four-rotor Enigma machine was allowed to be used a boat sent a message using all four rotors; realising what they’d done, they had to resend the message using the three-rotor setting. The two messages meant that the Bletchley Park staff could work out the wiring of the fourth wheel having never seen the machine and build the necessary bombe to break the code.
Turing himself was responsible for breaking the naval code, but getting cribs was proving hard, until military action helped him. A code book captured on U-110 gave the Bletchley Park team a list of the Kurzsignalen (Short Signal Code), which where 22 characters long and used to report sightings of possible Allied targets. The codes reduced transmission times and, therefore, the likelihood of being located through direction finding techniques.
Then on 29th or 30th October 1942, U-559 was depth-charged and forced to surface. Three British sailors, Able Seaman Colin Grazier, Lieutenant Francis Anthony Blair Fasson and NAAFI canteen assistant Tommy Brown, swam to the sinking U-Boat and captured code books with all of the current U-boat Enigma keys, plus the Wetterkurzschluessel (Weather Short Code Book). Between the code books, Turing and his team had a vital source of cribs to look out for.
