Enigma encoding machine

How Enigma machines worked

The machine is an electro-mechanical device that relies on a series of ‘rotors’ to scramble plaintext messages into incoherent ciphertext. Similar machines were first made in the early 20th century, and the first ‘Enigma’ was invented by German engineer Arthur Scherbius in 1918, who sought to sell it for commercial, rather than military, purposes.  

Above: Enigma machine with four rotors.

During the following years, the Enigma was redesigned and improved several times. This particular machine was made in 1944. It uses four code rotors and is housed in a wooden case. With it comes a rare survival, a smaller case containing five interchangeable spare rotors. This naval-type machine is the most advanced Enigma machine used in World War II; those used by the other military forces only had three rotors, and two spares.

Cracking the code

The Enigma code was eventually cracked by British Intelligence officers working at Bletchley Park near London, initially using methods developed by Polish mathematicians. The messages sent out each day used a different password, and discovering this password permitted the messages to be read. The machines used in the decoding work were called Bombes. They could only solve one problem, so were not really true computers.

The information received from deciphered material was codenamed ULTRA, as the fact that messages could be deciphered had to be closely guarded to prevent the encryption methods being changed. ‘Ultra’ secrets were deemed even more valuable than information classified ‘Most secret’, previously the highest security classification used.

This ability to decipher German military communications is often thought to have helped bring the war to a swifter end, although decades of secrecy delayed the recognition of the work carried out at Bletchley Park.