Free Seizing the Enigma by David Kahn Page A

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 atsea. 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