What was SIGABA and how did it help win WWII?

It altered the course of history, yet few have even heard of the SIGABA cipher machine. So, what was it? And what does SIGABA stand for? Read on to learn about one of the best kept secrets of World War II.

25 November 2022

SIGABA wasn’t a gun or a tank. It wasn’t a plane, a bomb or a warship. In fact, it resembled a bulky typewriter. Yet it arguably did more for the American war effort than any weapon in the US arsenal. Some historians even contend that it made a bigger impact on the outcome of World War II than the atomic bomb. So what was it?

Developed in the late 1930s by William Friedman, the US Army’s director of Signals Intelligence, and cryptologist Frank Rowlett, SIGABA was the main American message encryption machine of World War II. Like the German Enigmas and Japan’s Type B. Yet SIGABA was once described as ‘Enigma on steroids’. It was even said to have been the most advanced cipher machine the world had ever seen.

So, what was the SIGABA cipher WWII device? What does SIGABA stand for and how did it help win the war? Read on as we decipher the facts and reveal this remarkable story.

What Does SIGABA Stand For?

German signal troops sending coded messages (Photo: Historical via Getty Images)

It was known by many monikers: The Electronic Cipher Machine Mark II, CSP-888/889 and Converter M-134. The Germans called it simply The American Big Machine. However, history remembers it as SIGABA. So, what do these letters stand for? For those hoping for a dramatic reveal, the answer is somewhat anticlimactic.

All cipher machines during the war were named according to the same convention. According to this system, their names all began with the same prefix: ‘SIG’. It meant ‘Signals Intelligence’. Each ‘SIG’ would then be allocated randomly chosen suffixes. In the case of SIGABA it was ‘ABA’. So when we refer to SIGABA, we are talking about the SIG machine with the designation ABA.

How did the SIGABA Machine Work?

The famous Enigma machine (Photo: Ricky Deacon via Getty Images)

Historian David A. Hatch offered a succinct summary of SIGABA, describing it as ‘Enigma on steroids’. Indeed, both SIGABA and Enigma enciphered and deciphered typed-in messages by sending electrical currents through rotors. These rotors worked like a car’s mechanical mile counter. Thus, one set of rotors controlled the movements of another. For example, when one wheel completed a full turn, it caused the next one to move only a fraction of a turn.

At the start of each new message, these rotors moved to random locations, transforming the missive into seeming nonsense. The system relied on a daily changeable list of settings to prevent the enemy from cracking the code. But this alone didn’t work well enough, and Enigma codes were cracked.

The key to the success of the SIGABA cipher was in its complexity. Whereas Enigma machines had three or four rotors, SIGABA had 15. These were divided equally into three banks. From back to front, these were:

  • The main bank: worked in a similar way to Enigma.
  • The control wheels: used to govern the movement of the main bank.
  • The front wheels: didn’t move during the encipherment process but managed the control wheels which in turn controlled the main rotors.

Together, these created the SIGABA cipher, which was said to be unbreakable.

Impenetrable Intelligence

Self Destruct (Photo: CBS Photo Archive via Getty Images)

To avoid the risk of the SIGABA machine falling into the wrong hands, it was designed with an inbuilt self-destruct mechanism. If the operator was under fire and believed he was going to relinquish control of the machine to the enemy, he would trigger a 40lb thermite bomb.

A mixture of aluminium and iron oxide, known as thermite, would react and start to boil, producing searing temperatures of up to 1,400 degrees centigrade, slowly melting the inner workings of the machine.


PM Winston Churchill with General Dwight D. Eisenhower, 1944 (Photo: Hulton Archive via Getty Images)

The Americans used the SIGABA machine throughout World War II. They sent machines all over the world. It even encoded messages between Generals Eisenhower and Marshall as well as those between presidents Roosevelt and Truman and Prime Minister Winston Churchill.

In the war’s later years, the Americans modified a SIGABA machine to make it interoperable with a modified British Typex machine. Known as a CCM, or Combined Cipher Machine, it enabled more secure communication between the US and the British Armed Forces.

This was so successful that its use persevered beyond the War, with the newly-formed NATO employing it well into the 1950s. It was only declassified in 1996.

SIGABA Stood for Secrecy

English Army Detachment Releasing Pigeon carrying messages (Photo: Bettmann via Getty Images)

At a time when secrets meant lives, SIGABA helped the Americans keep theirs. It’s clear to see why some say that, without its cipher, WWII might have ended differently.


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