How the Bletchley Park code-breaking station came into existence
In part one of our feature, we look at how the Polish started to break the German Enigma code before passing the research onto the British.
THE POLISH INFLUENCE
Bletchley Park’s success would not have been possible if it hadn’t been for the work of the Polish mathematicians before them. On 1st September 1932 Marian Rejewski, working with Henryk Zyglaski and Jerzy Różycki, were given the task of solving the logical structure of the German military Enigma machine. The need was immense as Poland was in a desperate position and could be attacked at any time.
The mathematicians got an early break when they were given the list of cipher keys (Enigma settings) for the months of September and October. Going back through intercepted communications for this period, they managed to work out how the Enigma was wired internally, and how the rotors interacted with each other.
Next came a theme common in breaking Enigma: exploiting human error, which is now as it was then often the weakest link in any security system. To begin with the Poles used a flaw in the way that the message indicator was set.
This indicator was designed as a way for every single transmission to use a different initial rotor setting. It worked in addition to the daily key, which was set out in code books. This daily code contained the rotor order and rotors to be used, the plug board connections and, vitally, a universal initial rotor setting (the ground setting).
Using this method, Enigma operators would create a random initial rotor setting, such as ‘ABZ’, called the message indicator. They would then use the ground setting of the day to encrypt their new message indicator, which would get transmitted twice to ensure correct delivery. In our example, ‘ABZ’ could be transmitted as ‘HRDTSY’. Then, the message indicator would be then be used to encode the real message, which would be transmitted.
DOUBLE TROUBLE
This gave the Polish the information that when the message indicator was sent, the first and fourth letters (H and R), second and fifth (D and T), and third and sixth (S and Y) were the same letters.
By coming up with tables that exploited these relationships, it reduced the number of possible combinations down to 105,456. Although the codes were original broken manually by using perforated sheets, Marian Rejewski come up with an electro-mechanical device (the boma kryptologiczna), which would brute-force attack and try each of the 105,456 possible rotor combinations.
There could be several ‘right’ answers, but another mistake in Enigma made it easy to check: a letter could not have itself as its cypher text. In other words, if you typed ‘A’, you’d never get ‘A’ lighting up on the lampboard. Therefore the ciphertext could not have letters in the same position on the plaintext. Once one message had been decoded, the ground setting of the day was determined and all messages could be decrypted.