Top Secret! Encryption in the Second World War

The Enigma machine was used by the German Army, Air Force, and Navy—as shown here during the Battle of France. The SS, police, secret services, Reichspost (postal) service, railways, and German Foreign Office all used Enigma, too. Altogether, the Germans used about 50,000 Enigma machines during the war.

The encryption process involved code tables as well as the Enigma machine. At the start of every radio message, the starting position of the rotors would be sent (the message key) and the basic setting of the rotors and plugs confirmed (key identification). This information was encrypted manually using tables.

The Enigma machine was believed to be unbreakable because of its 100 quadrillion combinations. During the Second World War, the German armed forces had complete faith in the ingenious design of the Enigma machine—even though some of the enemy's unexplained successes should have given rise to doubt.

British citizen Alan Turing (1912–54) was an outstanding mathematician and computing pioneer. From 1938, he worked for the GC&CS secret service, which had set up a decryption "factory" at Bletchley Park.

At Bletchley Park, Turing developed mathematical models for decrypting Enigma messages. His Turing Bombe automated the decryption process so that the time required was reduced to just a few hours.

After February 1942, the German Navy began to use an improved version of Enigma with 4 rotors instead of 3. This was not broken by the Allies until December 1942. That decryption success led to the Allies' victory in the U-boat war and had a decisive impact on the outcome of the Second World War.

The Turing Bombe was used to work out the basic Enigma settings for that day—the sequence of the rotors and their starting positions. Each bombe had 108 rotors, replicating 36 Enigma machines.

By using a word that was likely to be included in the radio message, the Turing Bombe would try out all the possible settings in succession, at a rate of 60 rotations per minute. This process would often take just 2 hours.

Listening posts all over the UK were intercepting all German radio traffic. Intercepted messages such as this one were sent to Bletchley Park by teleprinter. The decrypted messages were checked at Bletchley Park for valuable information—such as this message from the Supreme Commander of the Wehrmacht to the surrounded Courland Army Group on February 12, 1945.

The manual backup procedure was used when no Enigma machine was available. Plain text was entered line by line into a table called a "cage" and read out in columns in a random sequence. The result was entered in the code sheet as a "book group" and pairs of letters standing below one another were replaced by encrypted letters using a substitution table.

German counterintelligence services used a machine called a subtraction device to decrypt intercepted radio messages. British agents used a 5-character code, to which the same digit was added each time. The intercepted groups of 5 characters were entered in the subtraction device and the digits from 0–9 were removed from them. The result was printed out on paper, so that meaningless combinations could be quickly spotted.

Rasterschlüssel 44 was the Germans' most secure cipher system. The Allies were able to decrypt 70% of Enigma messages, provided they already knew 40 letters in the text—but breaking this apparently simple manual encryption system took the British at least 2 weeks.

The mechanical C38 was designed by the Swedish cryptographer Boris Hagelin. During the war, he supplied the French, Italians, Japanese, and Americans with different versions of the C38.

During the Second World War, the C38 was the standard-issue cipher machine for the US armed forces, who called it the M-209. It was the American equivalent to the German Enigma, and 140,000 machines were in use.

The portable M209 is shown here being used with a messaging device to coordinate ground, sea, and air forces during the invasion of Lingayen on the Philippine island of Luzon.

Until the end of the Second World War, British secret service troops such as the SOE and commando groups used double transposition encryption.

The Hagelin C-52 was the most commonly used cipher machine in the Cold War. The C-52 uses configurable rotors which work with movable bars in a lug cage. A purely mechanical system, the C-52 achieved a key space of 4.13 sexdecilliards—more than the number of atoms in the universe.