Enigma online

The replicas of two extraordinary machines presented by the Enigma Cipher Center: Enigma and Rejewski’s Cryptologic Bomb are an introduction and an invitation to the story of the events that changed the course of World War II and paved the way for the digital world we live in. In this history, the innovative use of a machine against the cipher of another machine, which was the work of the Polish cryptanalysts, is of particular importance.

If you want to know more about the genius idea behind the invention of the Rejewski’s Cryptologic Bomb, after which Alan Turing, named his own machine “The Bombe” read the text below and visit the Enigma Cipher Center’s main exhibition in Poznań.

What was the Enigma machine?

The first projects for a cipher machine working on the basis of rotor mechanisms date back to World War I. One of its constructors was engineer Artur Scherbius who patented his own version of the machine, calling it “Enigma” (from the Greek word for mystery). His idea was that the machine would be available for purchase by the civilian population, with its intended market being businessmen wary of their trade secrets sent over the telegraph. Numerous Enigma models got released after that so there actually exists a whole family of cipher machines with rotor mechanisms at their core.

The primary recipient of the machines turned out to be the German army. The blitzkrieg planned by Germany in the 1930s demanded constant communication between different formations and thus an enciphering of the orders sent over the radio was a priority. The Enigma was first used by the German navy in 1926, and by land forces two years later.

In its outside appearance, the Enigma looked like a typewriter, additionally equipped with a panel of light-up letters. Under the lid, there was a set of connected rotors, each with the 26 letters of the alphabet. After a given letter key got pressed (ie. A), an electric impulse would pass through the set of rotors, and in the military version a plugboard as well, and light up another letter on the panel (ie. H). Crucially, after pressing the same letter again (ie. A), a different letter than before would light up (ie. Z). That was because, with each key pressed, the rotor advanced one position and consequently, the electrical circuit connected to a different output letter. After writing down a coded message (from the letters that lit up on the display panel) it would be transmitted by radio waves (using Morse code) to its receiver. After setting up the machine in an identical way as the sender had, the receiver would decode the message by typing it out and reading the letters that lit up.

Enigma presented a new method of machine ciphering in which the key was its initial settings. That included the choice and position of the rotors, rings and plugboard as well as the key input each time by the cryptographer. With the rotors rotating after every key press, the amount of possible combinations became unbelievably high. For that reason, the Enigma cipher was considered by many to be unsolvable.

The Polish Cipher Bureau decided to take on the task anyway and opted to employ mathematicians which was a novel approach at the time. For that purpose, a secret cryptology course opened in Poznań in 1929 for selected students from Poznań University. A Poznań branch of cryptanalyst forces was also formed and placed in the building of the Military Office in the same spot where, in a post-war building, the Enigma Cipher Centre is now located. Thus it was Poznań where the future three conquerors of the Enigma have begun their cryptologic career.

Rejewski’s Cryptologic Bomb

The Polish Cipher Bureau, and specifically a civilian employee by the name of Marian Rejewski, started working on the Enigma issue following his supervisor Maksymilian Ciężki’s orders in 1932 and eventually broke the code towards the end of December of that year. In spite of Germans constantly updating their coding procedures, the Polish team kept developing new devices to help with the deciphering throughout most of the 1930s, devices such as: the cyclometer, Zygalski sheets or Rejewski’s cryptologic bomb.

Rejewski’s foothold that enabled his successful attack attempt against the Enigma cipher, that is reconstructing its initial settings, was his observation of the correspondence between the first six letters of each dispatch. These were the twice coded three letter key that a cryptographer would come up with. Having analysed multiple dispatches from the same day, Rejewski was able to recreate the cycles for each of these keys’ letters which all had certain characteristics. Discovering these cycles considerably lowered the amount of possible rotor combinations used by the cryptographer. To make the process of checking each of these combinations faster, the first of the Polish machines was devised, that being the cyclometer. It was constructed with two sets of Enigma rotors and a switchboard connected to switches and small lightbulbs. They would indicate the length of a given cycle for each day which allowed for the reconstruction of the rotors’ initial settings.

After September of 1938 some major changes were introduced in the Enigma cryptographic methods. From then on, the initial settings of the Enigma were altered for every single dispatch. With the intercepted dispatches, the analysts could only reference a singular cycle (made up of one letter). The response to that was Rejewski’s bomb, the core elements of which were six sets of Enigma rotors, advancing with the help of an electric engine. There were cases in which a single letter in the dispatches from a given day would repeat in multiple sets of keys on each of the three positions. This rare characteristic would then be exploited and the machine was set up to search for that particular letter in a single cycle. Having found it, the rotating rotors immediately stopped. The rotor settings would then be written down and cryptanalysts would check if it gave a result that would enable reading the ciphertext. It was a huge achievement of the Warsaw company AVA to manufacture six such machines in a month.

It must be emphasised that the Poles’ innovative approach was to attempt using machines against another machine’s cypher for the very first time. In May of 1940 Germans had decided to give up on the double encryption of the dispatch key, rendering the last of Polish cryptanalyst methods, the Zygalski sheets. The English had to then approach the issue again, though their taking inspiration from the Polish solutions was apparent. It needs to be said that the genius British mathematician and cryptanalyst, Alan Turing, dubbed his own machine “The Bombe”, after the earlier Polish invention.