During the first two years of World War I, code systems were used for high-command and diplomatic communications, just as they had been for centuries, and cipher systems were used almost exclusively for tactical communications. Field cipher systems such as the U.S. Signal Corps’s cipher disk mentioned above, lacked sophistication (and security), however. Nevertheless, by the end of the war some complicated cipher systems were used for high-level communications, the most famous of which was the German ADFGVX fractionation cipher, described in the section Cryptography: Product ciphers.

The communications needs of telegraphy and radio and the maturing of mechanical and electromechanical technology came together in the 1920s to bring about a major advance in cryptodevices: the development of rotor cipher machines. Although the concept of a rotor had been anticipated in the older mechanical cipher disks, American Edward H. Hebern recognized in about 1917 (and made the first patent claim) that by hardwiring a monoalphabetic substitution in the connections from contacts on one side of an electrical disk (rotor) to contacts on the other side and then cascading a collection of such rotors, polyalphabetic substitutions of almost arbitrary complexity could be realized. A set of these rotors is usually arranged in a stack called a basket; the rotation of each of the rotors in the stack causes the next one to rotate, much as the wheels in an odometer advance 1/10 of a revolution for every full revolution of its driving wheel. In operation, the rotors in the stack provide an electrical path from contact to contact through all of the rotors. In a straight-through rotor system, closing the key contact on a typewriter-like keyboard sends a current to one of the contacts on the end rotor. The current then passes through the maze of interconnections defined by the remaining rotors in the stack and their relative rotational positions to a point on the output end plate, where it is connected to either a printer or an indicator, thereby outputting the ciphertext letter equivalent to the input plaintext letter.

Until 2003, Hebern was generally recognized as the inventor of the rotor encryption machine. In that year, scholars published research showing that in 1915, two years before Hebern’s work, a rotor machine had been designed and built by two Dutch naval officers, Lieut. R.P.C. Spengler and Lieut. Theo van Hengel, a second prototype built by a Dutch mechanical engineer and wireless operator, Lieut. W.K. Maurits, and the devices tested by the Dutch navy in the East Indies under the direction of Rear Adm. F. Bauduin. The navy declined to proceed with the project, however, and the participants did not immediately pursue a patent. At the end of World War I, Spengler and van Hengel sought to patent their idea, but the navy resisted declassifying their work. Meanwhile, Hebern had filed a patent claim in 1917, which held up through the years, and gradually the Dutch inventors were forgotten.

Starting in 1921 and continuing through the next decade, Hebern constructed a series of steadily improving rotor machines that were evaluated by the U.S. Navy and undoubtedly led to the United States’ superior position in cryptology as compared to that of the Axis powers during World War II. The 1920s were marked by a series of challenges by inventors of cipher machines to national cryptologic services and by one service to another, resulting in a steady improvement of both cryptomachines and techniques for the analysis of machine ciphers. At almost the same time that Hebern was developing the rotor cipher machine in the United States, European engineers, notably Hugo A. Koch of the Netherlands and Arthur Scherbius of Germany, independently discovered the rotor concept and designed machines that became the precursors of the best-known cipher machine in history, the German Enigma used in World War II. (See figure.)

Another type of rotor machine is much more like the Vernam encryption system (see Substitution ciphers, above). Such devices are pin-and-lug machines, and they typically consist of a collection of rotors having a prime number of labeled positions on each rotor. At each position a small pin can be set to an active or inactive position. In operation, all of the rotors advance one position at each step. Therefore, if the active pin settings are chosen appropriately, the machine will not recycle to its initial pin configuration until it has been advanced a number of steps equal to the product of the number of positions in each one of the rotors. The figure shows a machine of this type, the Hagelin M-209 (named for the Swedish engineer Boris Hagelin), which was used extensively by the U.S. military for tactical field communications during World War II. In the M-209 the rotors have 26, 25, 23, 21, 19, and 17 positions, respectively, so that the key period length is 101,405,850. (It is interesting to note that this length key would be exhausted in 1/100 of a second on an Internet backbone circuit today.) The relationship of this machine to the Vernam encryption system is not only through the way in which a lengthy binary sequence of active pin settings in the rotors is achieved by forming the product of six much shorter ones, but also in the way a symbol of plaintext is encrypted using the resulting key stream. Just behind the rotors is a “squirrel cage” consisting of 27 bars on each of which is a pair of movable lugs. Either or both of the lugs can be set in a position to be engaged and moved to the left on each step by a diverter actuated by the presence of an active pin on the corresponding rotor. The result is an effective gear wheel in which the number of teeth is determined by both the active pin settings and the movable lug settings. The number of teeth set determines the cyclical shift between one direct alphabet (plaintext) ABC…and a reverse standard alphabet ZYX….Thus, if no tooth were present, A would encrypt to Z, B to Y, and so forth, while one tooth present would cause A to encrypt to Y, B to Z, etc. This is strictly a Vernam-type encryption—i.e., encryption by subtraction modulo 26 of the key symbol from the plaintext symbol. To decrypt, the ciphertext is processed with the same pin settings that were used to encrypt it but with the cyclical shift set to occur in the opposite direction.

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In 1930 the Japanese Foreign Office put into service its first rotor machine, which was code-named Red by U.S. cryptanalysts. In 1935–36 the U.S. Army Signal Intelligence Service (SIS) team of cryptanalysts, led by William F. Friedman, succeeded in cryptanalyzing Red ciphers, drawing heavily on its previous experience in cryptanalyzing the machine ciphers produced by the Hebern rotor machines. In 1939 the Japanese introduced a new cipher machine, code-named Purple by U.S. cryptanalysts, in which rotors were replaced by telephone stepping switches. Because the replacement of Red machines by Purple machines was gradual, providing an enormous number of cribs between the systems to aid cryptanalysts, and because the Japanese had taken a shortcut to avoid the key distribution problem by generating keys systematically, U.S. cryptanalysts were able not only to cryptanalyze the Purple ciphers but also eventually to anticipate keys several days in advance. Functionally equivalent analogs to the Purple cipher machines were constructed by Friedman and his SIS associates, as seen in the figure, and used throughout the war to decrypt Japanese ciphers. Apparently no Purple machine survived the war. Another Japanese cipher machine, code-named Jade, was essentially the same as the Purple (see figure). It differed from the latter chiefly in that it typed Japanese kana characters directly.

The greatest triumphs in the history of cryptanalysis were the Polish and British solution of the German Enigma ciphers and of two teleprinter ciphers, whose output was code-named Ultra, and the American cryptanalysis of the Japanese Red, Orange, and Purple ciphers, code-named Magic. These developments played a major role in the Allies’ conduct of World War II. Of the two, the cryptanalysis of the Japanese ciphers is the more impressive, because it was a tour de force of cryptanalysis against ciphertext alone. In the case of the Enigma machines, the basic patents had been issued in the United States, commercial machines were widely available, and the rotor designs were known to Allied cryptanalysts from a German code clerk. Although such factors do not diminish the importance of the Ultra intercepts, they did make the cryptanalysis easier.