Cracking the Enigma

The Imitation Game was centered around a top secret British programme at Bletchely that aimed to decode encrypted German communication during the second world war. Towards the end, the cryptography team engaged by the Churchill government sought to break a German encryption device, the “Enigma”, which was believed to be one of the greatest in history.

To decode a particular German message that had been encrypted by Enigma, it had to be fed back into a similar Enigma device. Though the British were in possession of the same, they were rendered helpless by lack of knowledge as to the settings of the device. The ingenuity of the system lay in the fact that the Germans meticulously switched the settings every midnight, leaving less than 24 hours for the cryptographers to decode the day’s encrypted communication. Adding to their woes, the machine had around five rotors and ten plug board cables, giving rise to several trillion possible combinations.

Cracking the code was of paramount importance as details regarding secret convoys, U-boats in the Atlantic and surprise attacks by Germany on the Allied forces were communicated via the encrypted format afforded by Enigma.

The polish mathematician, Marian Rejewski, was the first person to achieve a major breakthrough on the Enigma, when he developed a method to crack the key of the machine once sufficient messages encrypted with the same key was collected during a time when the Engima used just 3 rotors. However, this method eventually failed when the Germans decided to change the procedure for choosing indicator settings. However, after this he went on to construct an electromechanical machine called bomba kryptologiczna (literally meaning cryptologic bomb), which attempted to brute force the key of the Enigma. The Germans later added 2 more rotors, exponentially increasing the number of possible keys and rendering brute force mostly useless.

The infamous cryptanalysis work at Bletchley Park started on 4^th September, 1939, when Britain declared war on Germany. Most of the work here, after receiving a jump-start due to the decision of Poland to share their research, was based on the principle of common words with known positions allowing identification of the key. (This is in contrast to the portrayal of the same in the movie The Imitation Game, where this realisation is the breakthrough that allows Alan Turing & Co to finally crack the Enigma.)  A British version of the bombe was designed by Alan Turing and built by Harold Keen, and was called 'Victory'. Over the next few years, over 200 such machines were built and placed in allied military bases so as to decrypt German communication effectively.

A rebuilt Victory machine, now residing at the Bletchley museum.

Due to the efforts of the entire team consisting of thousands working at Bletchley Park and because of an important mathematical input by Gordon Welchman, the Victory eventually succeeded in breaking several crucial messages transmitted over the Enigma, which later became a decisive factor in the Allied triumph over Germany in World War II.

Thus, one of the greatest encryption devices known to mankind, was broken due to the use of predictable words and repetitive communication, and set the wheels rolling for what eventually became the cyber world today.

Article written by Nivedith George and Aswin G