During the first two years of World War I, code sys-tems 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 previously, 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 "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 con-tacts on the other side and then cascading a collection of such rotors, polyalphabetic substitutions of almost arbi-trary 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, Lieutenant R.P.C.
Spengler and Lieutenant Theo van Hengel, a second pro-totype built by a Dutch mechanical engineer and wireless operator, Lieutenant W.K. Maurits, and the devices tested by the Dutch navy in the East Indies under the direction of Rear Admiral 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 improv-ing rotor machines that were evaluated by the U.S. Navy and undoubtedly led to the United States’ superior posi-tion 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, result-ing 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.
The German navy employed vari-ous versions of the Enigma cipher machine during the war, including this four-rotor model.
Hagelin design M-209 U.S. cipher machine used for tactical commu-nications during World War II.
Another type of rotor machine is much more like the Vernam encryption sys-tem (described previously in "Substitution Ciphers").
Such devices are pin-and-lug machines, and they typi-cally consist of a collection of rotors having a prime number of labeled positions on each rotor. At each posi-tion a small pin can be set to an active or inactive posi-tion. 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. One machine of this type, the Hagelin M-209 (named for the Swedish engineer Boris Hagelin), was used exten-sively by the U.S. military for tactical field communica-tions 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 sym-bol 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 cycli-cal shift between one direct alphabet (plaintext) ABC…
Although no Japanese Purple cipher machines survived the war, this is a functional analog of the Japanese machine that was oper-ational from 1939.
The Japanese Jade cipher machine was a variant of the Purple cipher machines in use during the war. It differed primarily in that Japanese kana characters could be typed directly on the keyboard.