December 7 marks the anniversary of the attack on Pearl Harbor, which thrust the United States into World War II. Although there were many developments that helped to hasten the war's end, among the most important were the advancements in cryptology.
Prior to the 20th century, ciphers were often based on substitutions or transpositions; one letter or symbol was substituted or transposed for a different letter of the alphabet. However, such ciphers were easy to crack because they failed to hide the statistical frequency of common combinations of letters such as "th" or "ng" in English.
Although polyalphabetic ciphers — those using more than one alphabet for substitutions — dated back to the 1400s, they still proved to be relatively easy to break. Codes using a different substitution for each letter were available, but required a very long key, which led to mistakes in transferring the information between the parties who needed it.
In the early 1900s, electro-mechanical rotor cipher machines were invented. Invented too late to be used in World War I, they were used heavily during World War II and even as late as the 1970s. The most famous of these was the German Enigma machine. Using a series of three rotors, or wheels, one or more wheels would advance after each letter was pressed, changing the letter substitution each time. This made encryption very complex and decoding difficult.
The Germans began using the Enigma machine in the late 1920s. By late 1932, the Poles had broken the Enigma code. In 1939 just a matter of weeks before Hitler invaded Poland, the Polish Cipher Bureau shared its technology with the French and the British, who continued to decode messages. When France was invaded, the British were aided by the Americans in their decryption efforts.
The weakness of the Enigma machine was its design which guaranteed that a letter would never be enciphered as itself. British mathematician, computer scientist and cryptanalyst Alan Turing was instrumental in cracking the Enigma code by developing the initial design of the bombe, an electromechanical device that was used to help determine the initial settings of the rotors, as well as other internal and external settings to break the code in use on any given day. It acted like several Enigma machines linked together to determine various combinations. Eventually the Germans added a fourth and fifth rotor, which greatly increased the number of possible combinations, but the bombe was modified further to help decrypt the code, and was quite successful.
The United States Navy started building its own bombes in 1943, primarily for use in decoding U-Boat messages.
Military intelligence gained from decrypting Enigma messages was known by the code name "Ultra." Winston Churchill told King George VI, "It was thanks to Ultra that we won the war." Dwight D. Eisenhower, then the western Supreme Allied Commander, said that Ultra was decisive to the Allied victory. These decryption efforts have been credited with shortening the war in Europe by as much as two years.
Today, encryption is a routine part of computer security. For every effort made to keep information secure, there is always a hacker who is looking for a way to break the code. The age-old battle of looking for ways to improve encryption methods, and looking for ways to break them, continues.