Authors: Hervie Haufler
Once the telegram was leaked to the press, headlines across the U.S. blared the incredible news. Any doubts about the message's authenticity were dispelled when Zimmermann admitted that he had sent it. With that, Wilson could no longer withstand the storm of rage the telegram stirred up; the U.S. declared war on Germany.
In March 1918, came the foremost cryptanalytic victory of the war. The German armies were closing in on Paris, preparing for the push that would seize the capital and drive France to make peace. German generals had been launching devastating surprise attacks because their cryptographers had devised a new cipher the French were unable to break. Known as the ADFGVX, it used only these letters of the alphabet because their Morse code equivalents were distinct from one another and less liable to be garbled in transmission. With German salients only thirty miles from Paris, close enough that the city was being bombarded by long-range Big Bertha artillery, French commanders were desperate to know where the next assault would fall. The task of breaking ADFGVX was left to France's most able cryptanalyst, young Lieutenant Georges Painvin. In an incredible feat of sleepless concentration, he broke the cipher and revealed when and where the Germans would strike. This time the French were ready for the German advance. The assault was beaten back and France was saved from defeat.
When the U.S. sent the American Expeditionary Force to France, Herbert O. Yardley, organizer of the first serious U.S. cryptographic program, went along to help with code work at the headquarters of the AEF commander, General John Pershing. Yardley was horrified to find the American forces relying on "schoolboy codes and ciphers" that, he was sure, the Germans were decoding as quickly as American operators. Nonetheless, the doughboys turned the course of the war toward triumph by their fresh vitality and overwhelming numbers, despite having their leaders' orders almost instantly known to the enemy.
The Great War was, indeed, a cryptographer's nightmare.
Well before the war's end, it was evident that a new order of military communications was required. The gasoline-powered mobility of modern armed forces needed radio to coordinate and direct their movements, which, in turn, called for faster and more secure methods of encryption and decryption than were possible with manual systems out of the past. It was time for machines to take on the tasks of cryptology.
Inventors Concentrate on Rotor Machines
Late in the war, the British put forth a code machine, the work of J. St. Vincent Pletts, that they recommended for immediate use by Allied commands. To convince their U.S. allies, they sent over a sample to be tested. The machine was delivered to American cryptanalyst William Friedman, along with five encoded messages the British were sure would prove undecipherable. Friedman broke them in three hours, ending this early try at machine encoding.
The need for mechanical systems was so evident, however, that almost simultaneously inventors in four different countries began work on machines, each of which relied on the same idea. This was the application of the electric-powered rotor, a revolvable code wheel.
Of the four inventors, the one whose development was to have the greatest consequence in World War II was the German, Arthur Scherbius. His work on a rotor device gained a boost when the Dutch inventor who had received the first patent on the machine, Hugo Alexander Koch, assigned the rights to Scherbius a year before he himself died. After going through several transformations, the Scherbius machine emerged as a device resembling an ungainly typewriter housed in a wooden box. It had a keyboard like a typewriter, but with only three rows of keys for the twenty-six letters, and none left over for numbers, punctuation or other extras. Atop the machine to the rear of the keyboard was a plate in which twenty-six round glass apertures were labeled with the letters of the alphabet and positioned above glow lamps. When the operator pressed down a key, rather than a skeletal arm rising to print an impression on paper, one of the glow lamps would illuminate a letter.
The trick was that the lighted letterâthe cipher letterâwas never the same as the depressed keyâthe plaintext letter. Pushing down a key fed a battery-powered electrical impulse into the machine's interior, and thereby hangs a tale of clever complexity.
The Scherbius machine depended primarily on three rotors on a single shaft to do the encoding and decoding. Each rotor was a small hockey-puck-like disk of insulating material. Around its rim were double rows of electrical contacts, twenty-six in number, representing letters of the alphabet. The contacts on one side of a rotor were wired in a random internal arrangement to those on the other side. As a result, the plaintext letters of the message delivered to one side emerged on the other side as different letters, transposed and scrambled. Thus, if the plaintext letter entered the right-hand rotor as
it might exit it as
Then, entering the second rotor as
it emerged as
And entering the third rotor as
it came out the far side as
On the left-hand wall of the machine was a fourth scrambling element: a fixed half rotor with thirteen contacts only on one side. This was the reflector, which the Germans called "the turnaround wheel." It bounced the electrical impulse back once again through the three rotors, rescrambling the order in the passage through each one.
The electrical surge did something else as well: it caused that first rotor to rotate one space, one twenty-sixth of a revolution. Otherwise, each plaintext letter entering the right side of the disk would invariably activate the same ciphertext letter on the other sideâeasy prey for cryptanalysts. By edging forward a notch each time a key was pressed, the entry letter's current flowed through a different contact on the cipher side. As a result, when a plaintext letterâ
sayâwas hit a second timeâ
âthe repeated plaintext letter became a different cipher letter. That is, with the first
enciphered as, say,
the second would be differentâ
say. When the first rotor completed its twenty-six-letter cycle, it triggered the second rotor to move forward a notch and, after
twenty-six-letter rotation, to activate the third rotor. In this way, the machine was constantly changing the interconnections, additionally altering the plaintext inputs.
As if that amount of letter scrambling weren't enough to foil crypt-analysis, Scherbius and his colleagues added a further complexity: the order of the rotors on their shaft could be changed. What had been the right-hand rotor could be switched to the left-hand slot or the middle one, and so on.
Another important feature of Scherbius's machine was the reciprocity of its lettering. If plaintext
lighted the glow lamp for ciphertext
then on the deciphering side of the cycle,
It was this reversibility that allowed the receiver to instantly decipher what the sender had transmitted.
Scherbius called his machine the Enigma. Ironically, considering its subsequent history, he is said to have derived the name from a musical composition,
in which the British composer Edward Elgar used melodic codes in characterizing some of his friends.
In seeking customers for his Enigma, Scherbius pointed out a critical advantage: the machine itself could be captured, but unlike a purloined codebook, it would still be useless to the captor. The reason was that in order to decode a message on the Enigma, it was necessary to know the starting positions of the rotors. This essential information was called the key. With the multirotor Enigma, the number of the key variations ran into the billions. To determine even one key, he argued, would take cryptanalysts years of effort.
His first attempts, in the 1920s, to market the Enigma to business customers as well as military chiefs met with rebuffs. The German navy considered the machine but turned him down. So did the commercial prospects he approached. But then English writers on the war, including Winston Churchill, gave Scherbius a lift. In Churchill's book
The World Crisis,
he revealed how British successes against the German fleet in the Great War stemmed in part from the breaking of German naval codes. His disclosures prompted German navy officers of the twenties to have second thoughts. They bought the Enigma and decided it was their cryptographic answer. The navy began using Enigmas in 1925. The army followed suit in 1928 and the newly reborn air force in 1935.
To make their Enigmas even more secure against cryptanalysis, the Germans introduced two major changes. The first was an increase in the number of rotors. They had their Enigmas built with slots to store two extra rotors. Their machines continued to operate with just three rotors, but the operator's ability to vary the sequence among the five available code wheels enormously increased the difficulties facing the would-be analyst. Later the navy upped the ante by adding an extra rotor and altering their machines to operate with four instead of three.
The second change was the introduction of an entirely new scrambling element, the plugboard, which looked like a miniature telephone switchboard. It included cables to facilitate the pairings of plugs and sockets for twenty-six letters. The operator could change these pairings to send current through the machine by entirely different paths.
With these changes, the Germans could instruct their Enigma operators on the sequence of rotors to insert, the start-up position of each rotor and the order of plugboard pairings. Now, when a German operator pressed down a key of his Enigma, the electrical impulse ran a most tortuous course. First it went through the plugboard maze of wiring, then proceeded one way through the rotors. At the end it was bounced back by that fixed-wheel reflector and returned by a different route through the rotors. Only then did it light the glow lamp.
In peacetime, changes in the settings were first made at quarterly intervals, then once a month and, later, once a week. When the war came, changes were made once a day or, in some cases, every eight hours.
The Enigma required at least two operators, one to strike the plaintext keys, the other to read and copy down the lighted ciphertext letters. For the fastest operation, extra operators were used, the final one transmitting the gobbledygook letter groups over the air.
Progressively altering and improving the Enigma, the Germans made it their all-purpose code machine. It was selected by the security police organizations, railroads and other governmental departments, in addition to the military services.
Thousands of Enigmas were put into use. During the course of the war the number of different keys rose to nearly two hundred, and at some stages of the war the various German networks employed fifty different keys simultaneously.
The Germans had good reason to believe their Enigmas were secure against cryptanalysis. Dr. Ray Miller, a computer scientist at the U.S. National Security Agency, has calculated the exact number of key settings faced by Enigma codebreakers. The possible permutations for the plugboards alone, he has determined, run to more than 500 trillion. And that was just one of the machine's five variable components. All the variables together multiply out to 3 x 10
. That number compares with only 10
as the estimated number of atoms in the entire observable universe. "No wonder," he concluded, "the German cryptographers had confidence in their machine!"
Did the Germans ever suspect that their enemies were reading their Enigma-encoded messages? At times when the Allies seemed to benefit from what seemed like amazing coincidences and incredible streaks of good fortune, questions were raised. Investigations were conducted. Always, at least until the very end of the war, the answers came back uniformly: the Enigma-based communications systems were inviolate. It was inconceivable that human minds could cope with such astronomical numbers of variations. If the Allies fared better than could be explained by brilliance or luck, the cause had to be secret agents, not penetration of the Enigma.
Alternatives to the Enigma
The first rotor code machine was developed by an American, Edward Ff. Hebern. He was also the inventor with the most grandiose ideas for his device.
Hebern began with two electric typewriters connected by twenty-six wires in random fashion. Strike the key on the first and the other would type out its enciphered equivalent. But since the connections were fixed, decryption was too easy.
He moved on to a rotor machine, filing his patent on it in 1921. This complex device used five rotors. He brought his machine to Washington and demonstrated it to the U.S. Navy's Code and Signals Section. Naval officials were "thrilled when he showed us what it could do." They seemed ready to commit to navywide use.
This positive reaction was good enough for Hebern. Without having secured a signed order, he planned big. He sold a million dollars' worth of stock in his new company and built a substantial factory to produce his machines. The navy, however, was not accustomed to moving so swiftly. Not until 1923 did it convince a board to investigate the machine. In the end, it ordered only two machines for six hundred dollars each. Hebern's company filed for bankruptcy and he ended up in court, sued by his stockholders. Even though he later sold thirty-five machines to the navy, Hebern never succeeded in establishing a viable code machine business.
In Stockholm another inventor resolved to try his luck with a rotor-type code machine. This was Arvid Damm, who already had patents for weaving looms. Although his code machines were clumsy constructions that invariably broke down during critical tests, he incorporated a company to market them. His smartest move was to add to his staff a young man named Boris Hagelin, son of a wealthy investor in the firm.
One day in 1924, when Damm was in Paris, word came that the Swedish military was considering a mass purchase of Enigma machines. Hagelin resolved not to let that happen. He made quick changes to simplify the Damm model, making it more like an Enigma. The Swedish army placed a large order.