Cracking the Code: Six of the Most Important Code Advances of the 20th Century

With advances in cipher technology in the 20th century, equivalent advances in cryptoanalysis were essential. New cipher technology meant that codes were produced by machine, not by hand, and therefore, had to be cracked by machine. Cryptography is the work of writing codes; cryptoanalysis is breaking codes.

In this article, we’ll explore the codebreaking of World War I and World War II, as well as take a quick look at the unbreakable, old-school code used by the United States in both World War I and World War II. Mass media has brought a great deal of attention to some of these activities, including codetalkers, Bletchley Park, and Alan Turing.


Room 40

Room 40 was part of British intelligence during World War I, and played a key role in decoding German communications during the War. Room 40, named for the space it occupied in the Old Admiralty Building, was created by the head of Naval Intelligence, Captain William Reginald “Blinker” Hall.

Prior to World War I, most governments, including Britain, had only minimal intelligence operations. These were typically short-lived, with the goal of attaining specific information. Intelligence was not an organized or ongoing activity. This changed with World War I.

Quite early in World War I, Britain acquired three German naval codebooks, providing the basis for the cryptography that would continue throughout the War. These were only three of the many codebooks the Germans used, and each was acquired differently—and none through military action. The division of Naval Intelligence was created in response to these codebooks, and with the understanding that cracking codes might be essential to wartime success. Early on, Naval Intelligence and Room 40 lacked both the people and resources needed to complete their tasks.

By January 1915, intelligence decrypted by Room 40 was providing the British with the information needed to protect British interests and reduce losses to the Germans. Room 40 did not just decode messages, but also used geolocation information from coded transmissions to deduce the position of German U-boats, and, in some cases, act to preserve British ships.

One of the best-known coded messages of the World War I era is the Zimmerman Telegram. The Zimmerman Telegram, sent from Germany to Mexico, was decoded by Room 40, and the message passed to the United States through a series of deceptions. Britain was also spying on the United States and didn’t want the extent of its intelligence efforts known.

The Zimmermann Telegram played a key role in the United States’ entrance into World War I, as it, for the first time, showed a clear intention by the Germans to offer violent opposition to the United States.