Seizing the Enigma (9 page)

The development of the wireless has been such that you can get the direction of one who speaks and go for him; so the German
daren’t open his mouth. But if he does, of course, the message is in cypher; and it’s the elucidation of that cypher which is one of the crowning glories of the Admiralty work in the late war. In my time they never failed once in that elucidation.

Subsequent indications were even more specific. In 1923, the official history of the Royal Navy in the war revealed various instances when intercepts had given the British an advantage. At the same time a dramatic and authentic story drew the attention of all to Britain’s cryptanalysis.

In his best-selling
The World Crisis
, the first two volumes of which were also published in 1923, Winston Churchill, who had been the civilian head of the Royal Navy at the start of the war, revealed, in his flamboyant style and with some poetic license as to facts, the basis of Britain’s codebreaking successes:

At the beginning of September, 1914, the German light cruiser
Magdeburg
was wrecked in the Baltic. The body of a drowned German under-officer was picked up by the Russians a few hours later, and clasped in his bosom by arms rigid in death, were the cipher and signal books of the German Navy and the minutely squared maps of the North Sea and Heligoland Bight. On September 6 the Russian Naval Attaché came to see me. He had received a message from Petrograd telling him what had happened, and that the Russian Admiralty with the aid of the cipher and signal books had been able to decode portions at least of the German naval messages. The Russians felt that as the leading naval Power, the British Admiralty ought to have these books and charts. If we would send a vessel to Alexandrov, the Russian officers in charge of the books would bring them to England. We lost no time in sending a ship, and late on an October afternoon Prince Louis [of Battenberg, first sea lord, whose name was later changed to Mountbatten] and I received from the hands of our loyal allies these sea-stained precious documents.

Churchill followed this with some colorfully told stories of how solved German intercepts had enabled the British to fight better at
sea. Soon a volume of the official German naval history acknowledged that “the German fleet command, whose radio messages were intercepted and deciphered by the English, played so to speak with open cards against the British command.”

Suddenly, the German navy saw that a mere change of codes was no longer enough. It needed to fundamentally transform its system of secret communications. It had to have a cryptosystem that would not give away any secrets even if captured. Perhaps a machine was the answer. The navy had been offered one half a dozen years before that promised security to messages whether or not the machine was in the hands of the enemy. The staff had rejected it as unsuitable, but now the navy saw things differently. It may have examined other cipher machines on the market, such as the wholly inadequate Kryha, but it turned back to Scherbius and began letting contracts.

By 1925, Chiffriermaschinen Aktien-Gesellschaft had started production of the first Enigma machines for the navy. They differed from the commercial model in several ways. The order of letters on the typewriter keyboard and on the illuminated panel was not the QWERTY of the commercial version but alphabetical. The rotor wiring was different. Though only three rotors were used in the machine at one time, five were supplied, providing a greater choice of keys and therefore greater security. Since the reflecting rotor could not be turned, only three toothed thumbwheels instead of four projected above the cover. Instead of twenty-six contacts, the naval Enigma had twenty-nine, adding to the normal alphabet the three umlauted letters ä, ö, and ü, included because the codebook in which plaintext was to be encoded before encipherment by the Enigma had umlauted codewords.

This pre-encoding and the extra codewheels were only two of the ways in which the navy sought to increase the security of messages enciphered in its new machine. Another measure sought to preclude the navy’s chief security concern: espionage. The navy required that only officers, whose honor presumably immunized them against the blandishments of money and women, could set rotor positions.

Another major security measure was aimed at blocking the only method that any German cryptanalyst could then conceive of for solving Enigma messages. Called superimposition, it would require having thirty or so messages, of which portions had been enciphered with the same succession of rotor positions; with very heavy traffic, this might happen. To avoid an accumulation of overlapping texts, the navy prescribed rotor starting positions that were far apart. These were listed in a booklet. The enciphering clerk would choose one and communicate it to the deciphering clerk by an indicator—a group of letters. The indicator was itself enciphered, and the randomness of the prescribed rotor starting positions eliminated the possibility of a cipher clerk’s making up a starting position that was not random, such as XXX or LIL.

A final security measure assigned messages different grades of security—general, officer, staff—with successively more complex cryptosystems and keys held by fewer people.

By the start of 1926, all of these systems had been prepared and Scherbius’s firm had delivered enough Enigmas for the navy to put the machine into service as its Funkschlüssel C (Radio Cipher C). The twenty-three-page manual for it, dated February 9, 1926, covered, in addition to a description of the machine and the method of enciphering and deciphering, such matters as how to test the bulbs and how to deal with ciphering errors.

The navy’s satisfactory experience with the Enigma during its first year became known to the army’s Chiffrierstelle, or Cipher Center. The officer in charge in 1926 and 1927 was Major Rudolf Schmidt, a World War I signals officer who had written the chapter on communications for a major study of the war. He and his cryptologists saw the merits of the Enigma. They made some changes to suit it better for army practice: twenty-six-contact rotors, only three rotors (perhaps to have less to carry in mobile warfare), a standard QWERTY keyboard, and a system of message keys that required no booklet,
only a set of keys that enabled the cipher operator to make up a different key for each message. On July 15, 1928, the Enigma went into the army’s service.

That year a single Enigma cost 600 reichsmarks, or $144 ($900 in 1991 dollars); volume purchases may have reduced this price. But the firm’s sales remained low. A few machines were sold to businesses, but the commercial market never materialized (nor did it for other cipher-machine makers). By the end of the decade the navy had bought no more than a couple of hundred machines, and the army about as many. Still, it was a start.

Then, one spring day in 1929, the team of a horse-drawn wagon that Scherbius was driving at his factory shied and smashed the wagon against a wall. Scherbius suffered severe internal injuries. On May 13, he died, only fifty years old. But his business survived.

By the mid-1930s the firm was manufacturing a variety of cryptographic machines. The army experimented with an eight-rotor printing version for a while. The most important change had come in 1930 with the army’s addition of a plugboard on the front of its machine. This consisted of a plate with twenty-six sockets, each representing a letter, that could be connected with one another by short cables with jacks on the end. The sockets were connected by wires to the keyboard and to the lamps, so that the enciphering and deciphering current passed through the plugboard. It added an extra substitution that overlay the rotor substitution. If on the plugboard the C socket and the R socket were joined by a cable and if without the plugboard the cipher letter for a plaintext
e
was C, the plugboard would convert the C to R. If the plugboardless cipher letter was R, the plugboard would replace this with C. The army connected only six pairs of letters, meaning that twelve letters were enciphered through the plugboard, the others being enciphered only with the rotors. But even twelve encipherments increased the number of keys—and so, theoretically, the number of trials a cryptanalyst would have to make—by billions. The plugboard was an excellent improvement.

In 1935, Hitler denounced the Versailles treaty and began his enormous expansion of Germany’s armed forces. They needed cipher machines, and they continued to buy Enigmas. Other agencies also purchased them: the railroad administration, the
Abwehr
(the military espionage service), and the
Sicherheitsdienst
, or SD (the Nazi party intelligence service).

During those rearmament years, both the army and the navy continually improved the Enigma and developed their systems of secret communication.

The navy alertly scanned the cryptologic horizons for
new
ideas. In the summer of 1930, for example, its cryptographers reported on a cipher machine devised by one Dr. Ruckhaber. “In its mechanical construction the method resembles in many points the not very successful Kryha system,” they wrote. Its mechanism slipped or jumped and caused many enciphering errors. Its output letters were harder to read than those of an illuminating system. Changing its setting took longer than changing the Enigma’s. It appeared easy to solve. The navy turned it down.

The
Reichsmarine
(its name was changed in 1935 to
Kriegsmarine
) developed its own cryptosystems, mostly for specialized uses. Some naval attachés held
Schlüssel A
(Code A), a code with a numerical superencipherment. The
Werftschlüssel
(Dockyard Cipher), a pencil-and-paper system, served shipyards and small ships. Early in 1939 the navy reworked and reissued the
Funkschlüssel H
(Radio Cipher H), which enciphered in pairs the letters of the nonsecret International Radio Telegraph Code. One edition, of 1,400 books in gray binding, served the merchant marine (
Handelsmarine
); the other, 800 in red, was for warships and naval posts. Shortly after war broke out, the navy prepared a
Wetterkurzschlüssel
(Short Weather Cipher) to abbreviate weather information so it could be transmitted “in the shortest possible time.”

The Enigma remained the navy’s basic and most widespread cryptosystem. And the navy proved itself not stiff-necked in continually evaluating it. In particular, it accepted the uncomfortable conclusions of a study by Lieutenant Henno Lucan, second radio officer of the battleship
Elsass
, that in neither physical nor cryptologic security did the Enigma meet modern requirements. At about the same time, the army proposed that the navy adopt the army version of the Enigma, with its twenty-six-contact rotors and with a plugboard, which the navy’s lacked. The army gave two reasons: the plugboard had greatly improved security, and a single machine would make the services cryptographically compatible.

In February 1930, the chief of the Naval Command requested that the B-Dienst look into the army’s proposal. The B-Dienst, successor to the codebreaking B- und E-Dienst of World War I, replied that the main questions to be answered were, did the army machine meet navy requirements and did the use of the same model in larger numbers imperil the navy’s cryptographic security?

On June 21, the B-Dienst offered a positive judgment of the army machine, chiefly because “it offers considerably greater security.” The improvement stemmed from the plugboard, which, the B-Dienst said, raised the number of possible enciphering circuits by 2 to 3 billion. This greatly outweighed the loss of 5,213 starting positions that the army machine’s smaller rotor would entail. Several years passed before the navy’s bureaucracy accepted this argument and before enough of the new machines and rotors were produced and distributed throughout the navy. Finally, in August 1934, the navy began using the army Enigma machine, with its plugboard and typewriter keyboard. But it sought an extra measure of security: each machine had a set, not of three rotors, like the army model, nor of five, like the earlier naval machine, but of seven. The instructions for the new machine—the
Funkschlüssel M
(Radio Cipher M, the M perhaps for
Marine
)—required that rotors I, II, and III serve for talking with the army, that IV and V stay in reserve, and that VI and VII be used when
the navy wanted to send messages to itself. The navy improved the system early in 1939. It recalled rotors VI and VII of all its Enigmas and cut a second notch in the alphabet rings. Notches now stood next to H and U. Each notch caused the rotor to the left to move one space when the notch reached a certain point in its rotor’s revolution. Rotors I to V and later the added rotor VIII each did this once in a revolution. When rotor VI or VII was used in the machine, its extra notch caused the rotor to the left to step twice in each revolution of VI or VII. Though this shortened the period, it also reduced the probability of a successful superimposition.

A few months later, the navy divided its cipher communications into two nets, one using a home waters key, one a foreign waters key. This lowered the volume of messages in each key and so lessened the chances for solution. In addition, the navy continued its three grades of messages. The general keys were widely held; enlisted men could handle them. Officer-grade messages used the same inner settings of the Enigma as the general grade but different plugboard settings, and if officer-grade messages were to be radioed, they were reenciphered with the general key. Staff-grade messages had their own inner settings and plugboard arrangement.

Paralleling these improvements in cryptographic security were those in physical and personnel security. On January 24, 1930, the Naval Command notified its four major units that “a surveillance of the machine and the rotor box [which held the rotors not in the machine], more comprehensive than before, especially on smaller vessels, is planned…. A sharp supervision of the personnel who have access to the machine is necessary.” The Naval Command noted that it intended (as Lieutenant Lucan had proposed) to secure the machine with a lock instead of just a lead seal.