They were encoded using the typewriter-like Enigma machine. The keyboard was wired so that typing one letter lit up a different one in a set of bulbs on top. Rotors altered the path of the electric circuit with every keystroke.

The machines were commercially available but modified for German military use. Without knowing the precise setting of a machine, there was no way to unpick the code.

By 1936, in the run-up to war, the German military was tightening its communications. In October that year, the senders began to reset the Enigma machines daily. Dermot Turing credits another Polish mathematician, Jerzy Różycki, with realizing that this altered the frequency of letters, revealing extra information.

The team developed tools to work through the hundreds of permutations, including punched cards and a mechanical device with rotors that mimicked Enigma, which, for uncertain reasons, the team called a bomba. Both concepts were later used and developed by Alan Turing.